Genetic Variation and Evolutionary Relationships Within a Group of Thirteen Species of Penaeid Prawns

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Abstract
Six Metapenaeus spp. [M. macleayi, M. bennettae, M. endeavouri, M. ensis, M. insolitus and M. eboracensis] and 7 Penaeus spp. [P. semisulcatus, P. monodon, P. esculentus, P. merguiensis, P. plebejus, P. latisulcatus and P. longistylus] common in Australian waters were studied at 37 genetic loci coding for soluble proteins and enzymes. The measure of genetic similarity (I) between the 2 genera averages 0.39 .+-. 0.08, and that between pairs of congeneric species has a mean of 0.69 .+-. 0.08 for Metapenaeus and 0.65 .+-. 0.08 for Penaeus. Closely related species, such as the 3 spp. of tiger prawns and the 3 spp. of king prawns, differed in at least 1/5 of their genes at the protein loci surveyed. All species had an extremely low average level of heterozygosity, ranging from 0.6-3.3%, with an average of only 14% of loci polymorphic. Since 3 spp. are wholly marine, it appears extremely unlikely that all species were subjected to bottlenecks in breeding population size sufficient to account for the low level of heterozygosity. The selective elimination of mutational variation at the majority of genetic loci may be the most likely explanation, requiring selection coefficients of approximately 100 times the rates of mutation. Successful and widespread marine invertebrate species occupying markedly heterogeneous physical and trophic environments may be characterized by extremely low levels of genetic variation.