At present, there is a general lack of information regarding the spatial genetic
architecture and genetic diversity of estuarine and coastal freshwater fish in
Australia or about the interacting intrinsic, extrinsic and historical influences
responsible for sculpting these patterns. This thesis represented the first
investigation of the phylogeographic structure and recent evolutionary histories
of teleost fishes from the coastal and estuarine environments of south-western
Australia, using the resolution afforded by mtDNA sequence data. Available
evidence indicated that, to different degrees, these species have limited
potential for dispersal amongst local assemblages from different water bodies.
As this theoretically reduces the confounding effects of recent gene flow on
extant genetic structure, these fishes were well suited to studying the
influences of historical factors. Historical influences were expected to be
particularly profound, given that these coastal environments underwent
massive modifications during Late Quaternary eustatic fluctuations.
The thesis consists of four major components, which explored different
aspects of interspecific and intraspecific phylogeny and p hylogeograp hy of
three teleost species, based on mtDNA control region and cytochrome b
fragments. First, the relationship between the endemic, 'strictly estuarine'
Leptatherina wallacei (Atherinidae) and the more widespread, 'estuarine &
marine' 6. presbyteroides was examined, with a view to establishing whether
6. wallacei represents a monophyletic or polyphyletic lineage and whether this
species was derived recently (i.e. in Holocene estuaries). Second, the
phylogeographic structure and genetic diversity of L. wallacei were investigated and compared with data from L. presbyteroides, with a view to
using this information to interpret the recent evolutionary histories of each
congener. Third, the divergence between assemblages of L. wallacei
inhabiting two isolated coastal lakes was used to estimate a maximal
substitution rate for the control region, which was then used to infer general
time frames for the divergence between the two Leptatherina species and
between the major phylogeographic partitions within each species. Fourth,
investigations were initiated into phylogeographic patterns and levels of
genetic diversity within and among assemblages of Pseudogobius olorum
(Gobiidae) from several coastal lakes and an estuary.
Phylogenetic analyses indicated that the two Leptatherina species were
characterised by exclusive and reciprocally-monophyletic lineages of
haplotypes from both mtDNA regions, supporting the monophyletic origins of
L. wallacei. Both 6. wallacei and 6. presbyteroides exhibited high levels of
genetic diversity and extensive overall subdivision (e.g. Qsr = 0.691 & 0.644
respectively for control region data). There was a profound phylogeographic
break in both species between all conspecific assemblages from the lower
west coast (LWC phylogroup) and all those from the south coast (SC
phylogroup), which suggested the influences of shared extrinsic and/or
historical factors. There was limited genetic structuring within the two major
phylogroups of either Leptatherina species, apparently reflecting recent
connectivity amongst local assemblages, with subsequent fragmentation and
insufficient time for lineage sorting. However, two major phylogeographic
breaks distinguished monophyletic control region phylogroups of L. wallacei from the isolated coastal Lake Clifton and Lake Walyungup, consistent with
their independent evolution following lacustrine entrapment during the
Holocene.
The divergence between these two isolated lacustrine assemblages of
Leptatherina wallaceiformed the basis for an estimate of the maximal
substitution rate of the control region. While these data were unable to
provide a precise estimate of the actual rate of molecular evolution, all the
evidence suggested that it was proceeding very rapidly. The maximal rate
estimate of 172.3% lineage-' MY-' was among the fastest ever reported.
Based on this rate, the two Leptatherina species diverged at least 1 SKya, thus
rejecting a Holocene origin for L. wallacei. The divergence between the LWC
and SC phylogroups of L. wallacei has been ongoing for at least GKya, while
the equivalent divergence in L. presbyteroides has been ongoing for at least
11 Kya. As the time frames of these divergences were consistent with periods
of massive environmental modifications associated with the end-Pleistocene
fall in sea level and the HMT, it was likely that these factors have played
important roles in sculpting the species' divergence and intra-specific genetic
structure. Although useful in temporally scaling genetic divergences within
and between the two Leptatherina species, wider application of this rate
estimate to questions regarding other taxa was limited. For example, evident
rate heterogeneity between the genera precluded its use with even the
relatively closely-related atherinid Atherinosoma elongafa.
Phylogeographic analyses identified high levels of genetic diversity and
extensive genetic subdivision (e.g. st = 0.652 for control region) amongst an
estuarine and several lacustrine assemblages of Pseudogobius olorum,
although phylogeographic structure was shallower than in either Leptatherina
species. There was increased divergence between three assemblages from
the lower west coast and two from the south coast, consistent with the
profound break evident in the Leptatherina. One lacustrine assemblage
appeared to represent a distinct lineage and a preliminary maximal rate
estimate (~61.4% lineage-1 MY-1) was calculated based on the minimum
divergence of this assemblage from its nearest conspecifics. Although slower
than the rate calculated for L. wallacei, this was still high for teleost fishes.
Overall, this study indicated that historical environmental factors, especially
those related to Quaternary eustatic changes, have played important roles in
sculpting the phylogeography and evolution of three teleost species from
south-western Australia. Moreover, as these species have differential
dependencies on estuarine environments (is. 'strictly estuarine' vs 'estuarine
& marine') and represented two different taxonomic groups (i.e. Atherinoidei &
Gobioidei), historical environmental factors may have exerted similar
influences on other coastal species in the region.
| Identifer | oai:union.ndltd.org:ADTP/221885 |
| Date | January 2003 |
| Creators | rhoddell@central.murdoch.edu.au, Richard James Hoddell |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Richard James Hoddell |
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