Natalie Baker

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            PhD Abstract

Levels and Patterns of Genetic Diversity in Wild Populations and
Cultured Stocks of Cherax quadricarinatus (redclaw crayfish)

This study investigated phylogenetic relationships among C. quadricarinatus
representing 17 discrete natural drainages across the natural range in
Australia and PNG, using 16s and COI gene sequences. Sequence
analysis of both genes resolved two distinct genealogical lineages in
Australia and three in PNG. The two divergent Australian lineages
concur with original taxonomic descriptions of Reik (1969) based on
external morphological differences. The three C. quadricarinatus
populations sampled in PNG were all genetically distinct from each
other, with one exhibiting a close association with an Australia lineage.
The immense physical barriers (rugged mountain ranges) to gene flow
in PNG will almost certainly have reduced dispersal capabilities for
C. quadricarinatus. During times of lowered sea levels in the past,
Australia and southern PNG were a single landmass with terrestrial
and freshwater organisms theoretically able to disperse over associated
land and via freshwater connections. The close genetic relationship
between PNG and Australian C. quadricarinatus support a recent
freshwater connection and hence gene flow between northern Australia
and PNG C. quadricarinatus populations.

Genetic differentiation among certain C. quadricarinatus lineages
exhibit as much genetic divergence at 16s RNA sequences as
taxonomically recognised sub-species in the Cherax genus. It is
therefore recommended that the taxonomy of C. quadricarinatus
in Australia and PNG should be reconsidered.

C. quadricarinatus specific microsatellite markers were developed
for this study. Five variable loci were employed to investigate the
extent of contemporary gene flow among fourteen C. quadricarinatus
wild river populations in northern Australia. High FST and genetic
distance estimates observed among pair wise comparisons of
C. quadricarinatus populations are consistent with limited or no
gene flow occurring among drainages. Speculation that C. quadricarinatus
may disperse between adjacent or nearby drainages at times of
flood, either across floodplains, or via flood plumes therefore seems
highly unlikely among the populations examined in the current study.
No significant correlation was observed between geographic distance
and genetic distance among C. quadricarinatus populations here.
C. quadricarinatus populations most closely resemble an island-like
model, where gene flow is independent of geographic distance among
populations and where genetic divergence occurs to a greater or lesser
extent as a result of genetic drift within otherwise isolated populations.

The growing economic potential of C. quadricarinatus culture, both
domestically and internationally, prompted expanding the current
study to examine genetic diversity levels in commercial C. quadricarinatus
stocks. The study employed five microsatellite markers to quantify
genetic diversity in four Australian and three C. quadricarinatus culture
stocks from overseas. Many C. quadricarinatus culture stocks also
showed deviations from HWE expectations. This was not a surprising
result given that the wild populations also deviated and domestication
can also influence HWE. Relatively high levels of genetic diversity were
observed. This probably results from intentional mixing of discrete river
strains for production of the first commercial stock. Genetic differentiation
estimates among culture stocks and assignment tests indicated that
overseas culture stocks are most likely derived from a the first commercial
culture stock developed in Australia and then disseminated widely
(the Hutchings stock). Robin Hutchings was a known supplier of live
C. quadricarinatus to many international culture initiatives. Assignment
of culture stocks back to their wild origins indicated that all C. quadricarinatus
culture stocks sampled possess alleles that originate from the Flinders River
(proportions ranged from 33-94%).

Domestication of C. quadricarinatus to date has not resulted in significant
reductions in levels of genetic diversity (heterozygosity or alleles richness)
when compared to wild populations sampled in this study. Comparing
culture stocks to wild populations to gauge their ‘genetic health’ may
not be a suitable scale for evaluating genetic diversity in culture stocks.
Wild populations are essentially evolving independently, are subjected to
harsh seasonal environmental fluctuations resulting in periodic population
crashes (genetic bottlenecks), with little or no recruitment from
neighbouring drainages (gene flow). This study does however indicate
that there is a large amount of genetic diversity distributed among wild
populations that has yet to be exploited in culture. Genetic diversity in
wild populations provides a resource for future stock improvement
programs for C. quadricarinatus culture and thus requires careful
conservation and appropriate management.