出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/07/18 06:02:02」(JST)
| この項目では、真菌について説明しています。その他の菌類については「菌」をご覧ください。 |
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| 約4億1000万年前 - 現世 （古生代デボン紀前期中盤［プラギアン〈cf.〉］- 新生代第四紀完新世末［サブアトランティック〈cf.〉］） |
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| Regnum Fungi (L., 1753) R.T. Moore, 1980 [1] |
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| Kingdom Fungus | ||||||
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Hibbett, D. S. et al.(2007)
伝統的な四大分類
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菌類（きんるい）とは、一般にキノコ・カビ・酵母と呼ばれる生物の総称であり、菌界（学名：Regnum Fungi ）に属する生物を指す。
細菌などと区別するために真菌（しんきん）とも呼ばれることもある。外部の有機物を利用する従属栄養生物であり、分解酵素を分泌して細胞外で養分を消化し、細胞表面から摂取する。
菌類に属する生物は、ほとんどが固着性の生物である。微視的には、細胞壁のある細胞からなり、先端成長を行うものが多い。これらは高等植物と共通する特徴であり、菌類が当初において植物と見なされた理由でもある。しかし、葉緑体を持たず、光合成も行わない従属栄養生物である。その点は動物と同じであるが、体外の有機物を分解し、細胞表面から吸収する、という栄養摂取の方法をとる。
形態的には単細胞の微生物であるものから、肉眼的大きさ以上に発達する多細胞生物までを含む。しかし、多細胞体を持つものにおいても、菌糸と呼ばれる1列に配置する細胞列までしか持たず、真の組織を発達させない。体が多数の菌糸から構成されているものは糸状菌（しじょうきん）と呼ばれ、単細胞のままで繁殖するものは酵母と呼ばれる。キノコ、カビ、あるいは糸状菌および酵母はいずれも分類上の単位ではない。
生殖には、胞子を形成するものが多い。生活史は様々であるが、無性生殖と有性生殖を含むものが多く、それぞれに異なった胞子を形成するものが多い。生活環においては、核が単相の状態が優占し、複相の期間は限られる。担子菌および子嚢菌は単相 (n) の一次菌糸が体細胞接合により二核の二次菌糸となる時期があり、他の多くの有性生殖を行う生物に見られる複相 (2n) に対してこれを重相 (n+n) 世代と呼ぶ。酵母は出芽または分裂により増殖し、細胞の融合を行う例もある[2]。
植物寄生のものが多く、農業上重要なものも多い。他方、菌根など、植物と共生するものも知られる。動物に寄生するものは少ないが、重要な病原体も含まれる。自由生活をするものはさまざまな生物の死体や排泄物などの有機物を栄養源とし、生態系において分解者として働くと考えられる。他に発酵に関わって重要なもの、抗生物質を産出するものなどがある。
菌界は古典的にはツボカビ類、接合菌類、子嚢菌類、担子菌類などから構成される。ツボカビ類は鞭毛をもつ遊走細胞を形成し、祖先的形質を持つ。
ツボカビ類(古典的な意味での)以外は生活史のどの部分でも鞭毛を形成しない。それらは有性生殖（接合後の減数分裂で生じる胞子のあり方）で分類される。接合菌は接合胞子嚢を形成するグループで、ケカビなどを含む。子嚢菌は子嚢の中に胞子をつくるグループで、ビール酵母などを含む。担子菌は担子器に胞子を外生する群で、キノコの多くを含む分類群である。
伝統的には、これに有性生殖の型が不明なものをまとめた不完全菌、それに菌類と藻類の共生体である地衣類を独立群とし、上記4群に併置した。また、胞子形成の共通性などから変形菌類を菌界に含めた。
しかしながら、20世紀終盤よりの生物分類全般の見直しの中で、これらに大きな見直しがなされており、2010年代現在でも変更が繰り返されている。ツボカビ類と接合菌類は特に変更の幅が大きく、他に新たに認められた群、菌界から排除された群も多い。また、近年の分子系統解析により、これまで原生動物とされてきた微胞子虫も特殊化した菌類の一群であると考えられている。変形菌は除外された。不完全菌、地衣類は独自の分類群として認めるのをやめ、菌類全体の体系の中に納められることとなった。それらについては後述の分類の項に詳細が解説されている。
菌類と細菌類は微生物として一括りに扱われる場合もあるが、前者は真核生物、後者は原核生物であり、細胞構造が全く異なる生物群である。
菌界は真核生物に含まれる界 (Kingdom) の一つであり、動物界や植物界などと同じレベルの分類群である。生物を二界に分類していたころは、菌類には運動性がなく細胞壁を持つことなどから植物に分類されていた。この場合、構造が単純であることもあって、葉緑体を失った退化的な植物である、と考えられることが多かった。しかし、菌類についての理解が深まるにつれ、細胞構造や分子遺伝学的な系統解析などの研究から得られる情報などから、植物とは異なる、独自の生物群であると考えられるようになり、5界説の頃より独立した界として広く認められるようになった[3]。現在の分子遺伝学的情報からは、植物よりも動物に近い系統であることがわかっている。動物と菌類を含む系統のことをオピストコンタという。
なお、かつてはその胞子形成の類似等から、変形菌類を菌界に含めて扱っていた。変形菌類、細胞性粘菌、ラビリンチュラ類をまとめて変形菌門（旧）とし、他の菌類を真菌門とするのが通例であった。また、卵菌類・サカゲツボカビ類なども菌類と考えられていたため、これらをツボカビ類とあわせて鞭毛菌亜門に位置づけていた。しかし、現在ではこれらは別の系統に属するものと判明したため、菌類として扱っていない。それらをまとめて偽菌類と呼ぶことがある。
一般的に、菌類にはツボカビ門、接合菌門、子嚢菌門、担子菌門の四群が含まれるとされてきたが、近年の分子系統解析により接合菌類が単系統でないこと、これまで原生動物とされてきた微胞子虫が菌類に含まれるであろうことが示されている。このうちで鞭毛細胞を持つのはツボカビ類のみである。水中生活をするものがあるのも大部分がこれで、他の群では水中生活のものはあるが、陸上のものが二次的に水中に入ったと考えられるものが多い。したがって、ツボカビ類がもっとも原始的なものと考えて良い。また、接合菌類は形態・構造に単純な面が多いため、これも比較的下等なものと見なされる。そして、子嚢菌、担子菌類がより高等なものと考えられていて、この2群をまとめたものをディカリアとする分類の仕方も提唱されている。しかし、これらの関係については明らかではない。
子嚢菌、担子菌にはそれぞれに酵母型、糸状菌型の生活をするものが含まれる。これらが進化の系列を示すものか、適応放散の結果であるかは判断が分かれる。中には、生活環の中でそれらの型を行き来するものがあるので、少なくともたとえば酵母型は単細胞だから下等、といった単純な判断はできない。
このほか、重要な菌類の群として、不完全菌類 (Deuteromycetes, Imperfect fungi, Fungi imperfecti) と呼ばれるグループが存在する。これらは無性生殖だけで繁殖しているように見える子嚢菌（しのうきん）または担子菌（たんしきん）である。体細胞分裂によって形成される分生子（ぶんせいし）と呼ばれる胞子により、あるいは胞子を作らずに菌糸の栄養成長のみによって、または酵母として増殖する。不完全菌類はその分生子形成様式などによって便宜的に学名が与えられているが、完全世代（有性生殖を行う世代）が発見・命名されればその学名がその生物の正式な名として使用される。不完全菌類としては同じ属に分類されていたものが、完全世代では別の属に分類されることもあり、不完全菌類としての分類はあくまで暫定的なものである。しかし、たとえばアオカビやコウジカビなど身近に見られるカビのほとんどはこれであり、また植物病原菌など実用上重要なものが多く含まれている。
なお、不完全菌の名は、かつては正式に分類群の名としても用いられたが、現在では次第に使わない方向に向かっており、代わりにアナモルフ菌 (Anamorphic fungi) や分生子形成菌 (Mitosporic fungi) 等の名が使われる。
地衣類は、コケ類と間違われやすいが、菌類の作った構造の内部に藻類が共生して成立している、複合的な生物体である。これらを分けることも不可能ではなく、それぞれに独立した生物と見なすことも可能である。しかし、実際には両者は強く結びついて生活しており、また両者が揃うことで形成される特殊な成分があったり、極めて特殊な環境で生活できたりと言った面から、それを独立した生物群と見なす考えもあった。しかし、その形態を構成するのは菌類であるから、その名は菌類の名として用いられる。現在ではむしろ菌類のあり方の一つと見なす考えが強くなり、現在では完全に菌類の分類体系に組み込まれている。担子菌もあるが、子嚢菌であることが多く、その中でも複数の分類群にまたがって存在する。不完全菌からなるものは、不完全地衣と呼ばれる。したがって複数の系統から平行的に地衣類が出現したと考えられている。
菌類は植物との関係が深く、動物との関係ははるかに薄い。例えば植物寄生菌には実に多くの種類が存在し、サビキンやクロボキンなど、綱レベルの大きな分類群が丸ごと植物寄生である例も見られる。それに比べると動物寄生のものははるかに少ない。また、その遺体を分解する場合にも、動物の遺体は主として細菌類によって分解され、植物の遺体は菌類が担当する傾向がある。また、共生関係においても現在ではほとんどの陸上植物が菌根を持っていることが知られている。また、この型の菌根が古生代から存在したらしい証拠も見つかっている。
他方、菌類の進化は主に陸上で起こったものと考えられる。接合菌、子嚢菌、担子菌はどれも大部分が陸生であり、水中生活のものはごくわずかである。その点、植物界の主要な群であるコケ類、シダ類、種子植物も陸上で進化したものであり、両者のそれは並行的である。このようなことから、菌類は植物と共進化してきたと考える見方がある。植物は陸上進出の段階で丈夫な繊維質を持つ茎や根を材木として発達させた。これを分解するように進化したのが子嚢菌や担子菌ではないかというのである。植物の側でも菌根などによって菌類の恩恵を受けているから、両者は共進化の関係にあるとも言える。
古典的な体系として、ウェブスターのものをあげておく[4]。下記の亜門を門として扱った例もある。現在においても、高校教科書などではほぼこれを踏襲するので、今後も見る機会はあると思われる。
これ以降の大きい変更としては、まず変形菌門が菌界から外されたことが挙げられる。上記の体系では真菌門の内容のみが菌界である。また、鞭毛菌に含めていたサカゲカビ類と卵菌類も系統が異なるものとして外され、それらはストラメノパイルに含まれている。
21世紀初頭の現在、菌類の分類体系には手が入り続けている。2007年に見直された分類体系では子嚢菌門、担子菌門、ツボカビ門、コウマクキン門、ネオカリマスティクス門（以上の三門が旧ツボカビ門）、グロムス菌門、微胞子虫門、および門としての分類の難しい4亜門（主に旧接合菌門に由来）に分類されている(Hibbett et al. 2007 [5])。
また、上記７門および４亜門に含まれない小群もある（ロゼラ類(Rozella)、フクロカビ類(Olpidium)、バシディオボルス類(Basidiobolus)）。また、菌界を広義に捉えなおし、オピストコンタ内の真菌に近いグループをこれに含める考えもある。（ヌクレアリア等）
菌類は栄養を吸収するために、酵素によって他の動植物を構成する高分子を分解している。 特に、セルロース、リグニン、コラーゲンといった他の生物にとって分解の難しい高分子を炭素、窒素、リンの低分子化合物に分解することができるので、それらの物質を生態系のサイクルに戻す分解者としての役割を担っている。
たとえば、森林内では生産者である植物の現存量は、そのかなりの部分が、消費者に回る前に材や落葉などの枯死（こし）部分として蓄積される。これら植物遺体は主成分がセルロース、リグニンであり、窒素、リンなどの含有量が少ない。そのため多くの動物はこれを直接利用することができない。しかし、これを菌類が分解し、なおかつ周囲から無機窒素化合物などを吸収してその体を作ることで、動物は植物遺体と菌類を同時に摂取し、それを餌として利用することが可能になるのである。
菌類は他の生物の病気の原因となるが、その一方、多くの菌類が他の生物と共生している。
地衣類は菌類と緑藻やシアノバクテリアとの共生体である。維管束植物の根と菌類との共生によって形成される器官は菌根と呼ばれる。菌根は植物が水分や養分を吸収する上で重要な役割を果たすことがあり、菌根の種類によって植物に対して主としてリンを供給するものや窒素を供給するもの、さらには有機物を供給するものも知られている。また，土壌病原菌から植物を防御する機能を持つ場合もあると推測されている。一方、菌類の側は植物から同化産物を供給されている。種子植物ではラン科やイチヤクソウ科、シダ植物ではマツバラン科やハナヤスリ科、ヒカゲノカズラ科の植物は発芽の初期に特定の菌類との共生が成立しないと生育できない。植物の葉などの組織内に共生している菌類は内生菌（エンドファイト）と呼ばれ、その機能についてはまだよく分かっていないが摂食阻害物質等の生成に寄与していると考えられるケースが知られている。アーバスキュラー菌根という型の菌根は陸上植物のひどく広範囲に見られるもので、やはり植物にとって有用な栄養素等の運搬に与っているらしい。
なお、ラン科のムヨウランやイチヤクソウ科のギンリョウソウなど、いくつかの種子植物は光合成色素を持たず、地下部の菌根に頼って生活している。これを、腐生植物という。菌根であるので、植物と菌類の共生と見ることもあるが、最近ではむしろ、植物が菌類を一方的に収奪している寄生とみなされている。かつてはネナシカズラなどと同じような生息基質への寄生と見て、土壌中の腐植質に寄生しているとして死物寄生という言葉もあった。最近の研究では、これらの植物が依存している菌類は主として他の植物と共生している菌根菌や植物病原菌、一部は木材腐朽菌であり、腐生植物は菌類を介して他の生きている植物や枯死植物から、間接的に栄養分を摂取していることが明らかになりつつある。イチヤクソウ科の植物は光合成をする種であっても栽培困難なものが多いが、これも菌類を介して周囲の菌根形成植物から栄養分を収奪して生活しているためである。そのため、外生菌根を形成した樹木とイチヤクソウ類を一緒に鉢植えにすると、長期間の栽培が可能であることが実証されている。
昆虫と菌類との共生も知られている。アンブロシアビートルと総称されるキクイムシは菌類を運搬するためにマイカンギアと呼ばれる器官を持ち，自身が樹幹内に掘った孔道の内側に持ち込んだ菌類を繁殖させ、それを摂食している。菌類の側から見ると、こうした昆虫は菌類を生育に適した環境に運搬していることになり、菌類の分散に寄与していると考えられる。また，熱帯に住むハチ目のハキリアリと、シロアリ目の高等シロアリの一部は、巨大な巣を作り、その中に外部から植物片を運び込み、かみ砕いて「苗床」を作り、そこで菌類を「栽培」し、食料としている。シロアリにおいては、外部の菌類がシロアリの卵に擬態して菌核を保護させるターマイトボールというものも発見されている。
人間は古くからキノコを食品として利用してきた。現在ではシイタケ、エリンギのように大々的に栽培され、身近なものになっているものもあれば、トリュフ、マツタケなどのように栽培が難しく、高級食材として扱われているものもある。
酵母はブドウ糖、ショ糖をエタノールに発酵する。この能力はビール、ワインなどの醸造に用いられている。また、カビや酵母はチーズを作るために重要な役割を果している。日本酒、焼酎、醤油、味噌など、日本古来の発酵食品では、コウジカビを穀物に培養し、繁殖させた麹（こうじ）を用いて醸造を行う。
そのほか、貴腐ワインの生産には果実につくハイイロカビが必要であるとか、食材を冷暗所に保管し、表面にカビを生やせて熟成させる（金華ハム、鰹節等）など、カビが関わる食品は様々である。
菌類には様々な有機化合物を生産するものがいる。例えば、アオカビの一種は抗生物質のペニシリンを生産する。 ベニテングタケは猛毒のアルカロイド、アミノ酸を含んでいる。マジックマッシュルームのように動物の中枢神経に作用し、幻覚症状を引き起こす成分を含んでいる菌類もある。
様々な植物に寄生する菌類が知られている。中には農作物に重大な被害を与えるものも多々ある。植物に寄生する菌類は様々な群に含まれる。代表的なものを以下に挙げる。
なお、卵菌類にも植物寄生菌があり、アブラナ科の白さび病など菌類の起こすものと似た病気が知られる。
菌類によってヒトやその他の動物が感染する病気（感染症）として、白癬菌による白癬（水虫、たむし、およびしらくも）やカンジダによるカンジダ症、クリプトコックスによるクリプトコックス症、アスペルギルスによるアスペルギルス症、プネウモキスチス（ニューモシスチス）によるニューモシスチス肺炎などがあり、臨床的に問題となっている。医学及び獣医学領域においては菌類を真菌と呼び、その学問を医真菌学と称する。真菌による感染症は一般に真菌症と呼ばれ、患部が皮膚の角質などに止まり真皮に及ばない表在性真菌症と、患部が真皮以降の皮下組織におよぶ深部表在性真菌症や、脳、肺、心臓などの内部臓器まで及ぶ深在性真菌症（全身性真菌症、内臓真菌症）に大別される。主に皮膚科領域で扱う前両者と内科系で扱う後者では病気の性質が大きく異なり、治療法および、使用可能な薬剤（抗真菌薬）も異なる。これらの病原菌は多糖類からなるキチン質の強固な細胞壁を持っているのみならず、人体と同じ真核生物であるため菌類の細胞だけに損傷を与えて人体組織に害の少ない薬物は非常に限られたものとなる。そのため、原核生物であり、非対称的に細菌のみに大きな損傷を与えることのできる抗生物質が多く発見されている細菌による感染症に比べ、治療が困難である(白癬菌による水虫の治療薬を開発すればノーベル賞が取れると言われるのはこのため)。また、深在性真菌症は日和見感染症の色彩が強く、診断も困難であることから症例は増加の一途にあり、致命率も高い。また、海外における風土病が重篤な輸入真菌症として国内で発症する事例も増加している。医学医療の高度化、国民の高齢化、および国際交流の普遍化を背景に、真菌症の教育、研究、および臨床を充実させることが期待される。
動植物への寄生を利用して、害虫や雑草を防ぐ生物農薬として使われる菌類がある。
この他に、菌類の生産する毒素（毒キノコやカビ毒）による中毒症や、アレルギー症といった病気の原因でもある。
| [ヘルプ] |
| ウィキスピーシーズにFungi（菌界）に関する情報があります。 |
| ウィキメディア・コモンズには、菌類に関連するメディアがあります。 |
| ウィクショナリーに菌類の項目があります。 |
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| Fungi Temporal range: Early Devonian–Recent (but see text) PreЄ
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| Clockwise from top left: Amanita muscaria, a basidiomycete; |
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| Scientific classification | |
| Domain: | Eukaryota |
| (unranked): | Opisthokonta |
| Kingdom: | Fungi (L., 1753) R.T. Moore, 1980[1] |
| Subkingdoms/Phyla/Subphyla[2] | |
Dikarya (inc. Deuteromycota)
Subphyla incertae sedis
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A fungus (/ˈfʌŋɡəs/; plural: fungi[3] or funguses[4]) is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds (British English: moulds), as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, protists, and bacteria. One major difference is that fungal cells have cell walls that contain chitin, unlike the cell walls of plants and some protists, which contain cellulose, and unlike the cell walls of bacteria. These and other differences show that the fungi form a single group of related organisms, named the Eumycota (true fungi or Eumycetes), that share a common ancestor (is a monophyletic group). This fungal group is distinct from the structurally similar myxomycetes (slime molds) and oomycetes (water molds). The discipline of biology devoted to the study of fungi is known as mycology (from the Greek μύκης, mukēs, meaning "fungus"). Mycology has often been regarded as a branch of botany, even though it is a separate kingdom in biological taxonomy. Genetic studies have shown that fungi are more closely related to animals than to plants.
Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. They may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They have long been used as a direct source of food, such as mushrooms and truffles, as a leavening agent for bread, and in fermentation of various food products, such as wine, beer, and soy sauce. Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals. Losses of crops due to fungal diseases (e.g., rice blast disease) or food spoilage can have a large impact on human food supplies and local economies.
The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 1.5 million to 5 million species, with about 5% of these having been formally classified. Ever since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christian Hendrik Persoon, and Elias Magnus Fries, fungi have been classified according to their morphology (e.g., characteristics such as spore color or microscopic features) or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits. Phylogenetic studies published in the last decade have helped reshape the classification of Kingdom Fungi, which is divided into one subkingdom, seven phyla, and ten subphyla.
A group of all the fungi present in a particular area or geographic region is known as mycobiota (plural noun, no singular), e.g., "the mycobiota of Ireland".[5]
The English word fungus is directly adopted from the Latin fungus (mushroom), used in the writings of Horace and Pliny.[6] This in turn is derived from the Greek word sphongos (σφογγος "sponge"), which refers to the macroscopic structures and morphology of mushrooms and molds;[7] the root is also used in other languages, such as the German Schwamm ("sponge") and Schimmel ("mold").[8] The use of the word mycology, which is derived from the Greek mykes (μύκης "mushroom") and logos (λόγος "discourse"),[9] to denote the scientific study of fungi is thought to have originated in 1836 with English naturalist Miles Joseph Berkeley's publication The English Flora of Sir James Edward Smith, Vol. 5.[7]
Fungal hyphae cells
Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: both fungi and plants are mainly immobile, and have similarities in general morphology and growth habitat. Like plants, fungi often grow in soil, and in the case of mushrooms form conspicuous fruit bodies, which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago.[10][11] Some morphological, biochemical, and genetic features are shared with other organisms, while others are unique to the fungi, clearly separating them from the other kingdoms:
Shared features:
Unique features:
Most fungi lack an efficient system for long-distance transport of water and nutrients, such as the xylem and phloem in many plants. To overcome these limitations, some fungi, such as Armillaria, form rhizomorphs,[27] that resemble and perform functions similar to the roots of plants. Another characteristic shared with plants is a biosynthetic pathway for producing terpenes that uses mevalonic acid and pyrophosphate as chemical building blocks.[28] However, plants have an additional terpene pathway in their chloroplasts, a structure fungi do not have.[29] Fungi produce several secondary metabolites that are similar or identical in structure to those made by plants.[28] Many of the plant and fungal enzymes that make these compounds differ from each other in sequence and other characteristics, which indicates separate origins and evolution of these enzymes in the fungi and plants.[28][30]
Fungi have a worldwide distribution, and grow in a wide range of habitats, including extreme environments such as deserts or areas with high salt concentrations[31] or ionizing radiation,[32] as well as in deep sea sediments.[33] Some can survive the intense UV and cosmic radiation encountered during space travel.[34] Most grow in terrestrial environments, though several species live partly or solely in aquatic habitats, such as the chytrid fungus Batrachochytrium dendrobatidis, a parasite that has been responsible for a worldwide decline in amphibian populations. This organism spends part of its life cycle as a motile zoospore, enabling it to propel itself through water and enter its amphibian host.[35] Other examples of aquatic fungi include those living in hydrothermal areas of the ocean.[36]
Around 100,000 species of fungi have been formally described by taxonomists,[37] but the global biodiversity of the fungus kingdom is not fully understood.[38] On the basis of observations of the ratio of the number of fungal species to the number of plant species in selected environments, the fungal kingdom has been estimated to contain about 1.5 million species.[39] A recent (2011) estimate suggests there may be over 5 million species.[40] In mycology, species have historically been distinguished by a variety of methods and concepts. Classification based on morphological characteristics, such as the size and shape of spores or fruiting structures, has traditionally dominated fungal taxonomy.[41] Species may also be distinguished by their biochemical and physiological characteristics, such as their ability to metabolize certain biochemicals, or their reaction to chemical tests. The biological species concept discriminates species based on their ability to mate. The application of molecular tools, such as DNA sequencing and phylogenetic analysis, to study diversity has greatly enhanced the resolution and added robustness to estimates of genetic diversity within various taxonomic groups.[42]
Mycology is the branch of biology concerned with the systematic study of fungi, including their genetic and biochemical properties, their taxonomy, and their use to humans as a source of medicine, food, and psychotropic substances consumed for religious purposes, as well as their dangers, such as poisoning or infection. The field of phytopathology, the study of plant diseases, is closely related because many plant pathogens are fungi.[43]
The use of fungi by humans dates back to prehistory; Ötzi the Iceman, a well-preserved mummy of a 5,300-year-old Neolithic man found frozen in the Austrian Alps, carried two species of polypore mushrooms that may have been used as tinder (Fomes fomentarius), or for medicinal purposes (Piptoporus betulinus).[44] Ancient peoples have used fungi as food sources–often unknowingly–for millennia, in the preparation of leavened bread and fermented juices. Some of the oldest written records contain references to the destruction of crops that were probably caused by pathogenic fungi.[45]
Mycology is a relatively new science that became systematic after the development of the microscope in the 16th century. Although fungal spores were first observed by Giambattista della Porta in 1588, the seminal work in the development of mycology is considered to be the publication of Pier Antonio Micheli's 1729 work Nova plantarum genera.[46] Micheli not only observed spores but also showed that, under the proper conditions, they could be induced into growing into the same species of fungi from which they originated.[47] Extending the use of the binomial system of nomenclature introduced by Carl Linnaeus in his Species plantarum (1753), the Dutch Christian Hendrik Persoon (1761–1836) established the first classification of mushrooms with such skill so as to be considered a founder of modern mycology. Later, Elias Magnus Fries (1794–1878) further elaborated the classification of fungi, using spore color and various microscopic characteristics, methods still used by taxonomists today. Other notable early contributors to mycology in the 17th–19th and early 20th centuries include Miles Joseph Berkeley, August Carl Joseph Corda, Anton de Bary, the brothers Louis René and Charles Tulasne, Arthur H. R. Buller, Curtis G. Lloyd, and Pier Andrea Saccardo. The 20th century has seen a modernization of mycology that has come from advances in biochemistry, genetics, molecular biology, and biotechnology. The use of DNA sequencing technologies and phylogenetic analysis has provided new insights into fungal relationships and biodiversity, and has challenged traditional morphology-based groupings in fungal taxonomy.[48]
An environmental isolate of Penicillium
Most fungi grow as hyphae, which are cylindrical, thread-like structures 2–10 µm in diameter and up to several centimeters in length. Hyphae grow at their tips (apices); new hyphae are typically formed by emergence of new tips along existing hyphae by a process called branching, or occasionally growing hyphal tips fork, giving rise to two parallel-growing hyphae.[49] The combination of apical growth and branching/forking leads to the development of a mycelium, an interconnected network of hyphae.[23] Hyphae can be either septate or coenocytic. Septate hyphae are divided into compartments separated by cross walls (internal cell walls, called septa, that are formed at right angles to the cell wall giving the hypha its shape), with each compartment containing one or more nuclei; coenocytic hyphae are not compartmentalized.[50] Septa have pores that allow cytoplasm, organelles, and sometimes nuclei to pass through; an example is the dolipore septum in fungi of the phylum Basidiomycota.[51] Coenocytic hyphae are in essence multinucleate supercells.[52]
Many species have developed specialized hyphal structures for nutrient uptake from living hosts; examples include haustoria in plant-parasitic species of most fungal phyla, and arbuscules of several mycorrhizal fungi, which penetrate into the host cells to consume nutrients.[53]
Although fungi are opisthokonts—a grouping of evolutionarily related organisms broadly characterized by a single posterior flagellum—all phyla except for the chytrids have lost their posterior flagella.[54] Fungi are unusual among the eukaryotes in having a cell wall that, in addition to glucans (e.g., β-1,3-glucan) and other typical components, also contains the biopolymer chitin.[55]
Fungal mycelia can become visible to the naked eye, for example, on various surfaces and substrates, such as damp walls and spoiled food, where they are commonly called molds. Mycelia grown on solid agar media in laboratory petri dishes are usually referred to as colonies. These colonies can exhibit growth shapes and colors (due to spores or pigmentation) that can be used as diagnostic features in the identification of species or groups.[56] Some individual fungal colonies can reach extraordinary dimensions and ages as in the case of a clonal colony of Armillaria solidipes, which extends over an area of more than 900 ha (3.5 square miles), with an estimated age of nearly 9,000 years.[57]
The apothecium—a specialized structure important in sexual reproduction in the ascomycetes—is a cup-shaped fruit body that holds the hymenium, a layer of tissue containing the spore-bearing cells.[58] The fruit bodies of the basidiomycetes (basidiocarps) and some ascomycetes can sometimes grow very large, and many are well known as mushrooms.
The growth of fungi as hyphae on or in solid substrates or as single cells in aquatic environments is adapted for the efficient extraction of nutrients, because these growth forms have high surface area to volume ratios.[59] Hyphae are specifically adapted for growth on solid surfaces, and to invade substrates and tissues.[60] They can exert large penetrative mechanical forces; for example, the plant pathogen Magnaporthe grisea forms a structure called an appressorium that evolved to puncture plant tissues.[61] The pressure generated by the appressorium, directed against the plant epidermis, can exceed 8 megapascals (1,200 psi).[61] The filamentous fungus Paecilomyces lilacinus uses a similar structure to penetrate the eggs of nematodes.[62]
The mechanical pressure exerted by the appressorium is generated from physiological processes that increase intracellular turgor by producing osmolytes such as glycerol.[63] Adaptations such as these are complemented by hydrolytic enzymes secreted into the environment to digest large organic molecules—such as polysaccharides, proteins, and lipids—into smaller molecules that may then be absorbed as nutrients.[64][65][66] The vast majority of filamentous fungi grow in a polar fashion—i.e., by extension into one direction—by elongation at the tip (apex) of the hypha.[67] Other forms of fungal growth include intercalary extension (longitudinal expansion of hyphal compartments that are below the apex) as in the case of some endophytic fungi,[68] or growth by volume expansion during the development of mushroom stipes and other large organs.[69] Growth of fungi as multicellular structures consisting of somatic and reproductive cells—a feature independently evolved in animals and plants[70]—has several functions, including the development of fruit bodies for dissemination of sexual spores (see above) and biofilms for substrate colonization and intercellular communication.[71]
The fungi are traditionally considered heterotrophs, organisms that rely solely on carbon fixed by other organisms for metabolism. Fungi have evolved a high degree of metabolic versatility that allows them to use a diverse range of organic substrates for growth, including simple compounds such as nitrate, ammonia, acetate, or ethanol.[72][73] In some species the pigment melanin may play a role in extracting energy from ionizing radiation, such as gamma radiation. This form of "radiotrophic" growth has been described for only a few species, the effects on growth rates are small, and the underlying biophysical and biochemical processes are not well known.[32] This process might bear similarity to CO2 fixation via visible light, but instead using ionizing radiation as a source of energy.[74]
Fungal reproduction is complex, reflecting the differences in lifestyles and genetic makeup within this diverse kingdom of organisms.[75] It is estimated that a third of all fungi reproduce using more than one method of propagation; for example, reproduction may occur in two well-differentiated stages within the life cycle of a species, the teleomorph and the anamorph.[76] Environmental conditions trigger genetically determined developmental states that lead to the creation of specialized structures for sexual or asexual reproduction. These structures aid reproduction by efficiently dispersing spores or spore-containing propagules.
Asexual reproduction occurs via vegetative spores (conidia) or through mycelial fragmentation. Mycelial fragmentation occurs when a fungal mycelium separates into pieces, and each component grows into a separate mycelium. Mycelial fragmentation and vegatative spores maintain clonal populations adapted to a specific niche, and allow more rapid dispersal than sexual reproduction.[77] The "Fungi imperfecti" (fungi lacking the perfect or sexual stage) or Deuteromycota comprise all the species that lack an observable sexual cycle.[78]
Sexual reproduction with meiosis exists in all fungal phyla (with the exception of the Glomeromycota).[79] It differs in many aspects from sexual reproduction in animals or plants. Differences also exist between fungal groups and can be used to discriminate species by morphological differences in sexual structures and reproductive strategies.[80][81] Mating experiments between fungal isolates may identify species on the basis of biological species concepts.[81] The major fungal groupings have initially been delineated based on the morphology of their sexual structures and spores; for example, the spore-containing structures, asci and basidia, can be used in the identification of ascomycetes and basidiomycetes, respectively. Some species may allow mating only between individuals of opposite mating type, whereas others can mate and sexually reproduce with any other individual or itself. Species of the former mating system are called heterothallic, and of the latter homothallic.[82]
Most fungi have both a haploid and a diploid stage in their life cycles. In sexually reproducing fungi, compatible individuals may combine by fusing their hyphae together into an interconnected network; this process, anastomosis, is required for the initiation of the sexual cycle. Ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not combine immediately after cell fusion, but remain separate in the hyphal cells (see heterokaryosis).[83]
In ascomycetes, dikaryotic hyphae of the hymenium (the spore-bearing tissue layer) form a characteristic hook at the hyphal septum. During cell division, formation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments. An ascus (plural asci) is then formed, in which karyogamy (nuclear fusion) occurs. Asci are embedded in an ascocarp, or fruiting body. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. After dispersal, the ascospores may germinate and form a new haploid mycelium.[84]
Sexual reproduction in basidiomycetes is similar to that of the ascomycetes. Compatible haploid hyphae fuse to produce a dikaryotic mycelium. However, the dikaryotic phase is more extensive in the basidiomycetes, often also present in the vegetatively growing mycelium. A specialized anatomical structure, called a clamp connection, is formed at each hyphal septum. As with the structurally similar hook in the ascomycetes, the clamp connection in the basidiomycetes is required for controlled transfer of nuclei during cell division, to maintain the dikaryotic stage with two genetically different nuclei in each hyphal compartment.[85] A basidiocarp is formed in which club-like structures known as basidia generate haploid basidiospores after karyogamy and meiosis.[86] The most commonly known basidiocarps are mushrooms, but they may also take other forms (see Morphology section).
In glomeromycetes (formerly zygomycetes), haploid hyphae of two individuals fuse, forming a gametangium, a specialized cell structure that becomes a fertile gamete-producing cell. The gametangium develops into a zygospore, a thick-walled spore formed by the union of gametes. When the zygospore germinates, it undergoes meiosis, generating new haploid hyphae, which may then form asexual sporangiospores. These sporangiospores allow the fungus to rapidly disperse and germinate into new genetically identical haploid fungal mycelia.[87]
Both asexual and sexual spores or sporangiospores are often actively dispersed by forcible ejection from their reproductive structures. This ejection ensures exit of the spores from the reproductive structures as well as traveling through the air over long distances.
Specialized mechanical and physiological mechanisms, as well as spore surface structures (such as hydrophobins), enable efficient spore ejection.[88] For example, the structure of the spore-bearing cells in some ascomycete species is such that the buildup of substances affecting cell volume and fluid balance enables the explosive discharge of spores into the air.[89] The forcible discharge of single spores termed ballistospores involves formation of a small drop of water (Buller's drop), which upon contact with the spore leads to its projectile release with an initial acceleration of more than 10,000 g;[90] the net result is that the spore is ejected 0.01–0.02 cm, sufficient distance for it to fall through the gills or pores into the air below.[91] Other fungi, like the puffballs, rely on alternative mechanisms for spore release, such as external mechanical forces. The bird's nest fungi use the force of falling water drops to liberate the spores from cup-shaped fruiting bodies.[92] Another strategy is seen in the stinkhorns, a group of fungi with lively colors and putrid odor that attract insects to disperse their spores.[93]
Besides regular sexual reproduction with meiosis, certain fungi, such as those in the genera Penicillium and Aspergillus, may exchange genetic material via parasexual processes, initiated by anastomosis between hyphae and plasmogamy of fungal cells.[94] The frequency and relative importance of parasexual events is unclear and may be lower than other sexual processes. It is known to play a role in intraspecific hybridization[95] and is likely required for hybridization between species, which has been associated with major events in fungal evolution.[96]
In contrast to plants and animals, the early fossil record of the fungi is meager. Factors that likely contribute to the under-representation of fungal species among fossils include the nature of fungal fruiting bodies, which are soft, fleshy, and easily degradable tissues and the microscopic dimensions of most fungal structures, which therefore are not readily evident. Fungal fossils are difficult to distinguish from those of other microbes, and are most easily identified when they resemble extant fungi.[97] Often recovered from a permineralized plant or animal host, these samples are typically studied by making thin-section preparations that can be examined with light microscopy or transmission electron microscopy.[98] Researchers study compression fossils by dissolving the surrounding matrix with acid and then using light or scanning electron microscopy to examine surface details.[99]
The earliest fossils possessing features typical of fungi date to the Proterozoic eon, some 1,430 million years ago (Ma); these multicellular benthic organisms had filamentous structures with septa, and were capable of anastomosis.[100] More recent studies (2009) estimate the arrival of fungal organisms at about 760–1060 Ma on the basis of comparisons of the rate of evolution in closely related groups.[101] For much of the Paleozoic Era (542–251 Ma), the fungi appear to have been aquatic and consisted of organisms similar to the extant chytrids in having flagellum-bearing spores.[102] The evolutionary adaptation from an aquatic to a terrestrial lifestyle necessitated a diversification of ecological strategies for obtaining nutrients, including parasitism, saprobism, and the development of mutualistic relationships such as mycorrhiza and lichenization.[103] Recent (2009) studies suggest that the ancestral ecological state of the Ascomycota was saprobism, and that independent lichenization events have occurred multiple times.[104]
It is presumed that the fungi colonized the land during the Cambrian (542–488.3 Ma), long before land plants.[105] Fossilized hyphae and spores recovered from the Ordovician of Wisconsin (460 Ma) resemble modern-day Glomerales, and existed at a time when the land flora likely consisted of only non-vascular bryophyte-like plants.[106] Prototaxites, which was probably a fungus or lichen, would have been the tallest organism of the late Silurian. Fungal fossils do not become common and uncontroversial until the early Devonian (416–359.2 Ma), when they occur abundantly in the Rhynie chert, mostly as Zygomycota and Chytridiomycota.[105][107][108] At about this same time, approximately 400 Ma, the Ascomycota and Basidiomycota diverged,[109] and all modern classes of fungi were present by the Late Carboniferous (Pennsylvanian, 318.1–299 Ma).[110]
Lichen-like fossils have been found in the Doushantuo Formation in southern China dating back to 635–551 Ma.[111] Lichens formed a component of the early terrestrial ecosystems, and the estimated age of the oldest terrestrial lichen fossil is 400 Ma;[112] this date corresponds to the age of the oldest known sporocarp fossil, a Paleopyrenomycites species found in the Rhynie Chert.[113] The oldest fossil with microscopic features resembling modern-day basidiomycetes is Palaeoancistrus, found permineralized with a fern from the Pennsylvanian.[114] Rare in the fossil record are the Homobasidiomycetes (a taxon roughly equivalent to the mushroom-producing species of the Agaricomycetes). Two amber-preserved specimens provide evidence that the earliest known mushroom-forming fungi (the extinct species Archaeomarasmius leggetti) appeared during the late Cretaceous, 90 Ma.[115][116]
Some time after the Permian–Triassic extinction event (251.4 Ma), a fungal spike (originally thought to be an extraordinary abundance of fungal spores in sediments) formed, suggesting that fungi were the dominant life form at this time, representing nearly 100% of the available fossil record for this period.[117] However, the relative proportion of fungal spores relative to spores formed by algal species is difficult to assess,[118] the spike did not appear worldwide,[119][120] and in many places it did not fall on the Permian–Triassic boundary.[121]
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Although commonly included in botany curricula and textbooks, fungi are more closely related to animals than to plants and are placed with the animals in the monophyletic group of opisthokonts.[122] Analyses using molecular phylogenetics support a monophyletic origin of the Fungi.[42] The taxonomy of the Fungi is in a state of constant flux, especially due to recent research based on DNA comparisons. These current phylogenetic analyses often overturn classifications based on older and sometimes less discriminative methods based on morphological features and biological species concepts obtained from experimental matings.[123]
There is no unique generally accepted system at the higher taxonomic levels and there are frequent name changes at every level, from species upwards. Efforts among researchers are now underway to establish and encourage usage of a unified and more consistent nomenclature.[42][124] Fungal species can also have multiple scientific names depending on their life cycle and mode (sexual or asexual) of reproduction. Web sites such as Index Fungorum and ITIS list current names of fungal species (with cross-references to older synonyms).
The 2007 classification of Kingdom Fungi is the result of a large-scale collaborative research effort involving dozens of mycologists and other scientists working on fungal taxonomy.[42] It recognizes seven phyla, two of which—the Ascomycota and the Basidiomycota—are contained within a branch representing subkingdom Dikarya. The below cladogram depicts the major fungal taxa and their relationship to opisthokont and unikont organisms. The lengths of the branches in this tree are not proportional to evolutionary distances.
The major phyla (sometimes called divisions) of fungi have been classified mainly on the basis of characteristics of their sexual reproductive structures. Currently, seven phyla are proposed: Microsporidia, Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Glomeromycota, Ascomycota, and Basidiomycota.[42]
Phylogenetic analysis has demonstrated that the Microsporidia, unicellular parasites of animals and protists, are fairly recent and highly derived endobiotic fungi (living within the tissue of another species).[102][125] One 2006 study concludes that the Microsporidia are a sister group to the true fungi; that is, they are each other's closest evolutionary relative.[126] Hibbett and colleagues suggest that this analysis does not clash with their classification of the Fungi, and although the Microsporidia are elevated to phylum status, it is acknowledged that further analysis is required to clarify evolutionary relationships within this group.[42]
The Chytridiomycota are commonly known as chytrids. These fungi are distributed worldwide. Chytrids produce zoospores that are capable of active movement through aqueous phases with a single flagellum, leading early taxonomists to classify them as protists. Molecular phylogenies, inferred from rRNA sequences in ribosomes, suggest that the Chytrids are a basal group divergent from the other fungal phyla, consisting of four major clades with suggestive evidence for paraphyly or possibly polyphyly.[127]
The Blastocladiomycota were previously considered a taxonomic clade within the Chytridiomycota. Recent molecular data and ultrastructural characteristics, however, place the Blastocladiomycota as a sister clade to the Zygomycota, Glomeromycota, and Dikarya (Ascomycota and Basidiomycota). The blastocladiomycetes are saprotrophs, feeding on decomposing organic matter, and they are parasites of all eukaryotic groups. Unlike their close relatives, the chytrids, most of which exhibit zygotic meiosis, the blastocladiomycetes undergo sporic meiosis.[102]
The Neocallimastigomycota were earlier placed in the phylum Chytridomycota. Members of this small phylum are anaerobic organisms, living in the digestive system of larger herbivorous mammals and in other terrestrial and aquatic environments enriched in cellulose (e.g., domestic waste landfill sites).[128] They lack mitochondria but contain hydrogenosomes of mitochondrial origin. As the related chrytrids, neocallimastigomycetes form zoospores that are posteriorly uniflagellate or polyflagellate.[42]
Members of the Glomeromycota form arbuscular mycorrhizae, a form of symbiosis wherein fungal hyphae invade plant root cells and both species benefit from the resulting increased supply of nutrients. All known Glomeromycota species reproduce asexually.[79] The symbiotic association between the Glomeromycota and plants is ancient, with evidence dating to 400 million years ago.[129] Formerly part of the Zygomycota (commonly known as 'sugar' and 'pin' molds), the Glomeromycota were elevated to phylum status in 2001 and now replace the older phylum Zygomycota.[130] Fungi that were placed in the Zygomycota are now being reassigned to the Glomeromycota, or the subphyla incertae sedis Mucoromycotina, Kickxellomycotina, the Zoopagomycotina and the Entomophthoromycotina.[42] Some well-known examples of fungi formerly in the Zygomycota include black bread mold (Rhizopus stolonifer), and Pilobolus species, capable of ejecting spores several meters through the air.[131] Medically relevant genera include Mucor, Rhizomucor, and Rhizopus.
The Ascomycota, commonly known as sac fungi or ascomycetes, constitute the largest taxonomic group within the Eumycota.[41] These fungi form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. This phylum includes morels, a few mushrooms and truffles, unicellular yeasts (e.g., of the genera Saccharomyces, Kluyveromyces, Pichia, and Candida), and many filamentous fungi living as saprotrophs, parasites, and mutualistic symbionts. Prominent and important genera of filamentous ascomycetes include Aspergillus, Penicillium, Fusarium, and Claviceps. Many ascomycete species have only been observed undergoing asexual reproduction (called anamorphic species), but analysis of molecular data has often been able to identify their closest teleomorphs in the Ascomycota.[132] Because the products of meiosis are retained within the sac-like ascus, ascomycetes have been used for elucidating principles of genetics and heredity (e.g., Neurospora crassa).[133]
Members of the Basidiomycota, commonly known as the club fungi or basidiomycetes, produce meiospores called basidiospores on club-like stalks called basidia. Most common mushrooms belong to this group, as well as rust and smut fungi, which are major pathogens of grains. Other important basidiomycetes include the maize pathogen Ustilago maydis,[134] human commensal species of the genus Malassezia,[135] and the opportunistic human pathogen, Cryptococcus neoformans.[136]
Because of similarities in morphology and lifestyle, the slime molds (mycetozoans, plasmodiophorids, acrasids, Fonticula and labyrinthulids, now in Amoebozoa, Rhizaria, Excavata, Opisthokonta and Stramenopiles, respectively), water molds (oomycetes) and hyphochytrids (both Stramenopiles) were formerly classified in the kingdom Fungi, in groups like Mastigomycotina, Gymnomycota and Phycomycetes. The slime molds were studied also as protozoans, leading to a ambiregnal, duplicated taxonomy.
Unlike true fungi, the cell walls of oomycetes contain cellulose and lack chitin. Hyphochytrids have both chitin and cellulose. Slime molds lack a cell wall during the assimilative phase (except labyrinthulids, which have a wall of scales), and ingest nutrients by ingestion (phagocytosis, except labyrinthulids) rather than absorption (osmotrophy, as fungi). Neither water molds nor slime molds are closely related to the true fungi, and, therefore, taxonomists no longer group them in the kingdom Fungi. Nonetheless, studies of the oomycetes and myxomycetes are still often included in mycology textbooks and primary research literature.[137]
The Eccrinales and Amoebidiales are opisthokont protists, previously thought to be zygomycete fungi. Other groups now in Opisthokonta (e.g., Corallochytrium, Ichthyosporea) were also at given time classified as fungi. The genus Blastocystis, now in Stramenopiles, was originally classified as a yeast. Ellobiopsis, now in Alveolata, was considered a chytrid. The bacteria were also included in fungi in some classifications, as the group Schizomycetes.
The Rozellida clade, including the "ex-chytrid" Rozella, is a genetically disparate group known mostly from environmental DNA sequences that is a sister group to fungi. Members of the group that have been isolated lack the chitinous cell wall that is characteristic of fungi.
The nucleariids, protists currently grouped in the Choanozoa (Opisthokonta), may be the next sister group to the eumycete clade, and as such could be included in an expanded fungal kingdom.[138]
Although often inconspicuous, fungi occur in every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles[139] and in many food webs. As decomposers, they play an essential role in nutrient cycling, especially as saprotrophs and symbionts, degrading organic matter to inorganic molecules, which can then re-enter anabolic metabolic pathways in plants or other organisms.[140][141]
Many fungi have important symbiotic relationships with organisms from most if not all Kingdoms.[142][143][144] These interactions can be mutualistic or antagonistic in nature, or in the case of commensal fungi are of no apparent benefit or detriment to the host.[145][146][147]
Mycorrhizal symbiosis between plants and fungi is one of the most well-known plant–fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in mycorrhizal relationships with fungi and are dependent upon this relationship for survival.[148]
The mycorrhizal symbiosis is ancient, dating to at least 400 million years ago.[129] It often increases the plant's uptake of inorganic compounds, such as nitrate and phosphate from soils having low concentrations of these key plant nutrients.[140][149] The fungal partners may also mediate plant-to-plant transfer of carbohydrates and other nutrients. Such mycorrhizal communities are called "common mycorrhizal networks".[150] A special case of mycorrhiza is myco-heterotrophy, whereby the plant parasitizes the fungus, obtaining all of its nutrients from its fungal symbiont.[151] Some fungal species inhabit the tissues inside roots, stems, and leaves, in which case they are called endophytes.[152] Similar to mycorrhiza, endophytic colonization by fungi may benefit both symbionts; for example, endophytes of grasses impart to their host increased resistance to herbivores and other environmental stresses and receive food and shelter from the plant in return.[153]
Lichens are formed by a symbiotic relationship between algae or cyanobacteria (referred to in lichen terminology as "photobionts") and fungi (mostly various species of ascomycetes and a few basidiomycetes), in which individual photobiont cells are embedded in a tissue formed by the fungus.[154] Lichens occur in every ecosystem on all continents, play a key role in soil formation and the initiation of biological succession,[155] and are the dominating life forms in extreme environments, including polar, alpine, and semiarid desert regions.[156] They are able to grow on inhospitable surfaces, including bare soil, rocks, tree bark, wood, shells, barnacles and leaves.[157] As in mycorrhizas, the photobiont provides sugars and other carbohydrates via photosynthesis, while the fungus provides minerals and water. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism; in most cases the resulting organism differs greatly from the individual components. Lichenization is a common mode of nutrition; around 20% of fungi—between 17,500 and 20,000 described species—are lichenized.[158] Characteristics common to most lichens include obtaining organic carbon by photosynthesis, slow growth, small size, long life, long-lasting (seasonal) vegetative reproductive structures, mineral nutrition obtained largely from airborne sources, and greater tolerance of desiccation than most other photosynthetic organisms in the same habitat.[159]
Many insects also engage in mutualistic relationships with fungi. Several groups of ants cultivate fungi in the order Agaricales as their primary food source, while ambrosia beetles cultivate various species of fungi in the bark of trees that they infest.[160] Likewise, females of several wood wasp species (genus Sirex) inject their eggs together with spores of the wood-rotting fungus Amylostereum areolatum into the sapwood of pine trees; the growth of the fungus provides ideal nutritional conditions for the development of the wasp larvae.[161] Termites on the African savannah are also known to cultivate fungi,[142] and yeasts of the genera Candida and Lachancea inhabit the gut of a wide range of insects, including neuropterans, beetles, and cockroaches; it is not known whether these fungi benefit their hosts.[162] The larvae of many families of fungicolous flies, particularly those within the superfamily Sciaroidea such as the Mycetophilidae and some Keroplatidae feed on fungal fruiting bodies and sterile mycorrhizae.[163]
Many fungi are parasites on plants, animals (including humans), and other fungi. Serious pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus Magnaporthe oryzae,[164] tree pathogens such as Ophiostoma ulmi and Ophiostoma novo-ulmi causing Dutch elm disease,[165] and Cryphonectria parasitica responsible for chestnut blight,[166] and plant pathogens in the genera Fusarium, Ustilago, Alternaria, and Cochliobolus.[146] Some carnivorous fungi, like Paecilomyces lilacinus, are predators of nematodes, which they capture using an array of specialized structures such as constricting rings or adhesive nets.[167]
Some fungi can cause serious diseases in humans, several of which may be fatal if untreated. These include aspergilloses, candidoses, coccidioidomycosis, cryptococcosis, histoplasmosis, mycetomas, and paracoccidioidomycosis. Furthermore, persons with immuno-deficiencies are particularly susceptible to disease by genera such as Aspergillus, Candida, Cryptoccocus,[147][168][169] Histoplasma,[170] and Pneumocystis.[171] Other fungi can attack eyes, nails, hair, and especially skin, the so-called dermatophytic and keratinophilic fungi, and cause local infections such as ringworm and athlete's foot.[172] Fungal spores are also a cause of allergies, and fungi from different taxonomic groups can evoke allergic reactions.[173]
Mycotoxins are secondary metabolites (or natural products), and research has established the existence of biochemical pathways solely for the purpose of producing mycotoxins and other natural products in fungi.[28] Mycotoxins may provide fitness benefits in terms of physiological adaptation, competition with other microbes and fungi, and protection from consumption (fungivory).[174][175] Many fungi produce biologically active compounds, several of which are toxic to animals or plants and are therefore called mycotoxins. Of particular relevance to humans are mycotoxins produced by molds causing food spoilage, and poisonous mushrooms (see above). Particularly infamous are the lethal amatoxins in some Amanita mushrooms, and ergot alkaloids, which have a long history of causing serious epidemics of ergotism (St Anthony's Fire) in people consuming rye or related cereals contaminated with sclerotia of the ergot fungus, Claviceps purpurea.[176] Other notable mycotoxins include the aflatoxins, which are insidious liver toxins and highly carcinogenic metabolites produced by certain Aspergillus species often growing in or on grains and nuts consumed by humans, ochratoxins, patulin, and trichothecenes (e.g., T-2 mycotoxin) and fumonisins, which have significant impact on human food supplies or animal livestock.[177]
Ustilago maydis is a pathogenic plant fungus that causes smut disease in maize and teosinte. Plants have evolved efficient defense systems against pathogenic microbes such as U. maydis. A rapid defense reaction after pathogen attack is the oxidative burst where the plant produces reactive oxygen species at the site of the attempted invasion. U. maydis can respond to the oxidative burst with an oxidative stress response, regulated by the gene YAP1. The response protects U. maydis from the host defense, and is necessary for the pathogen’s virulence.[178] Furthermore, U. maydis has a well-established recombinational DNA repair system which acts during mitosis and meiosis.[179] The system may assist the pathogen in surviving DNA damage arising from the host plant’s oxidative defensive response to infection.[180]
Cryptococcus neoformans is an encapsulated yeast that can live in both plants and animals. C. neoformans usually infects the lungs, where it is phagocytosed by alveolar macrophages.[181] Some C. neoformans can survive inside macrophage, which appears to be the basis for latency, disseminated disease, and resistance to antifungal agents. One mechanism by which C. neoformans survives the hostile macrophage environment is by up-regulating the expression of genes involved in the oxidative stress response.[181] Another mechanism involves meiosis. The majority of C. neoformans are mating "type a". Filaments of mating "type an" ordinarily have haploid nuclei, but they can become diploid (perhaps by endoduplication or by stimulated nuclear fusion) to form blastospores. The diploid nuclei of blastospores can undergo meiosis, including recombination, to form haploid basidiospores that can be dispersed.[182] This process is referred to as monokaryotic fruiting. this process requires a gene called DMC1, which is a conserved homologue of genes recA in bacteria and RAD51 in eukaryotes, that mediates homologous chromosome pairing during meiosis and repair of DNA double-strand breaks. Thus, C. neoformans can undergo a meiosis, monokaryotic fruiting, that promotes recombinational repair in the oxidative, DNA damaging environment of the host macrophage, and the repair capability may contribute to its virulence.[180][182]
The human use of fungi for food preparation or preservation and other purposes is extensive and has a long history. Mushroom farming and mushroom gathering are large industries in many countries. The study of the historical uses and sociological impact of fungi is known as ethnomycology. Because of the capacity of this group to produce an enormous range of natural products with antimicrobial or other biological activities, many species have long been used or are being developed for industrial production of antibiotics, vitamins, and anti-cancer and cholesterol-lowering drugs. More recently, methods have been developed for genetic engineering of fungi,[183] enabling metabolic engineering of fungal species. For example, genetic modification of yeast species[184]—which are easy to grow at fast rates in large fermentation vessels—has opened up ways of pharmaceutical production that are potentially more efficient than production by the original source organisms.[185]
Many species produce metabolites that are major sources of pharmacologically active drugs. Particularly important are the antibiotics, including the penicillins, a structurally related group of β-lactam antibiotics that are synthesized from small peptides. Although naturally occurring penicillins such as penicillin G (produced by Penicillium chrysogenum) have a relatively narrow spectrum of biological activity, a wide range of other penicillins can be produced by chemical modification of the natural penicillins. Modern penicillins are semisynthetic compounds, obtained initially from fermentation cultures, but then structurally altered for specific desirable properties.[186] Other antibiotics produced by fungi include: ciclosporin, commonly used as an immunosuppressant during transplant surgery; and fusidic acid, used to help control infection from methicillin-resistant Staphylococcus aureus bacteria.[187] Widespread use of antibiotics for the treatment of bacterial diseases, such as tuberculosis, syphilis, leprosy, and others began in the early 20th century and continues to date. In nature, antibiotics of fungal or bacterial origin appear to play a dual role: at high concentrations they act as chemical defense against competition with other microorganisms in species-rich environments, such as the rhizosphere, and at low concentrations as quorum-sensing molecules for intra- or interspecies signaling.[188] Other drugs produced by fungi include griseofulvin isolated from Penicillium griseofulvum, used to treat fungal infections,[189] and statins (HMG-CoA reductase inhibitors), used to inhibit cholesterol synthesis. Examples of statins found in fungi include mevastatin from Penicillium citrinum and lovastatin from Aspergillus terreus and the oyster mushroom.[190]
Certain mushrooms enjoy usage as therapeutics in folk medicines, such as Traditional Chinese medicine. Notable medicinal mushrooms with a well-documented history of use include Agaricus subrufescens,[191][192] Ganoderma lucidum,[193] and Ophiocordyceps sinensis.[194] Research has identified compounds produced by these and other fungi that have inhibitory biological effects against viruses[195][196] and cancer cells.[191][197] Specific metabolites, such as polysaccharide-K, ergotamine, and β-lactam antibiotics, are routinely used in clinical medicine. The shiitake mushroom is a source of lentinan, a clinical drug approved for use in cancer treatments in several countries, including Japan.[198][199] In Europe and Japan, polysaccharide-K (brand name Krestin), a chemical derived from Trametes versicolor, is an approved adjuvant for cancer therapy.[200]
Baker's yeast or Saccharomyces cerevisiae, a unicellular fungus, is used to make bread and other wheat-based products, such as pizza dough and dumplings.[201] Yeast species of the genus Saccharomyces are also used to produce alcoholic beverages through fermentation.[202] Shoyu koji mold (Aspergillus oryzae) is an essential ingredient in brewing Shoyu (soy sauce) and sake, and the preparation of miso,[203] while Rhizopus species are used for making tempeh.[204] Several of these fungi are domesticated species that were bred or selected according to their capacity to ferment food without producing harmful mycotoxins (see below), which are produced by very closely related Aspergilli.[205] Quorn, a meat substitute, is made from Fusarium venenatum.[206]
Edible mushrooms are well-known examples of fungi. Many are commercially raised, but others must be harvested from the wild. Agaricus bisporus, sold as button mushrooms when small or Portobello mushrooms when larger, is a commonly eaten species, used in salads, soups, and many other dishes. Many Asian fungi are commercially grown and have increased in popularity in the West. They are often available fresh in grocery stores and markets, including straw mushrooms (Volvariella volvacea), oyster mushrooms (Pleurotus ostreatus), shiitakes (Lentinula edodes), and enokitake (Flammulina spp.).[207]
There are many more mushroom species that are harvested from the wild for personal consumption or commercial sale. Milk mushrooms, morels, chanterelles, truffles, black trumpets, and porcini mushrooms (Boletus edulis) (also known as king boletes) demand a high price on the market. They are often used in gourmet dishes.[208]
Certain types of cheeses require inoculation of milk curds with fungal species that impart a unique flavor and texture to the cheese. Examples include the blue color in cheeses such as Stilton or Roquefort, which are made by inoculation with Penicillium roqueforti.[209] Molds used in cheese production are non-toxic and are thus safe for human consumption; however, mycotoxins (e.g., aflatoxins, roquefortine C, patulin, or others) may accumulate because of growth of other fungi during cheese ripening or storage.[210]
Many mushroom species are poisonous to humans, with toxicities ranging from slight digestive problems or allergic reactions as well as hallucinations to severe organ failures and death. Genera with mushrooms containing deadly toxins include Conocybe, Galerina, Lepiota, and, the most infamous, Amanita.[211] The latter genus includes the destroying angel (A. virosa) and the death cap (A. phalloides), the most common cause of deadly mushroom poisoning.[212] The false morel (Gyromitra esculenta) is occasionally considered a delicacy when cooked, yet can be highly toxic when eaten raw.[213] Tricholoma equestre was considered edible until being implicated in serious poisonings causing rhabdomyolysis.[214] Fly agaric mushrooms (Amanita muscaria) also cause occasional non-fatal poisonings, mostly as a result of ingestion for use as a recreational drug for its hallucinogenic properties. Historically, fly agaric was used by different peoples in Europe and Asia and its present usage for religious or shamanic purposes is reported from some ethnic groups such as the Koryak people of north-eastern Siberia.[215]
As it is difficult to accurately identify a safe mushroom without proper training and knowledge, it is often advised to assume that a wild mushroom is poisonous and not to consume it.[216][217]
In agriculture, fungi may be useful if they actively compete for nutrients and space with pathogenic microorganisms such as bacteria or other fungi via the competitive exclusion principle,[218] or if they are parasites of these pathogens. For example, certain species may be used to eliminate or suppress the growth of harmful plant pathogens, such as insects, mites, weeds, nematodes, and other fungi that cause diseases of important crop plants.[219] This has generated strong interest in practical applications that use these fungi in the biological control of these agricultural pests. Entomopathogenic fungi can be used as biopesticides, as they actively kill insects.[220] Examples that have been used as biological insecticides are Beauveria bassiana, Metarhizium spp, Hirsutella spp, Paecilomyces (Isaria) spp, and Lecanicillium lecanii.[221][222] Endophytic fungi of grasses of the genus Neotyphodium, such as N. coenophialum, produce alkaloids that are toxic to a range of invertebrate and vertebrate herbivores. These alkaloids protect grass plants from herbivory, but several endophyte alkaloids can poison grazing animals, such as cattle and sheep.[223] Infecting cultivars of pasture or forage grasses with Neotyphodium endophytes is one approach being used in grass breeding programs; the fungal strains are selected for producing only alkaloids that increase resistance to herbivores such as insects, while being non-toxic to livestock.[224]
Certain fungi, in particular "white rot" fungi, can degrade insecticides, herbicides, pentachlorophenol, creosote, coal tars, and heavy fuels and turn them into carbon dioxide, water, and basic elements.[225] Fungi have been shown to biomineralize uranium oxides, suggesting they may have application in the bioremediation of radioactively polluted sites.[226][227][228]
Several pivotal discoveries in biology were made by researchers using fungi as model organisms, that is, fungi that grow and sexually reproduce rapidly in the laboratory. For example, the one gene-one enzyme hypothesis was formulated by scientists using the bread mold Neurospora crassa to test their biochemical theories.[229] Other important model fungi are Aspergillus nidulans and the yeasts Saccaromyces cerevisiae and Schizosaccharomyces pombe, each of which with a long history of use to investigate issues in eukaryotic cell biology and genetics, such as cell cycle regulation, chromatin structure, and gene regulation. Other fungal models have more recently emerged that each address specific biological questions relevant to medicine, plant pathology, and industrial uses; examples include Candida albicans, a dimorphic, opportunistic human pathogen,[230] Magnaporthe grisea, a plant pathogen,[231] and Pichia pastoris, a yeast widely used for eukaryotic protein expression.[232]
Fungi are used extensively to produce industrial chemicals like citric, gluconic, lactic, and malic acids,[233] and industrial enzymes, such as lipases used in biological detergents,[234] cellulases used in making cellulosic ethanol[235] and stonewashed jeans,[236] and amylases,[237] invertases, proteases and xylanases.[238] Several species, most notably Psilocybin mushrooms (colloquially known as magic mushrooms), are ingested for their psychedelic properties, both recreationally and religiously.
An Austria-based company and a university joined forces to develop a working incubator that cultivates fungi that is safe for human consumption and also able to digest plastic while it's growing.[239]
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| リンク元 | 「fungi」「myco」 |
| 拡張検索 | 「central nervous system fungal infection」「antifungal of imidazole derivative」 |