Population Structure and Phylogeography of Four Freshwater Species in Southeast Queensland, Australia
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Freshwater species are expected to show higher levels of genetic structuring than those inhabiting estuarine or marine environments because it is difficult for freshwater species to move between river systems. However, freshwater species that exhibit some tolerance to salt water may be capable of marine dispersal and therefore may show lower levels of genetic structuring. The species that are restricted to freshwater are likely to have genetic structures that reflect the history of those drainages. Various studies have attempted to use genetic data to infer past geomorphological changes. For example, stream rearrangements were thought to have influenced the distribution of genetic diversity in the purple spotted Gudgeon, Morgunda morgunda (Hurwood and Hughes, 1998). The main aim of this study was to resolve the relative significance of contemporary and historical events in structuring populations of freshwater species in southeast Queensland river systems. The study revolved around four freshwater species namely the ornate rainbow fish Rhadinocentrus ornatus, the empire gudgeon Hypseleotris compressa, the endemic freshwater shrimp Macrobrachium australiense and freshwater shrimp Macrobrachium tolmerum, and examined patterns of genetic variation using allozyme and mtDNA analysis and also previously established hypotheses of biogeographic association among these regions was tested. It was hypothesized that R. ornatus and M. australiense, which are restricted to freshwater, would display genetic structure similar to the species which had already been examined i.e., would show similarities among streams along the Sunshine Coast region only (Hughes et al., 1999), because these creeks were thought to have had a confluence before they reached the sea, during the last 10, 000 BP. The results for R. ornatus and M. australiense, were different from expectations, In contrast to earlier studies, there was evidence of restricted gene flow among sites within the Sunshine Coast region based on highly significant FST values. This was a surprising finding but suggested that gene flow among subpopulations is extremely limited in these two species in both past and present times. The high levels of genetic structure in R. ornatus and M. australiense suggest that they may have different dispersal behaviour from the previously studied species Oxleyan Pygmy Perch (Nannoperca oxleyana) (Hughes et al., 1998), even though they inhabit the same environment. The apparent difference between these two obligate freshwater species and Oxleyan Pygmy Perch (N. oxleyana) studies may be because R. ornatus and M. australiense may not have moved between adjacent streams even though they shared a confluence. Other reasons could be that they may also be more sensitive to elevated salinities than Oxleyan Pygmy Perch (N. oxleyana). These elevated salinities would have occurred at the lower reaches of the streams and thus could have restricted dispersal from one creek to the other for these two obligate freshwater species (R. ornatus and M. australiense). It was hypothesized that H. compressa and M. tolmerum would show lower levels of genetic structure than R. ornatus and M. australiense throughout the study areas because they can tolerate some marine conditions (Chapter 3 & 4). Genetic data for H. compressa supported this hypothesis, indicating that subpopulations of this species had low levels of mtDNA genetic differentiation, with many recently derived haplotypes which were widespread along the southeast Queensland coast. This suggested that there was existing gene flow among and between regions or recent demographic change in the populations sampled. On the other hand, genetic data on M. tolmerum did not support this hypothesis completely, because although allozyme analysis suggested gene flow between regions, mtDNA data suggested that gene flow was limited. The study demonstrated the importance of current and past events in shaping the genetic population structure of freshwater species of southeast Queensland river systems. The species that were thought to tolerate brackish water showed higher levels of connectivity among rivers than those that were restricted to freshwater, although M. tolmerum showed lower levels of connectivity than predicted. For R. ornatus and M. australiense, both contemporary and historical movements are extremely limited across the region. To conclude, the present study showed that R. ornatus and M. australiense have a genetic structure typical of freshwater species. On the other hand, H. compressa had a genetic structure more typical of estuarine or marine species, while M. tolmerum, had a pattern intermediate between freshwater and estuarine species.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environmental Studies
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