Evolution of sexual polymorphisms in plants

There are at least four types of stylar polymorphisms: heterostyly, stigma-height dimorphism, enantiostyly and flexistyly in flowering plants. The occurrence of stylar polymorphisms in numerous insect-pollinated flowering plant families suggest convergent evolution due to the adaptation of floral morphology to pollination behaviors of insects. Little is known about the molecular genetic bases and the functional advantages of these polymorphisms. Controversies still persist about their evolutionary pathways and maintenance. By incorporating both genetics components and ecological factors, I investigate the evolution of sexual polymorphisms. In the second chapter, I use both analytical and simulation models of population dynamics to explore the ecological and genetic mechanisms that might maintain the stigma-height dimorphism and explain the associations of morph ratios with population size, or the geographical and ecological conditions of the populations. I conclude that for self-sterile species, population size-dependent L-biased morph ratios in stigma-height dimorphic populations are due either to a greater degree of disassortative mating in the L-morph and pollen limitation in large populations, or to a greater ovule fertilization probability in L-morph and pollen limitation in small populations. For self-fertile species, strongly L-biased morph ratios independent of population size may be due to lethality of homozygous S-morph. Despite the wide range of mating systems, most sexual species are either hermaphroditic or dioecious, with the majority of them being hermaphrodites whereas dioecy is found in only about 6% of angiosperm species. In the third chapter, I construct a mathematical model that focuses on the competition for germination sites to understand the ecological conditions and phenotypic traits that may favor dioecy over hermaphroditism during colonization competitions. I demonstrate that increasing seed production provides a greater benefit to dioecy than increasing seed dispersal and dioecy should most often arise from hermaphroditic lineages with limited seed dispersal, perennial life history, and low plant density.