Plant sexuality

Plant sexuality deals with the wide variety of sexual reproduction systems found across the plant kingdom. That plants employ many different strategies to engage in sexual reproduction was used, from just a structural perspective, by Carolus Linnaeus (1735) to propose a system of classification of flowering plants, and later this subject received attention from Charles Darwin (1877). Flowers, the reproductive organs 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.

• Individual sexual organ (a flower in angiosperms):
  • Bisexual - Reproductive organ with both male and female equivalent parts (stamens and pistil in angiosperms; also called a perfect flower); another term widely used is hermaphrodite.
  • Unisexual - Reproductive structure that is either functionally male or functionally female. In angiosperms this condition is also called imperfect.

• Individual plant:
  • Hermaphrodite - A plant that has only hermaphrodite reproductive structures. In angiosperm terminology a synonym is monoclinous from the Greek "one bed".
  • Monoecious - having unisexual flowers, conifer cones, or functionally equivalent structures of both sexes appearing on the same plant; from Greek for "one household".
  • Dioecious - having unisexual flowers, conifer cones, or functionally equivalent structures occuring on different individuals; from Greek for "two households".
  • 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.
  • Trimonoecious (polygamous) - male, female, and hermaphrodite structures all appear on the same plant.

• 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.
  • Trioecious (or subdioecious) - male, female, and hermaphrodite plants are all in the same population.

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 5% are 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.