The Role of Mating System Divergence in Plant Speciation

Understanding the formation of new species, the process of speciation, is among the most challenging problems in evolutionary biology. Rates of speciation vary widely among flowering plants and are strongly associated with the degree of reproduction via self-fertilization versus mating with other individuals (known as the plant 'mating system'). This project integrates new evolutionary theory, field and laboratory experiments, and genomic analyses to test key hypotheses about the role of mating system changes in plant speciation. The project investigates the processes that determine where and when strong barriers to mating occur. It will clarify whether evolutionary changes in rates of self-fertilization directly promote reproductive isolation between diverging species. The experimental work focuses on two subspecies of the California annual plant, Clarkia xantiana, which differ in mating system. One subspecies is primarily self-fertilizing the other is primarily outcrossing. These plants are ideal for study because they exhibit features of newly formed species. However, they have only recently diverged and can still interbreed. Moreover, their flowers are substantially different where they co-occur, a signature of the speciation process. The proposed work will also involve extensive outreach to the public via an innovative program called, 'Market Science', where researchers present topics to and engage directly with the public at local farmers' markets. This highly successful program will be expanded to include new topics on evolutionary biology. The proposed project will also directly involve undergraduates in authentic research in the classroom.

The proposed work address three primary objectives. Aim 1 examines the role of natural selection in the evolution of reproductive character displacement between sister outcrossing and selfing lineages. This work involves the development of novel theoretical models of character displacement that incorporate divergence in mating system. It also involves large-scale field experiments that quantify the role of individual traits in conferring reproductive isolation and determine their genetic basis. Aim 2 examines the ecological and genetic costs of hybrid formation between sister outcrossing and selfing lineages. This work involves field experiments and genomic analyses that quantify the reproductive fitness of hybrids. Aim 3 synthesizes Aims 1 and 2 by asking about how reproductive isolating mechanisms collectively influence genome-wide patterns of variation and introgression between the two taxa. This integrative work will advance evolutionary research by calling attention to the potential for mating system to influence speciation broadly in nature.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.