Polyploidy and genetic diversity in the Lepidosperma costale species complex (Cyperaceae)

Lepidosperma species, or sword sedges (Cyperaceae: Schoeneae), are an important component of many ecosystems in southern Australia. In Western Australia in particular, several Lepidosperma spp. are threatened by resource extraction. This thesis explores the role of polyploidy and hybridisation in the evolution of the L. costale species complex to inform the conservation of these taxa. Flow cytometry (Chapter 2) revealed that the species complex contained four DNA ploidy levels: diploid, triploid, tetraploid, and pentaploid. These ploidy levels were largely disjunct at the landscape and microgeographic scales suggesting that ploidy levels should not be mixed when restoring L. costale sensu lato populations.

To explore genetic relationships among cytotypes and populations, a suite of novel chloroplast microsatellite (Chapter 3) and nuclear microsatellite (Chapter 4) markers were developed. Application of these markers (Chapter 5) revealed that diploid populations contained high genetic diversity and that there was little differentiation among diploid populations. In contrast, polyploid populations contained lower within-population diversity and high differentiation among populations. Interestingly, multi-locus genotypes were repeated in most polyploid populations suggesting that polyploidisation is associated with a shift to facultative clonal reproduction in the L. costale species complex. Furthermore, several polyploid populations contained divergent chloroplast haplotypes, which indicated that some individuals may be of allopolyploid origin.

To investigate the hypothesised hybrid origin of these individuals, DNA sequencing of nuclear ribosomal and chloroplast DNA regions was combined with analysis of a novel low-copy nuclear marker (partial arogenate dehydrogenase gene). DNA sequencing (Chapter 6) confirmed that some individuals were of allopolyploid origin, and that they had formed through hybridisation with three phylogenetically distinct Lepidosperma spp. Collectively, the results of this thesis have highlighted the complex genetic patterns and cryptic diversity contained within polyploid lineages of Lepidosperma. These novel insights will inform conservation and have revealed that polyploidy and hybridisation may be important drivers of diversification in this species-rich genus.