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Plant sexuality

Plant sexuality deals with the wide variety of sexual reproduction systems found across the plant kingdom. This article describes morphological aspects of sexual reproduction of plants.

That plants employ many different strategies to engage in sexual reproduction was used, from just a structural perspective, by Carolus Linnaeus (1735 and 1753) to propose a system of classification of flowering plants. Later this subject received attention from Christian Konrad Sprengel (1793) who described plant sexuality as the "revealed secret of nature" and, for the first time, understood the biotic and abiotic interactions of the pollination process. Charles Darwin's theories of natural selection are based on his work. Flowers, the reproductive structures of angiosperms, are more varied than the equivalent structures of any other group of organisms, and flowering plants also have an unrivalled diversity of sexual systems (Barrett, 2002). But sexuality and the significance of sexual reproductive strategies is no less important in all of the other plant groups. The breeding system is the single most important determinant of the mating structure of nonclonal plant populations. The mating structure in turn controls the amount and distribution of genetic variation, a central element in the evolutionary process (Costich, 1995).

Terminology

The complexity of the systems and devices used by plants to achieve sexual reproduction has resulted in botanists and evolutionary biologists proposing numerous terms to describe structures and 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 (taken from Molner, 2004), generalized to fit more than just plants that have flowers, and expanded to include other terms and better 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 - Reproductive structure with both male and female equivalent parts (stamens and pistil in angiosperms; also called a perfect or complete flower); other terms widely used are hermaphrodite, monoclinous, and synoecious.
  • Unisexual - Reproductive structure that is either functionally male or functionally female. In angiosperms this condition is also called diclinous, imperfect or incomplete.

Individual plant

  • Hermaphrodite - A plant that has only hermaphrodite reproductive units (flowers, conifer cones, or functionally equivalent structures). In angiosperm terminology a synonym is monoclinous from the Greek "one bed".
  • Monoecious - having unisexual reproductive units (flowers, conifer cones, or functionally equivalent structures) of both sexes appearing on the same plant; from Greek for "one household". Individuals bearing flowers of both sexes at the same time are called simultaneously or synchronously monoecious. Individuals that bear only flowers of a single sex at one time are called consecutively monoecious; "protoandrous" 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 - having unisexual reproductive units (flowers, conifer cones, or functionally equivalent structures) occurring on different individuals; from Greek for "two households". Individual plants are not called dioecious: they are either gynoecious or androecious.
  • Because many dioecious conifers show a tendency towards monoecy (that is, a female plant may sometimes produce small numbers of male cones or vice versa), these species are termed subdioecious (McCormick & Andresen, 1963).
  • In angiosperm terminology,

'diclinous ("two beds") includes all species with unisexual flowers, although particularly those with only unisexual flowers, i.e. the monoecious and dioecious species.

  • Gynoecious - has only female reproductive structures; the "female" plant.
  • Androecious - has only male reproductive structures; the "male" plant.
  • Gynomonoecious - has both hermaphrodite and female structures.
  • Andromonoecious - has both hermaphrodite and male structures.
  • Subandroecious - plant has mostly male flowers, with a few female or hermaphrodite flowers.
  • Subgynoecious - plant has mostly female flowers, with a few male or hermaphrodite flowers.
  • Trimonoecious (polygamous) - male, female, and hermaphrodite structures all appear on the same plant.
 
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 with no pollen

Plant population

  • Hermaphrodite - only hermaphrodite plants.
  • Monoecious - only monoecious plants.
  • Dioecious - only dioecious plants.
  • Gynodioecious - both female and hermaphrodite plants present.
  • Androdioecious - both male and hermaphrodite plants present.
  • Subdioecious - population of primarily unisexual (dioecious) plants, with a few monoecious individuals.
  • Trioecious - male, female, and hermaphrodite plants are all in the same population.

Some plants use a method known as self-incompatibility to ensure genetic diversity within the species. In these plants, the male organs cannot fertilize the female parts of the same plant.

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, female, or hermaphrodite, the latter consisting of two anthers and an ovary ('c' below). A male flower can be morphologically male ('a' below) or a hermaphrodite flower with anthers and a rudimentary gynoecium ('b' below; functionally 'male'). Ash flowers can also be morphologically female ('e' below) or hermaphrodite and functionally female ('d' below; with vestigial anthers).

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(Illustration from Binggeli and Power, 1999)

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; about 70% are hermaphroditic, while only about 23% dioecious and 7% are monoecious. About 7% of species exhibit gynodioecy or androdioecy, while 10% contain both unisexual and bisexual flowers (Molner, 2004).

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.

References

  • Barrett, S. C. H. (2002). The evolution of plant sexual diversity. Nature Reviews Genetics 3(4): 274-284.
  • Binggeli, P., & Power, J. (1999). Gender variation in ash (Fraxinus excelsior L.)
  • Costich, D. E. (1995). Gender specialization across a climatic gradient: experimental comparison
of monoecious and dioecious Ecballium. Ecology 76 (4): 1036-1050.
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