School of BioSciences - Theses
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ItemPhylogeography of Box Eucalypts with Disjunct Distributions in South-East Australia( 2021)Despite Australia generally being considered a continent of ancient and stable landscapes, the south-east has experienced major environmental changes during the Pliocene and Pleistocene. In addition to the slow continent-wide decline in rainfall and several global glacial-interglacial cycles, there have been more localised events that have impacted the vegetation of the region including marine inundation of the Murray Basin, uplift of the Padthaway High that dammed the Murray River and formed the large Lake Bungunnia, and volcanic activity in the Newer Volcanics Province of western Victoria and south-eastern South Australia. A record of the impacts of these events on the distribution of plants has been left in the patterns of genetic diversity and relatedness across the landscape. The eucalypts, >850 species of shrubs and trees in the genera Angophora, Corymbia and Eucalyptus, are one of the most widespread and species rich groups of woody plants in Australia, and almost define what is thought of as the ‘bush’. They occur in habitats ranging from wet forests of the east coast and south-west corner of the continent all the way through the climatic gradients to the inland deserts. In my thesis I focus on E. sect. Adnataria, one of the most diverse groups within the eucalypts with ~120 species, and, in south-eastern Australia, the species it contains dominate large areas of open forest and woodlands. This makes members of E. sect. Adnataria good candidates on which to undertake phylogeographic studies to build understanding of the biogeographical history of southeastern Australia, as I have done in this thesis. In chapter 2 I explore the phylogeography of E. behriana using ddRADseq data, showing that the populations of the species west of the lower Murray Basin were isolated earlier than those to the east. This is hypothesised to be related to the marine inundation and formation of Lake Bungunnia in the Murray Basin over the last several million years. The isolation of the populations at Long Forest and around West Wyalong are shown to have recently experienced geneflow with the larger populations of the Victorian Goldfields, Wimmera, and Murray Mallee in the east. This more recent isolations of eastern populations are hypothesised to be due to climate changes under the global glacial cycle, with a possible contribution from recent volcanic activity in the Newer Volcanics Province. I set out to resolve the relationships and taxonomy within the E odorata complex in chapter 3, however I didn’t manage to achieve this using both ddRADseq and DArTseq, with many relationships remaining unresolved, and many questions remaining unanswered. What I show is that most E. viridis populations, E. aenea and E. castrensis form a lineage sister to the remained of the clade, which includes an E. viridis population previous described as E. viridis var. latiuscula from south-eastern Queensland. I also show hybridisation is common within the complex and with taxa outside it, especially the closely related Grey Boxes, which has contributed to the diversity of morphology in the group. Chapter 4 returns to focus on E. behriana, as in this chapter I present a case study on the pitfalls and flaws in using plastid DNA for phylogeography in the absence of substantial outgroup sampling. I show how, by analysing the ingroup only, a geographically sensible and well resolved pattern of population relationships can be established. However, by drastically expanding my sampling to include extensive representation of co-occurring outgroups, I show these patterns become meaningless due to the large cyto-nuclear discrepancy in the eucalypts. A further phylogeographic study is presented in chapter 5, that of E. baueriana. This taxon occurs in coastal regions of south-east Australia, and I show there is a deep divergence between a south-western lineage and a north-eastern one located in eastern Gippsland. While the cause of this disjunction is not clear, we hypothesise it is related to the occurrence of gallery rainforests in the region that prevent E. baueriana form colonising it’s preferred riparian habitat. The fragmentation of the distribution of the two main lineages is hypothesised to be related to glacial-interglacial climatic cycles; in particular, the changes they cause to sea-levels.