separatingSexes · Many paths to separate sexes: the genomics of sex-determination in the Hawaiian Wikstroemia

Life on earth has undergone several major evolutionary transitions, but perhaps none so striking or relevant to our daily lives as the origin of separate sexes. Yet, despite intense study for over 100 years, understanding how and why genes determining sex evolve - and the consequences for the chromosomes harboring them - remains an open problem in evolutionary biology. One obstacle is that most previous work has focused on ancient sex chromosome systems, like those found in mammals, where recombination suppression and genetic degeneration has obliterated any signs or signals of how genetic sex-determination evolved in the first place. But here, flowering plants (Angiosperms) come to the rescue! Compared to animals, Angiosperms exhibit a staggering diversity of reproductive forms, and have transitioned from combined (hermaphroditism) to separate sexes (dioecy) many times in their evolutionary history. The goal of this project is to understand the genomic changes giving rise to at least two and possibly three parallel transitions from hermaphroditism to dioecy via the evolution of new sex chromosomes within an endemic Hawaiian radiation of the flowering plant genus Wikstroemia (Thymelaeaceae; 12 spp.). Using a combination of genomics, bioinformatics, gene expression, and greenhouse experiments, I aim to achieve three key objectives: (1) locate and fully annotate sex-linked regions of the genome associated with each form of dioecy; (2) clarify the systematics of this radiation and study the biogeography of sex-linked genes, and the relative times of origin and whether/how recombination suppression has evolved for each form; and (3) determine what genes/developmental pathways are involved in sex-determination in these closely related lineages. This fascinating radiation offers a unique snapshot of repeated evolution of genetic sex-determination from a common genetic starting point, shedding new light on how and why evolutionary transitions to separate sexes occur.