Close-up of an
Echinopsis spachiana flower, showing both carpels (only the styles and stigmas are visible) and stamens, making it a perfect flower.
Plant sexuality covers the wide variety of sexual reproduction systems found across the plant kingdom. This article describes morphological aspects of sexual reproduction of plants.
Among all living organisms, flowers, which are the reproductive units of angiosperms, are the most varied physically and show the greatest diversity in methods of reproduction.[1] Carolus Linnaeus (1735 and 1753) proposed a system of classification of flowering plants based on plant structures; since plants employ many different morphological adaptations involving sexual reproduction, flowers played an important role in that classification system. Later on Christian Konrad Sprengel (1793) studied plant sexuality and called it the "revealed secret of nature" and for the first time it was understood that the pollination process involved both biotic and abiotic interactions. Charles Darwin's theories of natural selection utilized this work to promote his theory of evolution. Plants that are not flowering plants (green alga, mosses, liverworts, hornworts, ferns and gymnosperms such as conifers) also have complex interplays between morphological adaptation and environmental factors in their sexual reproduction. The breeding system, or how the sperm from one plant fertilizes the ovum of another, is the single most important determinant of the mating structure of nonclonal plant populations. The mating structure or morphology of the flower parts and their arrangement on the plant in turn controls the amount and distribution of genetic variation, a central element in the evolutionary process.[2]
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Contents
- 1 Terminology
- 1.1 Individual reproductive unit (a flower in angiosperms)
- 1.2 Individual plant sexuality
- 1.3 Plant population
- 2 Flower morphology
- 3 Evolution
- 4 See also
- 5 References
- 6 External links
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Terminology
The flowers of angiosperms are determinate shoots that have sporophylls. The parts of flowers are named by scientists and show great variation in shape, these flower parts include sepals, petals, stamens and carpels. As a group the sepals form the calyx and as a group the petals form the corolla, together the corolla and the calyx is called the perianth. In flowers which possess indistinguishable calyx and corolla, the individual units are then called "tepals". The stamens collectively are called the androecium and the carpels collectively are called the gynoecium.
The complexity of the systems and devices used by plants to achieve sexual reproduction has resulted in botanists and evolutionary biologists using numerous terms to describe physical structures and functional strategies. Dellaporta and Calderon-Urrea (1993) list and define a variety of terms used to describe the modes of sexuality at different levels in flowering plants. This list is reproduced here,[3] generalized to fit more than just plants that have flowers, and expanded to include other terms and more complete definitions.
The Alder is
monoecious. Shown here: maturing male flower catkins on right, last year's female catkins on left
Individual reproductive unit (a flower in angiosperms)
- Bisexual or perfect flowers have both male (androecium) and female (gynoecium) reproductive units, including stamens, carpels, and an ovary. Flowers that contain both androecium and gynoecium are called androgynous or hermaphroditic. Examples of plants with perfect or bisexual flowers include the lily, rose, and most plants with large showy flowers, though perfect flower does not have to have petals or sepals. Other terms widely used are monoclinous, and synoecious. A complete flower is a perfect flower with petals and sepals.
- Unisexual: Either functionally male or functionally female. In angiosperms this condition is also called diclinous, imperfect, or incomplete.
Individual plant sexuality
Many plants have complete flowers that have both male and female parts, others only have male or female parts and still other plants have flowers on the same plant that are a mix of male and female flowers. Some plants even have mixes that include all three types of flowers, where some flowers are only male, some are only female and some are both male and female. A distinction needs to be made between arrangements of sexual parts and the expression of sexuality in single plants versus the larger plant population. Some plants also undergo what is called Sex-switching, like Arisaema triphyllum which express sexual differences at different stages of growth. In some arums smaller plants produce all or mostly male flowers and as plants grow larger over the years the male flowers are replaced by more female flowers on the same plant. Arisaema triphyllum thus covers a multitude of sexual conditions in its lifetime; from nonsexual juvenile plants to young plants that are all male, as plants grow larger they have a mix of both male and female flowers, to large plants that have mostly female flowers.[4] Other plant populations have plants that produce more male flowers early in the year and as plants bloom later in the growing season they produce more female flowers. In plants like Thalictrum dioicum all the plants in the species are either male or female.
Terminology:
- Hermaphrodite. In hermaphroditic species, each reproductive unit (as in flower, conifer cone or functional equivalent) of each individual has both male and female structures. In angiosperm terminology a synonym is monoclinous from the Greek "one bed".
- Monoecious. In monoecious species, each individual has reproductive units that are merely female and reproductive units that are merely male. The name derives from Greek "monos" (one) and "ecos" (home). Individuals bearing separate flowers of both sexes at the same time are called simultaneously or synchronously monoecious. Individuals that bear flowers of one sex at one time are called consecutively monoecious; plants may first have single sexed flowers and then later have flowers of the other sex. Protoandrous or protandrous describes individuals that function first as males and then change to females; protogynous describes individuals that function first as females and then change to males.
- Dioecious. In dioecious species, each individual has reproductive units that are either merely male or merely female. That is, no individual plant of the population produces both microgametophytes (pollen) and megagametophytes (ovules).[5] From Greek for "two households". [Individual plants are not called dioecious; they are either gynoecious (female plants) or androecious (male plants).]
- Androecious, plants producing male flowers only, produce pollen but no seeds, the male plants of a dioecious population.
- Gynoecious, plants producing female flowers only, produces seeds but no pollen, the female of a dioecious population. In some plant populations, all individuals are gynoecious with non sexual reproduction used to produce the next generation.
- Subdioecious, a tendency in some dioecious populations to produce individuals that are not clearly male or female. The population produces normally male or female plants but some may be monoecious, hermaphroditic, or monoecious/hermaphroditic, with plants having perfect flowers, both male and female imperfect flowers, or some combination thereof, such as female and perfect flowers. Flowers may be in some state between purely male and female, with female flowers retaining non-functional male organs or vice versa. The condition is thought to represent a transition between hermaphroditism and dioecy.[6][7]
- Gynomonoecious has both hermaphrodite and female units.
- Andromonoecious has both hermaphrodite and male units.
- Subandroecious has mostly male flowers, with a few female or hermaphrodite flowers.
- Subgynoecious has mostly female flowers, with a few male or hermaphrodite flowers.
- Polygamy, plants with male, female, and perfect (hermaphrodite) flowers on the same plant, called trimonoecious or polygamomonoecious plants, (see next section for use for plant populations).[8] A polygamous inflorescence has both unisexual and bisexual flowers.[9]
- Trimonoecious (polygamous) - male, female, and hermaphrodite floral morphs all appear on the same plant.
- Diclinous ("two beds"), an angiosperm term, includes all species with unisexual flowers, although particularly those with only unisexual flowers, i.e. the monoecious and dioecious species.
Holly (
Ilex aquifolium) is dioecious: (above) shoot with flowers from male plant; (top right) male flower enlarged, showing stamens with pollen and reduced, sterile stigma; (below) shoot with flowers from female plant; (lower right) female flower enlarged, showing stigma and reduced, sterile stamens (staminodes) with no pollen
Plant population
Often a species or a population within a species will have the same form of sexuality. Specific terms can then be used to describe the sexual expression of the group as a whole.
- Hermaphrodite, plants whose flowers have both male and female parts.
- Monoecious (meaning "one house" in Greek) plants have separate male and female flowers on the same plant. These plants are often wind pollinated. Examples of monoecious plants include corn (Zea mays),[10] birch and pine trees,[11] and most fig species.[12]
- Dioecious (meaning "two houses" in Greek), all plants are either female or male. The American Holly (Ilex opaca) is a famous example.
- Androdioecious, both male and hermaphrodite plants present.
- Gynodioecious, both female and hermaphrodite plants present. In some plants, strictly female plants are produced by the degeneration of the tapetum, a shell-like structure in the anther of a flower where the pollen cells form,[13]
- Gynoecy plants are all females in a population, often regulated by environmental factors like temperature, photo period or water availability.
- Polygamous, when there is a mix of hermaphrodite and unisexual plants in the natural population.[14]
- Subdioecious, population of unisexual (dioecious) plants, with monoecious individuals too.[15]
- Trioecious, sometimes used in place of subdioecious when male, female, and hermaphrodite plants are more equally mixed within the same population.
- Polygamodioecious, having bisexual and male flowers on some plants, and bisexual and female flowers on others.[16]
About 11% of all angiosperms are strictly dioecious or monoecious. Intermediate forms of sexual dimorphism, including gynodioecy and androdioecy, represent 7% of the species examined of a survey of 120,000 plant species. In the same survey, 10% of the species contain both unisexual and bisexual flowers.[17]
The majority of plant species use allogamy, also called cross-pollination or outcrossing, as a means of breeding. Many plants are self-fertile and the male parts can pollinate the female parts of the same flower and/or same plant. Some plants use a method known as self-incompatibility to promote outcrossing. In these plants, the male organs cannot fertilize the female parts of the same plant; other plants produce male and female flowers at different times to promote outcrossing.
Sexual reproduction (involving meiosis and outcrossing) is the most common mode of reproduction among higher plants. As summarized by Bernstein and Bernstein, pp. 272-274.[18] About 55% of higher plant species are principally sexual This includes plants that are dioecious and dichogamous. An additional 7% are partially sexual, i.e. partially self-fertilizing and partially cross-fertilizing. About 15% undergo meiosis, but are principally self-fertilizing, and significant out-crossing is lacking (autogamy). Only about 8% of higher plant species reproduce exclusively by non-sexual means. These include plants that reproduce vegetatively by runners or bulbils, or by apomixis. The selective advantage of outcrossing sex (the predominant form of reproduction in plants) appears to be the masking of deleterious recessive mutations (reviewed by Bernstein and Bernstein, pp. 272-289[18].
Dichogamy is common in flowering plants, and occurs when bisexual (perfect) flowers (or sometimes entire plants) produce pollen when the stigmas of the same flower is not receptive of the pollen. This promotes outcrossing by limiting what is called autopollination or self-pollination or selfing.[19] These plants are called dichogamous. Some plants have bisexual flowers but the pollen is produced before the stigma of the same flower is receptive of pollen, these are described as protandrous flowers; in a similar way, protogyny describes flowers that have stigmas that can accept pollen before the same flower or plant sheds its pollen.[8]
- Autogamy. Self-fertilization. Fusion of male and female gametes, usually from the same individual.
- Apomixis: Asexual reproduction through seeds
Flower morphology
A species such as the ash tree (Fraxinus excelsior L.), demonstrates the possible range of variation in morphology and functionality exhibited by flowers with respect to gender. Flowers of the ash are wind-pollinated and lack petals and sepals. Structurally, the flowers may be either male or female, or even hermaphroditic, consisting of two anthers and an ovary. A male flower can be morphologically male or hermaphroditic, with anthers and a rudimentary gynoecium. Ash flowers can also be morphologically female, or hermaphroditic and functionally female.
The Asteraceae or sunflower family with close to 22,000 species,[20] have highly modified inflorescences that are flowers collected together in heads composed of a composite of individual flowers called florets. Heads with florets of one sex, when the flowers are pistillate or functionally staminate, or made up of all bisexual florets, are called homogamous and can include discoid and liguliflorous type heads. Some radiate heads may be homogamous too. Plants with heads that have florets of two or more sexual forms are called heterogamous and include radiate and disciform head forms, though some radiate heads may be heterogamous too.
Evolution
Angiosperms
It is thought that flowering plants evolved from a common hermaphrodite ancestor, and that dioecy evolved from hermaphroditism. Hermaphroditism is very common in flowering plants; over 85% are hermaphroditic, whereas only about 6-7% are dioecious and 5-6% are monoecious.[21][22]
A fair degree of correlation (though far from complete) exists between dioecy/sub-dioecy and plants that have seeds dispersed by birds (both nuts and berries). It is hypothesized that the concentration of fruit in half of the plants increases dispersal efficiency; female plants can produce a higher density of fruit as they do not expend resources on pollen production, and the dispersal agents (birds) need not waste time looking for fruit on male plants. Other correlations with dioecy include: tropical distribution, woody growth form, perenniality, fleshy fruits, and small, green flowers.[23]
Plant growth regulators can be used to alter flower and plant sexuality, in cucumbers ethephon is used to delay staminate flowering and transforms monoecious lines into all-pistillate or female lines. Gibberellins also increase maleness in cucumbers. Cytokinins have been used in grapes that have undeveloped pistils to produce functional female organs and seed formation.
See also
- Botany
- Evolution of sexual reproduction
- Meiosis
- Apomixis
References
- ^ Barrett, S. C. H. (2002). The evolution of plant sexual diversity. Nature Reviews Genetics 3(4): 274-284.
- ^ Costich, D. E. (1995). Gender specialization across a climatic gradient: experimental comparison of monoecious and dioecious Ecballium. Ecology 76 (4): 1036-1050.
- ^ Molnar, S. (2004). Plant Reproductive Systems, internet version posted February 17, 2004.
- ^ Ewing, J. W., & Klein, R. M. (1982). Sex Expression in Jack-in-the-Pulpit. Bulletin of the Torrey Botanical Club 109 (1): 47-50. doi:10.2307/2484467
- ^ Angiosperm sexual systems
- ^ Correlation between male and female reproduction in the subdioecious herb Astilbe biternata (Saxifragaceae) - Olson and Antonovics 87 (6): 837 - American Journal of Botany
- ^ Strittmatter, L.I.; Negrón-Ortiz, V.; Hickey, R.J. (2002). "Subdioecy in Consolea spinosissima (Cactaceae): breeding system and embryological studies". American Journal of Botany 89 (9): 1373–1387. http://www.amjbot.org/content/89/9/1373.abstract.
- ^ a b Geber, Monica A. (1999), Gender and sexual dimorphism in flowering plants : with 29 tables, Berlin: Springer, pp. 4, ISBN 3-540-64597-7, http://books.google.com/?id=pUo2T34ppKUC&pg=PA4&dq=polygamy+plants
- ^ Davis, P.H.; Cullen, J. (1979), The identification of flowering plant families, including a key to those native and cultivated in north temperate regions, Cambridge: Cambridge University Press, pp. 106, ISBN 0-521-29359-6
- ^ Kiesselbach, T.A. (1999), The structure and reproduction of corn, Cold Spring Harbor, N.Y: Cold Spring Harbor Laboratory Press, pp. 3, ISBN 978-0-87969-556-9, OCLC 245875754
- ^ "?". http://www.ext.vt.edu/departments/envirohort/factsheets2/landsnurs/feb88pr6.html.[dead link]
- ^ Waldbauer, Gilbert (2003), What good are bugs? : insects in the web of life, Cambridge, Mass.: Harvard University Press, pp. 33–34, ISBN 978-0-674-01027-7, OCLC 50198798
- ^ Delannay, Xavier (1979), "Evolution of male sterility mechanisms in gynodioecious and dioecious species of Cirsium (Cynareae, Compositae)", Plant Systematics and Evolution 132 (4): 327, doi:10.1007/BF00982395
- ^ Males outcompete hermaphrodites for seed siring success in controlled crosses in the polygamous Fraxinus excelsior (Oleaceae) - Morand-Prieur et al. 90 (6): 949 - American Journal of Botany
- ^ Barrett S.C.H.; Case A.L.; Peters G.B. Gender modification and resource allocation in subdioecious Wurmbea dioica (Colchicaceae) Journal of Ecology, Volume 87, Number 1, January 1999 , pp. 123-137(15)
- ^ Gleason & Cronquist (1963), Manual of vascular plants of Northeastern United States and adjacent Canada, Princeton, N.J., pp. XX, ISBN 0-442-02722-2
- ^ S. L. Dellaporta, and A. Calderon-Urrea. Sex Determination in Flowering Plants Plant Cell 5: Pages 1241-1251.
- ^ a b Bernstein C, Bernstein H. (1991). Aging, Sex, and DNA Repair. Academic Press, Inc., San Diego ISBN 0-12-092860-4.
- ^ http://www.publish.csiro.au/?act=view_file&file_id=BT9950451.pdf
- ^ Asteraceae in Flora of North America @ efloras.org
- ^ Rieger, R., A. Michaelis, and M.M. Green (1991). Glossary of Genetics, Fifth Edition. Springer-Verlag. ISBN 0-387-52054-6
- ^ Heilbuth, J.C. (2000). Lower species richness in dioecious clades. American Naturalist 156: 221-241
- ^ Vamosi, J.C., & Vamosi, S.M. (2004). The role of diversification in causing the correlates of dioecy. Evolution 58: 723-731
- Binggeli, P., & Power, J. (1999). Gender variation in ash (Fraxinus excelsior L.)
- Darwin, C. (1877). The Different Forms of Flowers on Plants of the Same Species.
- Dellaporta, S.L. and A. Calderon-Urrea. (1993). Sex determination in flowering plants. The Plant Cell 5: 1241-1251.
- Linnaeus, C. (1735). Systema Naturae.
- Renner, S.S., & Ricklefs, R.E. (1995). Dioecy and its correlates in the flowering plants. American Journal of Botany 82: 596-606.
External links
- Images of sexual systems in flowering plants at bioimages.vanderbilt.edu
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