Review of Genetic Studies ofDreissenaspp.

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Abstract
SYNOPSIS. Temperate fresh waters of eastern North America were invaded in the late 1980s by two bivalve species, the zebra mussel (Dreissena polymorpha) and the quagga mussel (D. bugensis). The point of origin of zebra mussels is unknown because they are widespread throughout western and eastern Europe, but the quagga mussel is confined to the Dnieper and Bug rivers in the Ukraine. Both species are highly variable in shell morphology. Zebra mussel color and pattern morphs defy classification, but at least one distinct pattern is found only in the Aral-Caspian region and North American populations. An extremely pale, laterally compressed quagga morph is found in eastern Lake Erie and southern Lake Ontario, but has not been noted in the Ukraine. Allozyme electrophoresis has revealed extraordinarily high levels of genetic variability in the zebra mussel. Average heterozygosities range from 27.0 to 43.5%, compared to 9.7 to 14.5% in the quagga mussel. Two surveys of North American and European populations indicated that little variability was lost when zebra mussels were transported to North America. Therefore, either the invader population was comprised of a large number of individuals, and/or more than one source population contributed to the invasion. No genetic differentiation among populations has occurred in North America in the nine years since the first invasion. Zebra mussels, like many bivalves, demonstrate heterozygote deficiencies at some loci. A weak correlation has been found between individual heterozygosity and shell length. The need for identification of enzymes, loci, and allelic mobilities to allow comparison of genetic data among studies is stressed, as is the need for vouchering specimens. Fruitful areas for future research include elucidation of dreissenid taxonomy and status of putative subspecies, monitoring for the potential invasion of the saline tolerant D. rostriformis grimmi into North American estuarine waters, and ecological and physiological comparison of extreme morphs such as the normal and profundal quagga mussels.

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