Evolution of combined versus seperate sexes in flowering plants

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Despite the central role of sex in organismal biology, surprisingly little is known about how and why reproductive systems involving separate sexes have arisen. Even less is understood about the evolutionary processes that underlie the genetic control of sexual systems. The sexual systems of plants are of specific interest not just because of their obvious agricultural importance but also because of their complexity and the unique opportunity they present to study the gradual evolution of separate sexes in general. Plant groups that exhibit variability in their sexual systems and that have different modes of sex determination are the ideal context in which to study the evolution of sex determining chromosomes. We have selected a small Eurasian clade of Cucurbitaceae, comprised of the genera Bryonia and Ecballium, with which to study the evolution of separate and combined sexes in plants and the evolution of sex-determining chromosomes. Species in this clade are either monoecious or dioecious, and sexual systems can even vary between populations, apparently depending on habitat. Previous work in a lab has clarified the phylogenetic and phylogeographic relationships within this group. This provides us with a strong comparative framework that we can use to study the organization of sex determining regions on the chromosomes among the various species. The proposed study will answer questions about the evolution of sex-linked DNA regions, and the origin and composition of sex chromosomes. We request funding (1) to sequence up- and down-stream from AFLP markers linked to the male sex in Bryonia dioica and analyse those sequences in a molecular phylogenetic framework and, (2) to construct a BAC library of B. dioica in order to perform Chromosome Painting via fluorescence in-situ hybridization (FISH). The ultimate goal of this research program is to contribute to a better understanding of the evolution of separate and combined sexes and of sex-determination in plants.