Thesis / Master of Science (MS)

In the plant kingdom, only about one tenth of the species have different sexes. Since plant sex chromosomes are in general younger (and not always distinguishable from autosomes) than animal sex chromosomes, it is interesting to analyze them. This data can then be used in agricultural research in terms of growing exlusively one type of sex or using sex-linked genes for breeding. Because of polyploidization and long-term selection of only agriculturaly advantageous features, it seems fit to find a model plant which has not been subjected to artificial selection and could be used for the study of natural genome evolution. In this study, we chose Mercurialis annua and its close relative Mercurialis huetii for comparative analysis of their genomes and sex chromosomes which seem to be undergoing the first stage of degeneration.

The Mercurialis annua L. (Euphorbiaceae) species complex comprises a group of closely related lineages that present a wide range of sexual-systems, making it a valuable model for the study of plant sexual-system evolution. Within this polyploid complex, diploid populations are dioecious, and polyploid populations either monoecious or androdioecious (males coexist with functional hermaphrodites). The primary aim of this thesis was to use patterns of genetic diversity to elucidate the evolutionary origin and maintenance of the sexual-system diversity in M. annua. The phylogeny of the M. annua complex was reconstructed using chloroplast and ITS DNA sequence. This, in conjunction with morphometric analysis, showed that both hexaploid M. annua, and a novel species from the Canary Islands (newly described here as Mercurialis canariensis), were allopolyploid in origin. Such an origin for hexaploid M. annua suggests that androdioecy may have been able to arise in this group as a consequence of hybridisation between a monoecious lineage, tetraploid M. annua, and a dioecious lineage, M. huetii. Artificial crosses were used to show that hexaploid M. annua has disomic marker inheritance, and a statistical approach was developed to quantify genetic diversity and differentiation in polyploids with disomic inheritance. Strong gradients in genetic (allozyme) diversity at a pan-European scale were used to infer the existence of separate glacial refugia for dioecious and monoecious races of M. annua, at the eastern and western ends of the Mediterranean basin, respectively. A metapopulation model had previously been proposed to explain the ecological maintenance of androdioecy in M. annua. Here, population-level patterns of genetic diversity were used as an indirect test of this model. The discovery of lower within-population diversity, and of greater genetic differentiation between populations, for monoecious populations than for androdioecious populations was consistent with the metapopulation model, and suggests that androdioecy is maintained by the occurrence of regular local extinction.

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.