J. Chaplin

Are assemblages of black bream (Acanthopagrus butcheri) in different estuaries genetically distinct

Samples of the estuarine-spawning teleost Acanthopagrus butcheri were collected from nine estuaries and a coastal lake, located in the Pilbara and South-western drainage divisions of Western Australia and distributed along a coastline covering a distance of nearly 2,000 km. The patterns of allozyme variation in these samples were used to explore the extent to which there was variation in the genetic compositions of black bream assemblages in geographically-isolated estuarine systems, and whether or not any such variation could be related to the geographical location or type of estuary. Although only three of 36 scorable loci (Gpi-1, Ldh and Mdh-2) exhibited variation that could be used for analysis, there was considerable variation in allele frequencies at these loci among the different samples (mean FST = 0.166). Much of the detected variation was attributable to differences between the samples collected from the two drainage divisions, which are located in very different climatic regions. Furthermore, the genetic compositions of samples from neighbouring estuaries were typically more similar to each other than to those of samples collected from more distantly-located systems. However, the assemblages in one west coast and two south coast estuaries, that are closed to the ocean for extensive periods of time during the year, all showed very similar genetic compositions. Nevertheless, it is crucial to recognise that, pairwise comparisons of samples collected from the different estuaries, both within and between the two drainage divisions, almost invariably showed statistically significant differences in allele frequencies at one or more loci. Thus, our results indicate that the local populations of black bream in individual estuaries are genetically distinct, which is probably a consequence of both a limited movement by individuals between estuaries and the effects of differences in regional and local environmental conditions.

Cyprinotus incongruens (Ostracoda): an ancient asexual?

Although unisexual ostracods are common, their evolutionary history is now known only from inferences gained through examination of the fossil record. Here we use mtDNA, allozyme and genome size analyses to investigate the origins of unisexuality, polyploidy and clonal diversity in the freshwater ostracod Cyprinotus incongruens. Our genetic evidence suggests that transitions to polyploidy have been common in this taxon and may sometimes involve internal genome‐fusion events, in contrast to the usual origin of animal polyploids through interspecific hybridization. Both the extent and congruent patterns of allozyme and mtDNA divergence amongst clones of C. incongruens are consistent with its persistence as an asexual taxon for approximately 5 million years. However, the detected patterns of genetic variation might also reflect the origins of this ostracod through a series of independent transitions to asexuality by several closely related ancestral taxa. The results make it clear that efforts to demonstrate the antiquity of asexual taxa, solely from surveys of the extent of genetic divergence among their component lineages, will ordinarily be ambiguous, requiring confirmation through investigations which search the genome for the genetic consequences of long abandoned recombination.

Biannual spawning periods and resultant divergent patterns of growth in the estuarine goby Pseudogobius olorum: temperature-induced?

Samples of juveniles and adults of the goby Pseudogobius olorum were collected from seven sites in the shallows of the upper Swan Estuary, Western Australia, using a 3 mm-mesh seine net on one or two occasions in each month between September 1983 and April 1985. The mean gonadosomatic index of female fish rose from very low values in winter (June–August) to a sharp peak in mid-spring (October), reflecting the rapid maturation of ovaries over, this period. Ovaries with post-ovulatory follicles and ovaries that were undergoing degeneration were present, in November and December, but were then either rare or absent in those members of the corresponding cohort which survived into January and February. Female fish with advanced oocytes and mature ovaries were not found in December and January, but were present in February to April. The above trends exhibited by ovarian maturity indices, together with the appearance of larvae and small fish in both spring and autumn, demonstrate that P. olorum spawns in both spring and autumn and at best to only a limited extent in summer. Length-frequency and gonadal data show that the progeny of the spring-spawning group frequently spawn in the following autumn, when they are ∼ 5 mo old, and that those of the autumn-spawning group frequently spawn in the following spring, when they are ∼ 7 mo old. Some representatives of these two spawning groups survive through the winter and summer, respectively, to breed in a second season. Growth of the progeny of the spring-spawning group was relatively rapid between late spring and mid-autumn, whereas that of the autumn-spawning group was negligible during winter, but then inceased markedly in spring. It is proposed that the biannual spawning periods in each year by P. olorum in the Swan Estuary developed as a result of a rise in water temperature over the last few thousand years. Such a rise would have brought forward further into spring and extended later into autumn the periods when the water temperatures lie within the range (20 to 25°C) at which P. olorum typically spawns. However, mid-summer is now characterised by water temperatures >25°C, which are considered less conducive to reproductive success.

Biological and Genetic Characteristics of Restocked and Wild Acanthopagrus butcheri (Sparidae) in a Southwestern Australian Estuary

Acanthopagrus butcheri was restocked in an estuary in which it had become depleted. The restocked fish were cultured in 2001 and 2002 using broodstock from that estuary. These fish, whose otoliths had been stained with alizarin complexone, were released into the estuary and their biological performance tracked for seven to eight years. The 2002 cohort, introduced at circa four months old in autumn, survived far better than the 2001 cohort, introduced at circa seven months old in winter, when freshwater discharge peaks and temperatures are low. While restocked fish matured and grew nearly as fast as wild fish, the increase in density was accompanied by a reduced growth of wild fish. Genetic comparisons, using seven microsatellite loci, demonstrated that the expected heterozygosity and relatedness of restocked and wild A. butcheri, which is naturally characterized by low levels of genetic polymorphism, were similar. Although culturing did not demonstrably increase the level of inbreeding, it did result in the loss of some rare alleles. The biological and genetic results, together with the contribution of restocked A. butcheri to the commercial catch for this species in the estuary rising to 62–74% by 2007–2010, demonstrates the efficacy of using restocking to replenish depleted A. butcheri stocks.

Population genetic structures of three congeneric species of coastal pelagic fishes (Arripis: Arripidae) with extensive larval, post-settlement and adult movements

The population genetic structures of three congeneric coastal pelagic marine fishes (Arripis trutta, A. truttaceus and A. georgianus) were investigated to determine whether these structures were consistent with the apparently high gene flow life histories of these species. This investigation used fragment length polymorphisms at two to four nDNA intron loci (amplified by EPIC-PCR) in samples of each species collected from across their entire Australian distributions. The results revealed no evidence of genetic subdivision in any species based on an analysis of variation either across samples (with multilocus FST values ranging from effectively zero to 0.005), or between pairs of samples. These findings, when considered in combination with other available evidence, are consistent with the view that each of these species represents a rare example of a coastal fish species that is genetically homogeneous over a vast area, including the entire geographic range (A. truttaceus and A. georgianus) or Australian distribution (A. trutta).

Contrasting population structures of three Pristis sawfishes with different patterns of habitat use

This research demonstrates how population structure differs in elasmobranchs with different patterns of habitat use. Population structure was assessed using data at microsatellite loci in three species of Pristis sawfishes in northern Australian waters. Statistically significant population structure was found in each of P. clavata (FST = 0.021, F′ST = 0.151, P < 0.001) and P. zijsron (FST = 0.026, F′ST = 0.130, P < 0.001), which spend their entire life in marine waters. In contrast, there was no evidence of significant population structure in P. pristis, which uses freshwater rivers as juveniles and marine waters as adults (FST = 0.004, F′ST = 0.029, P = 0.210). When combined with the results of mtDNA analyses from a previous study, the results suggested that dispersal in P. pristis is male-biased, whereas both male and female gene flow are restricted at large spatial scales in each of P. clavata and P. zijsron in Australian waters. The present study has provided the first evidence of sex-biased dispersal in a sawfish.

Development and characterisation of novel microsatellite loci for the baldchin groper (Choerodon rubescens) and cross-amplification in seven other labrid species

We describe the development and characterisation of novel microsatellite loci for the baldchin groper, Choerodon rubescens. The purpose was to identify loci that can be used to resolve uncertainties about the population (stock) structure of this fish species, which is endemic to a narrow region of the west coast of Australia and showing evidence of overfishing in some locations. Of 22 loci characterised, 12 appear to be ideally suited for population-level analyses. Utilising data obtained from four sampling locations across the distribution of C. rubescens, the total number of alleles observed at each of the 12 loci ranged from 2 to 24, while the overall values of expected heterozygosity ranged from 0.19 to 0.89. Cross-amplification of the 12 loci in seven other labrid species was often successful, especially in congeners.

Isolation and characterisation of novel microsatellite markers from Pagrus auratus and cross amplification in Acanthopagrus butcheri

We describe the isolation and characterisation of microsatellite loci for Pagrus auratus and cross amplification in another sparid, Acanthopagrus butcheri. Of 35 loci tested, 12 appear to be appropriate for population genetic studies of P. auratus, which are essential to inform management responses to evidence of overexploitation of this species. Three were ideal for A. butcheri and, when combined with published loci, can be used to assess the genetic implications of restocking depleted populations of this species.