Evolution and ecology of plant sexual systems

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Evolutionary transitions that occur repeatedly over the course of history are of inherent interest to biologists. One such transition involves a fundamental change in the way organisms reproduce and has occurred at least 100 times in the flowering plants alone: the evolution of separate sexes from hermaphroditism. During this transition, individuals cease being able to reproduce as both female and male parents (i.e., hermaphrodites) and instead act solely as mothers or fathers to their offspring (i.e., dioecy). Why should individuals give up one of their sex roles to become specialist females or males? Biologists going back to Darwin have thought about this question, but two key issues remain unclear - issues that can begin to be resolved using the combination of modern molecular genetics and experimental manipulations that characterizes the research proposed here. First, the importance of ecological conditions in evolutionary transitions to dioecy is widely recognized but poorly understood. I will provide the first empirical investigation of the effects of environmental conditions on a key step in the evolution of dioecy - the evolution of males - using experiments and genetic analysis of the native plant Sagittaria latifolia. This species is unusual in having both dioecious and hermaphroditic populations, making it an ideal experimental system. Second, there is evidence that dioecious lineages contain fewer species than hermaphrodite lineages, but the processes responsible are largely unknown. Much can be learned about the effect of combined vs. separate sexes on patterns of diversity using lineages that have undergone rapid diversification in their sexual systems. The Canary Islands radiation of Echium appears to be such a lineage, with one quarter of species having dimorphic breeding systems. Using genetic and field studies of Echium, I will investigate the historical and contemporary processes that shape the evolution of combined vs. separate sexes. The combination of approaches proposed here will provide (a) novel insights into the evolution of dioecy, and (b) training for students in high-throughput molecular genetics, and advanced experimental design and statistics - highly sought skills across a range of biological careers.