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. To date, we have sequenced the genomes of males and females of 6 Hawaiian species Wikstroemia. Over the coming few years, we will be adding whole genome sequence data for hundreds of individuals from dozens of species throughout the Pacific, as well as as transcriptomic data for around 100 individuals from Hawaiian taxa. The aims of this project will include: i) Assemble reference genomes for each of the 6 Hawaiian species we have already collected sequence data for ii) Collect genomic data and construct a genus-level phylogeny for Wikstroemia, including as many representative species from the Pacific basin as possible in order to study the biogeography and colonization of the Hawaiian islands iii) Collect population-level genomic sequence data for the extant Hawaiian species and characterize the sex-linked regions of the genomes unique to each form of dioecy. iv) To perform segregation analyses using experimental greenhouse crosses and a combination of DNA and transcriptomic data in order to confirm sex-linked regions identified using bioinformatic approaches, and to identify candidate sex-determining genes for each form of dioecy.