MICROEVOLUTION AND GENETIC DIFFERENTIATION IN THE BRINE SHRIMP Artemia, 2001

PhD Thesis by I. Kappas, School of Biological Sciences, University of Wales, Swansea, UK, 339 pp.

Summary:

Intraspecific genetic differentiation between populations of Artemia franciscana from San Francisco Bay (SFB) and Vietnam (feral) as well as evolutionary divergence between Asian Artemia species have been investigated using allozymes, reproductive characters and mitochondrial DNA.

Allozyme analysis (20 loci) of A. franciscana, SFB and derived (over a period of one to seven years) Vietnamese populations reveals considerable genetic differences between samples, cultured at combinations of two salinities (80ppt, 120 ppt) and three temperatures (26°C, 30°C, 34°C). Combined probability values (pooled over all polymorphic loci) of tests for allele frequency homogeneity between all salinity/temperature combinations range from < 10^-4 to 0.033. In addition, cluster analysis applied to all levels of comparison (within and between temperatures and salinities) shows evidence of differentiation of Vietnamese Artemia from the wild (SFB) stock as early as within a year after inoculation.

Rapid divergence between SFB and Vietnamese Artemia is also evident from an analysis of reproductive (fecundity) traits. Vietnamese populations seem to be better adapted to increased temperature, as gauged by the significantly higher reproductive output they display compared to San Francisco Bay Artemia. Correlations between individual heterozygosity and fitness also confirm, albeit in a few cases, the existence of a strong genetic component involved in the determination of reproductive profiles.

Similar patterns of genetic differentiation to those obtained by allozymes and reproductive traits are revealed by mitochondrial DNA RFLP markers. However, unlike allozymes, there is strong evidence of a reduction in haplotype diversity from 40.6% in SFB Artemia to 10.5% in the established Vietnamese strain. Although genetic drift could account for much of the depletion in gene diversity, firm indications are obtained of selective forces shaping the gene pool in the tropical conditions of Vietnam.

Finally, a recently described species (A. tibetiana) is allozymically characterised for the first time and compared with A. franciscana, SFB and A. sinica. A. tibetiana exhibits high levels of mean expected heterozygosity (0.138) and proportion of polymorphic loci (45%) compared with other bisexual species. Genetic distance between A. tibetiana and A. franciscana is 0.731 and that between the former and A. sinica is 0.116. The specific status of Artemia from Tibet is evaluated in conjunction with other evidence and suggestions are made for a more thorough examination of patterns of evolutionary divergence in the genus.

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