9411869 Schemske The great diversity of reproductive systems is a distinctive feature of higher plants. Self-incompatibility, a genetic system that prevents fertilization by self-pollen in hermaphroditic plants, is one of a number of mechanisms that promote outcrossing in higher plants. It is estimated to be present in over half of all angiosperm species. This phenomenon has important implications for plant breeding, and as a result its physiological and molecular bases have been the focus of extensive research. Self- incompatibility also plays an important role in the evolution of higher plants. The loss of self-incompatibility and evolution of selfing is thought to be a common evolutionary pattern in a number of angiosperm families. Yet despite an extensive literature on the major ecological and genetic factors favoring self-vs. cross fertilization, little is know about the specific factors involved in the origin or loss of self-incompatibility. The co-occurrence of two closely related annuals in the genus Linanthus presents an opportunity to investigate the relationship between self-incompatibility and mating system evolution. The two species are ecologically and morphologically similar, yet they are distinguished by their breeding systems: Linanthus parviflorus is self-incompatible, while an undescribed sympatric species is self- fertilizing. In comparing a number of ecological and genetic aspects of the two species, the proposed research will address the functional significance of self-incompatibility, focussing on unique features of the phenomenon. For instance, while a number of mechanisms of higher plants prevents self-fertilization, self- incompatibility is unique in that it also may prevent mating between close relatives. The evolutionary significance of this will be investigated. Another focus of the research will be the relationship between self-incompatibility and morphological floral characters that influence the deposition of se lf-pollen on the stigma. Information gained from this study will contribute to our understanding of the factors that influence the loss of self- incompatibility, a critical turning point in the course of mating system evolution in higher plants.
