関: prothallium

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/06/26 02:16:34」(JST)

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Liverwort Prothallus

A prothallium, or prothallus (from Latin pro = forwards and Greek θαλλος (thallos) = twig) is usually the gametophyte stage in the life of a fern or other pteridophyte. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well.

The prothallium develops from a germinating spore. It is a short-lived and inconspicuous heart-shaped structure typically 2-5 millimeters wide, with a number of rhizoids (root-like hairs) growing underneath, and the sex organs: archegonium (female) and antheridium (male). Appearance varies quite a lot between species. Some are green and conduct photosynthesis while others are colorless and nourish themselves underground as saprotrophs.

Alternation of generations

Prothallus of the tree fern Dicksonia antarctica (note new moss plants for scale)

Spore-bearing plants, like all plants, go through a life-cycle of alternation of generations. The fully grown sporophyte, what is commonly referred to as the fern, produces genetically unique spores in the sori by meiosis. The haploid spores fall from the sporophyte and germinate by mitosis, given the right conditions, into the gametophyte stage, the prothallus. The prothallus develops independently for several weeks; it grows sex organs that produce ova (archegonia) and flagellated sperm(antheridia). The sperm are able to swim to the ova for fertilization to form a diploid zygote which divides by mitosis to form a multicellular sporophyte. In the early stages of growth, the sporophyte grows out of the prothallus, depending on it for water supply and nutrition, but develops into a new independent fern, which will produce new spores that will grow into new prothallia etc., thus completing the life cycle of the organism.

Theoretical advantages of alternation of generations

It has been argued that there are two important evolutionary advantages to the alternation of generations plant life-cycle.^{[citation needed]} Firstly, by forming a haploid gametophyte rather than limiting the haploid stage to gametes, there is only one allele for any genetic trait. Thus, all alleles will be expressed during vegetative growth because no allele may be masked by a dominant counterpart (there is no counterpart). The benefit of this is that any mutation that causes a lethal, or harmful, trait expression cannot be masked and will cause the gametophyte to die; thus, the trait cannot be passed on to future generations, preserving the strength of the gene pool.^{[citation needed]} Also, crossing-over during meiosis in the formation of spores, and sexual reproduction in the gametophytes, produces genetic diversity.

External links

Liverwort Sporophyte
Fern Life-Cycle

English Journal

Photosynthesis-dependent but neochrome1-independent light positioning of chloroplasts and nuclei in the fern Adiantum capillus-veneris.

Sugiyama Y1, Kadota A.
Plant physiology.Plant Physiol.2011 Mar;155(3):1205-13. doi: 10.1104/pp.110.171553. Epub 2011 Jan 19.
Chloroplasts change their positions in the cell depending on the light conditions. In the dark, chloroplasts in fern prothallia locate along the anticlinal wall (dark position). However, chloroplasts become relocated to the periclinal wall (light position) when the light shines perpendicularly to th
PMID 21248076

Analysis of expressed sequence tags in prothallia of Adiantum capillus-veneris.

Yamauchi D1, Sutoh K, Kanegae H, Horiguchi T, Matsuoka K, Fukuda H, Wada M.
Journal of plant research.J Plant Res.2005 Jun;118(3):223-7. Epub 2005 Jun 7.
The analysis of expressed sequences from a diverse set of plant species has fueled the increase in understanding of the complex molecular mechanisms underlying plant growth regulation. While representative data sets can be found for the major branches of plant evolution, fern species data are lackin
PMID 15940394

Analysis of fern spore banks from the soil of three vegetation types in the central region of Mexico.

Ramírez-Trejo Mdel R1, Pérez-García B, Orozco-Segovia A.
American journal of botany.Am J Bot.2004 May;91(5):682-8. doi: 10.3732/ajb.91.5.682.
The vertical structure of fern spore banks was studied in a xerophilous shrubland, montane rain forest, and pine-oak forest in Hidalgo, Mexico, using the emergence method. Soil samples were collected in April 1999 at depths of 0-10, 10-20, and 20-30 cm. Viable spores decreased significantly with dep
PMID 21653423

Japanese Journal

スギナの葉緑体核様体に及ぼすシプロフロキサシンの影響

泉好弘,富松美樹
大分大学教育福祉科学部研究紀要 30(1), 21-27, 2008-04
… To determine the effects of the antibiotics that inhibit the bacterial DNA gyrase activity on the chloroplast nucleoids of Equisetum arvense, the numbers of the chloroplast nucleoids inantibiotic-treated prothallia were counted. … In untreated prothallia, many chloroplast nucleoids were randomly distributed in the chloroplasts. … On the other hand, the chloroplasts of antibiotic-treated prothallia had a few chloroplast nucleoids. …
NAID 120001698464