The identification of taxa and discernment of evolutionary relationships within the family Dreissenidae have been confounded by morphological plasticity as well as prior lack of a comprehensive DNA sequence data analysis. We thus analyzed the phylogenetic relationships of putative taxa (species and subspecies) in the genus Dreissena in relation to its nearest living relatives(Mytilopsisleucophaeata and Congeria kusceri) using DNA sequence data from the nuclear 28S RNA gene and threemitochondrial genes: cytochrome c oxidase subunit I (COI), 16S RNA, and cytochrome (cyt) b oxidase. Relationships resolved by maximum likelihood and Bayesian phylogenetic trees are robust and congruent and support division of Dreissena into three subgenera: Dreissena, Pontodreissena, and Carinodreissena. The subgenus Pontodreissena contains two species: Dreissena caputlacus and Dreissena rostriformis. Putative subspecies once proposed for D. rostriformis lack genetic divergence and likely should no longer be recognized; these include D. r. “bugensis” (the quagga mussel), D. r. “grimmi, ” D. r. “distincta,” and D. r. “compressa.” The Pontodreissenathen comprisesthe sistergroup(nearestrelative) to a clade comprising the other two subgenera (Dreissena and Carinodreissena). The subgenus Carinodreissena contains the valid taxa Dreissena carinata and Dreissena blanci; both inhabit ancient lakes in the Balkan Peninsula. We consider the once recognized Dreissena “stankovici” and Dreissena “presbensis” to be synonyms of Dreissena carinata; DNA and morphological evidence supports this conclusion. The subgenus Dreissena includes two species, Dreissena polymorpha and Dreissena anatolica. D. anatolica is endemic to Turkey in lakes north of the Mediterranean, and D. polymorpha (the zebra mussel) has been widely introduced throughout much of Eurasia and North America, spreading from its native distribution in the Pontocaspian region.

We additionally analyze population genetic variation for invasive and native populations of the zebra mussel D. polymorpha (using 11 nuclear DNA microsatellite loci) and the quagga mussel D. r. “bugensis” (using 9 microsatellite loci) across North America and Eurasia and compare our results with previous studies that used other markers. Results reveal significant genetic structuring of introduced populations from Eurasia and North America for both species. North American invasions of both species were founded from multiple source populations and a large number of propagules, showing no founder effects and substantial genetic diversity. In contrast, recently colonized quagga mussel populations from the Colorado River and California exhibit some founder effects. Genetic compositions of both species have changed over time at given colonization sites, with some populations adding alleles from adjacent populations, some losing them, and most retaining closest similarity to their original composition. In conclusion, these genetic data comprise a valuable baseline for resolving present and future invasion pathways for dreissenids, as well as interpreting patterns of distributions in their native ecosystems.