Barriers to Self-Fertilization
Flowers can be staminate (bearing only male reproductive organs), pistillate (bearing only female reproductive organs), or perfect (bearing male and female reproductive organs). Individual plants can be monoecious (bearing staminate and pistillate flowers), dioecious (staminate and pistillate flowers borne on separate plants), or even trioecious (staminate, pistillate, and perfect flowers borne on separate plants). Within dioecy, various conditions can be found in different species; gynodioecy, for example, is the term applied to the breeding system of species in which individuals bear either female or hermaphrodite flowers (Richards, 1997). Almost three-quarters of all plant species produce perfect flowers. Approximately 5 percent are dioecious, and slightly more than 5 percent are monoecious (Molnar, 2004).
Pollination can occur within the flowers of a single plant, among different flowers of a single plant, and among flowers of different plants. A plant that is self-fertile and self-pollinating is called autogamous if pollination and fertilization take place within the same flower. A plant is geitonogamous if pollination and fertilization take place between flowers of the same plant, whereas a plant that is cross-pollinated and cross-fertilized is xenogamous. It is common for plants to receive mixtures of self and outcross (nonself) pollen grains, especially if the male and female parts are in the same flower (Plate 1—a perfect or hermaphrodite flower).
Perpetual self-fertilization could be problematic for plants because of the many potential genetic complications associated with inbreeding (Charlesworth and Charlesworth, 1987). Accordingly, adaptations that reduce the likelihood of selfing exist in many taxa. Dioecy and monoecy promote outcrossing,
and that they achieved ecological dominance 100 million to 70 million years ago (Davies et al., 2004). Chief among the many explanations offered for their spectacular ascendancy is the development of mutualistic associations with animals for the dispersal of pollen (Baker and Hurd, 1968; Faegri and van der Pijl, 1979; Labandeira et al., 1994; Stebbins, 1950, 1974) and seeds (Herrera, 1989; Kevan, 1984; van der Pijl, 1982). Mutualistic associations with animals provide mobility of gametes to otherwise predominantly sessile terrestrial plants, which allows for greater genetic variation in plants as well as access to a wider range of ecological opportunities through seed dispersal. For flowering plants, use of an animal partner to transport pollen increases the area in which potential mates can be found and promotes outcrossing, the merger of gametes from genetically distinct individuals. Increasing genetic variability through recombination associated with outcrossing is key