The evolution, ecology and genetics of sex determination in Mercurialis annua

The allocation of resources to male or female progeny, or to male or female reproductive function more generally, is one of the most important life history decisions a sexually reproducing individual must ever make. Sex determination is thus a fundamental process, yet the mechanisms which control it are surprisingly diverse. In this thesis, I examine sex determination in the plant species Mercurialis annua L. (Euphorbiaceae). I assess the mechanism of sex determination operating in dioecious and androdioecious populations of M. annua and also investigate the conservation and evolution of sex-determining mechanisms across the annual mercury clade, the lineages of which display exceptional variation in sexual system. First, using crosses, I establish that sex in dioecious M. annua is controlled by a single-locus genetic mechanism, consistent with recent work that identified a single male-linked DNA marker in the species. My search for new sex-linked genes revealed none, however, suggesting that M. annua possesses at most a small non-recombining region around sex-determining loci. Why many dioecious plants lack heteromorphic sex chromosomes is still poorly understood and I consider explanations for this. I extend my investigation by comparing genetic diversity between loci that differ in their linkage to the sex-determining locus. I find a single male-linked marker to possess significantly lower diversity than autosomal loci, but no difference in the diversity of partially sex-linked and non-sex-linked genes. I also assess the conservation of a sex-linked marker among annual mercury lineages and conduct crosses between lineages to examine the conservation of sex determination. My findings indicate a conserved mechanism of single-locus genetic sex determination and I consider the role polyploidisation and hybridisation have played in sexual system evolution and the modification of sex-determining mechanisms in the clade. Finally, I assess the presence of environmental sex determination in androdioecious M. annua, concluding that although male frequency is not influenced by growing density, a degree of sexual lability exists in the lineage.