Chromosomes and Sex Differentiation

Abstract

Plant species can be classified into two major groups: those that permit self-pollination (autogamy) and those that inhibit self-pollination. In mostly self-pollinating species, harmful recessive mutations with a large effect are efficiently eliminated by selection, while slightly deleterious mutations accumulate as a consequence of the reduced effective population size and effective recombination rates (Wright et al. 2008). In contrast, plants that prevent autogamy are able to mask and retain in their genomes harmful recessive mutations with large effects in spite of more efficient selection against slightly deleterious mutations in this group. In cosexual plants, various mechanisms, such as dichogamy, heterostyly or self-incompatibility, prevent self-pollination. Another mechanism is the evolution of unisexual flowers. Populations can be distinguished according to the localization of unisexual flowers: monoecious (male and female on the same plant), gynomonoecious (hermaphrodite and female flowers on the same plant), andromonoecious (male and hermaphrodite flowers on the same plant), dioecious (male and female flowers on different plants), gynodioecious (female and cosexual individuals), androdioecious (male and cosexual individuals), or trioecious (male, female, and cosexual individuals), as reviewed by Dellaporta and Calderon-Urrea (1993). Gymnosperms are mostly monoecious, but also comprise a relatively high percentage of dioecious species. There are c. 1,010 species of gymnosperms, of which 36 %, namely all 300 species of cycads, Ginkgo biloba, and approximately 80 Gnetales are dioecious (Ming et al. 2011). In contrast, dioecy has been reported in only about 6 % of angiosperm species (Renner and Ricklefs 1995). Interestingly, dioecy is more widespread in tropical species, and an exceptionally high percentage of the dominant woody species of tropical forests are dioecious (Matallana et al. 2005). New cases of dioecy continue to be found because of the phenomenon of cryptic dioecy (Mayer and Charlesworth 1991).