The evolution of gametophytic self-incompatibility: A theoretical perspective

Self-incompatibility (SI) is a genetic mechanism in plants whereby individuals discriminate between self and non-self pollen, allowing non-self pollen to germinate while arresting self pollen germination. In plants with a gametophytically determined self-incompatibility reaction, a pollen grain\u27s compatibility type is determined by its own haploid genotype. Because it appears that other SI systems have arisen only within lineages that already have a gametophytic system, I focus my attention initially on the origin of gametophytic self-incompatibility (GSI). In most of these SI systems, the incompatibility reaction is genetically controlled by a single multi-allelic locus. ^ Plant evolutionary biologists have increasingly recognized the need to understand better the relationship among variation in floral features, patterns of pollen movement, and the evolution of mating systems. Empirical studies have shown that patterns of pollen transfer affect both rates of self-fertilization and success as an outcross pollen parent. Theoretical studies have begun to focus on pollen transfer dynamics and their impact on mating system evolution. ^ While much has been learned about the relationship between selfing rates, inbreeding depression, and the evolution of SI systems, previous studies have not included explicit models of the pollination process. In my dissertation, I propose a model for the evolution of GSI, based on an explicit model of the pollination process. I first considered the initial evolution of GSI, focusing on the addition of parameters that describe ecological events (i.e., pollination) associated with the mating system. I then expanded the basic model to more accurately reflect the hypothesized evolution of the system—that it likely arose from a previously existing recognition system, and that the SI system is typically multi-allelic. ^ I also investigated the relationship of GSI genes to physiologically similar genes that are not involved in SI responses. The GSI genes from the Solanaceae, Scrophulariaceae and Rosaceae show structural homology to one another, but they also have a striking degree of similarity to physiologically similar genes from fungi and other plants. Using several computer-based methods, I reconstructed the relationship among the amino acid and nucleotide sequences of both the GSI genes and their physiologically similar counterparts.