Elizabeth Ballment

Genetic mapping of the black tiger shrimp Penaeus monodon with amplified fragment length polymorphism

Abstract We report construction of an initial genetic linkage map for the black tiger shrimp, Penaeus monodon. Mapping was carried out using polymorphic markers derived from 23 Amplified Fragment Length Polymorphism (AFLP) primer pairs. These were analysed on three reference families of known pedigree. A total of 673 polymorphic AFLP loci that conformed to expected Mendelian segregation ratios were scored in three families, and these were used to construct separate male and female linkage maps for each family. AFLP markers that consisted of a segregating fragment of the same size, amplified with the same primer pair in two or more of the reference families, were considered to be common markers. 116 such common AFLP markers were used to construct a common linkage map across the three families. This linkage map has 20 linkage groups covering a total genetic distance of 1412 cM. Future directions for genetic mapping in P. monodon are discussed in light of these initial data.

Mitochondrial DNA variation in Indo-Pacific populations of the giant tiger prawn, Penaeus monodon

Surveys of mitochondrial DNA (mtDNA) variation in the giant tiger prawn, Penaeus monodon, using restriction fragment length polymorphisms have provided the first clear evidence that the Indo‐West Pacific region is a site of accumulation of genetic diversity rather than a site of origin of genetic diversity. No haplotyes were found in common between a group of five southeast African populations and a group of five Australian (including Western Australia) and three southeast Asian populations. The dominant haplotype was different in the Australian and southeast Asian population groups. Genetic diversity (π) was greatest in Indonesia (π averaged 0.05), less in the Philippines and Australia (π averaged 0.01), and markedly less in the southeast African and the West Australian populations (π averaged 0.003). The high diversity of the southeast Asian populations resulted from the occurrence in those populations of a set of haplotypes found only in southeast Asia but derived from the southeast African haplotypes. These genetic variants therefore evolved in the Indian Ocean and later migrated into the Indo‐West Pacific region. Low genetic variation in the geographically marginal populations in southeast Africa and Western Australia is considered to be the result of bottlenecks, but mismatch distributions suggest that large population sizes have been maintained in Indonesian populations for long periods.

Genetic structure of Penaeus monodon in Australia: concordant results from mtDNA and allozymes

Abstract Preliminary analyses of mtDNA restriction fragment length polymorphisms confirmed results, first revealed by allozyme surveys, that Australian P. monodon from the west coast differ genetically from east coast populations. The levels of variation in the mtDNA were high relative to the allozymes. These data suggest mtDNA analyses will prove useful in further describing P. monodon population structure, for example, in testing whether the distribution patterns of rare allozyme variants reflect differentiation on smaller geographical scales, or are the result of some stochastic effect.

Population genetics of the fissiparous holothurian Stichopus chloronotus (Aspidochirotida) on the Great Barrier Reef, Australia

Abstract Population genetic structure was studied in one nearshore and two offshore populations of Stichopus chloronotus, a common holothurian species on Indo-Pacific coral reefs. Genetic variation at five polymorphic loci was examined using allozyme electrophoresis. The nearshore population consisted almost exclusively of male individuals, and more males than females were found in all populations studied. Deviations of heterozygosity from that predicted under Hardy-Weinberg equilibrium indicated that asexual reproduction occurred in all populations. Estimates of the level of asexual reproduction using the ratios of the number of sexually produced individuals to sample size, observed genotypic diversity to expected genotypic diversity, and number of genotypes to sample size confirmed that this reproductive mode was more important at the nearshore reef compared to the two offshore reefs. There were large differences in genotypic frequencies between males and females. F-statistics on clonal genotypic frequencies were not statistically significant between populations for neither females or males, suggesting high dispersal of larvae between reefs. A higher mortality of females during larval or early post-settlement stages, or reduced dispersal capability of female larvae are the most likely reasons for biased sex ratios.

Interspecific hybridization of the tiger prawns Penaeus monodon and Penaeus esculentus

Interspecies hybrids were produced by artificial insemination of Penaeus monodon females with P. esculentus males. Successful spawnings were achieved from two of the 13 matings attempted. Hatch rates were low (< 4%) but 85% of the vigorous larvae produced survived to PL1. The hybrid identity of the larvae was confirmed by allozyme analysis at five diagnostic loci. Tank-reared juveniles grew at an overall rate of approximately 0.09 g day−1 to a weight of 10 g in tanks until the prawns were stressed at 4.5 months. A number of viral diseases were expressed subsequently, including IHHNV, and the experiment was terminated. There was no indication of hybrid vigour in growth rates, and the high growth rate of P. monodon was not combined simply with the colour patterns of P. esculentus. A variety of body colour and pattern variants, ranging between those of P. monodon and P. esculentus, was observed in the hybrids.

POPULATION GENETICS AND TAXONOMY OF CAULERPA (CHLOROPHYTA) FROM THE GREAT BARRIER REEF, AUSTRALIA1

Allozyme variation was examined in seven species and four varieties of Caulerpa sampled from the Great Barrier Reef (GBR) region, Australia. Differences between species were greater than those between populations of the same taxon sampled from different geographical locations, and typically included fixed gene differences (no alleles found in one taxon shared by the other taxon) at two or more loci. Three varieties of C. racemosa, vars imbricata, laetevirens, and racemosa, and a peltate morph of C. racemosa were as strongly distinguished genetically as the six other species examined: C. cupressoides, C. lentillifera, C. peltata, C. serrulata, C. sertularioides, and C. taxifolia. Cluster analysis did not place all of the C. racemosa varieties together and linked them with other species, but cladistic analyses showed the allozyme data gave little useful phylogenetic information. Eight of 13 plants identified initially as C. serrulata were distinguished at several loci, indicating the presence of an undetermined cryptic taxon. Population genetic analysis of polymorphism, which occurred in some taxa, demonstrated strong spatial differentiation among populations of C. cupressoides, C. racemosa vars laetevirens and racemosa, C. serrulata, and C. taxifolia and significant but variable degrees of clonality and/or inbreeding within these populations. Allozymes proved to be a useful tool for defining species boundaries and investigating population structure in Caulerpa, but not for determining phylogenetic relationships within the genus.

Growth of Penaeus monodon×Penaeus esculentus tiger prawn hybrids relative to the parental species

Abstract Interspecies hybrids were produced by artificial insemination of Penaeus monodon females with P. esculentus males. Successful spawnings (those in which some eggs hatched) were achieved from nine of the 17 P. monodon × P. esculentus matings attempted (53%). Mean egg numbers produced ranged from 158,000 to 438,000 but hatch rates were low ( −1 ), was the same as those of pure P. monodon (0.047 g day −1 ) and significantly greater than that of P. esculentus larvae (0.033 g day −1 ). The relative growth rate among parental species and the hybrids was determined accurately by rearing progeny of pure and hybrid matings in parallel under controlled conditions. The colour pattern of the hybrids was intermediate between that of P. monodon and P. esculentus . Sex ratio was significantly skewed in favour of males in the hybrids (proportion of males was 0.86 compared with 0.56 in the parental species), possibly suggesting females are the heterogametic sex in penaeids. The hybrids had the fast growth rate of P. monodon , and some of the attractive colour pattern of P. esculentus .

Genetic variation in the green alga Caulerpa taxifolia

Abstract Allozyme surveys of 12 populations of Caulerpa taxifolia collected from the central Great Barrier Reef (GBR) in north-east Australia, Moreton Bay in east Australia, and the western Mediterranean, including populations with fine and robust morphologies, revealed little overall genetic differentiation. Exceptions to the general rule were detected, nonetheless, among the allozymes of oxaloacetate-decarboxylating and non-oxaloacetate-decarboxylating malate dehydrogenase (ME and MDH, respectively). Variation in the allozymes of ME, in particular, indicated that populations with robust form in eastern Australia may be genetically more similar to populations with similar morphology in the Mediterranean than to populations with fine morphology in the central GBR. All Mediterranean populations, and the robust form from Australia were monomorphic for allele b, while the fine Australian populations had a number of different alleles, including allele b, but most commonly alleles c, d, g, and i. Comparative surveys of one eastern Mediterranean and two western Atlantic populations of Caulerpa mexicana demonstrated clear genetic differentiation from C. taxifolia, with fixed gene differences at several loci. There was little differentiation among populations of C. mexicana. These results suggest that allozyme markers could assist species identification of Caulerpa species on broad geographical scales, but also raises the question of whether there is high gene exchange today throughout the range of Caulerpa species.

Genetic differences among black-lipped pearl oyster (Pinctada margaritifera) populations in the western Pacific☆

Abstract Seven populations of the black-lipped pearl oyster, Pinctada margaritifera , from the western Pacific were assayed for allozyme variation at 17 polymorphic loci from mantle biopsies. Levels of within population variation were high (mean number of alleles per locus averaged 6.8, mean direct-count heterozygosities averaged 0.575). Gene frequencies within populations conformed to those expected under random mating. Significant genetic differentiation between populations within reef groups and additionally between the Great Barrier Reef (GBR) and the Pacific islands (Kiribati and the Cook Islands) was revealed by hierarchical F ST analysis. The average numbers of migrants per generation (N e m) among the Pacific islands and among the GBR populations was 6–8 and twice that between the Pacific islands and the GBR. This suggests a relatively high gene flow among populations in Kiribati and the Cook Islands on an evolutionary time scale. However, the occurrence of significant differences in gene frequencies among populations within reef groups suggests that genetic surveys of local populations should be undertaken prior to making any transfers of pearl oysters from other reefs in order to protect these genetic resources.