The evolution of apomixis in the Asplenium monanthes fern complex

Abstract

Asexually reproducing eukaryotes provide a window into the evolution and maintenance of sexual reproduction, and are challenging our concept of a species. In plants, asexual reproduction (apomixis) is known to cause taxonomic problems and has been indicated to have a disparately high frequency in homosporous ferns. The reasons for such a high frequency of apomictic taxa are unclear, and only add to the enigmatic nature of these ferns. Contemporary studies in ferns are focussing on such questions, and are providing valuable insights into the evolutionary dynamics of apomictic ferns. In this thesis, I investigate the evolution of apomixis in the Asplenium monanthes complex. First, I perform a biosystematic study of the complex, based upon plastid and nuclear DNA sequence data, reproductive mode and polyploidy. I present evidence for reticulate evolution and multiple apomictic lineages. Second, I address the problem of species delimitation in an apomictic species complex. I performed a comparative analysis of AFLP data and sequence data using a variety of species delimitation methods. Results supported the inferences of independent lineages and reticulate evolution made in the previous chapter, but the AFLP data did not support inferences of parentage. Third, I investigated the evolution of genome size in the complex based on DNA C-value data, and tested the utilisation of spore size data to infer ploidy level based on the relationship between genome size and spore size. Genome size variation was shown not only to be due to polyploidy, but also due to expansion of the monoploid genome / chromosome size. Moreover, the evolution of spore size and genome size were not correlated, indicating that spore size is not a good indicator of ploidy level in apomictic complexes. Finally, I investigate the origins of apomixis in A.monanthes based upon AFLP data, sequence data and DNA C-values. I find evidence that the observed genetic and karyological diversity is explained by a single origin of apomixis followed by the spread of apomixis by hybridisation with closely related sexual species by the male function. There is also evidence for post genetic divergence by other mechanisms such as genetic segregation, somatic mutation and unequal meiosis. This thesis presents the first thorough investigation of this complex and has increased our understanding of the evolutionary dynamics of apomixis in ferns.