Genetic differentiation in North American and European populations of the cladoceran Bythotrephes

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Bythotrephes cederstroemi, a Palearctic cladoceran (Crustacea) zooplankter, has recently invaded all of the Laurentian Great Lakes. We used cellulose acetate gel electrophoresis to examine the genetic structure of these newly founded populations and to compare it to the structure of established European populations of B. cederstroemi and a congener, Bythotrephes longimanus. All populations of Bythotrephes had low intrapopulation genetic variation, with 1–12 multiple‐locus genotypes per population. Genotype frequencies were often different from Hardy‐Weinberg expectations for North American populations, primarily due to heterozygote excesses. Interpopulation variation was low within drainage basins but high among basins. Great Lakes populations were most similar to Finnish populations, while Swedish and German populations formed a separate group. North American populations appear to have been founded from a single European population, probably located in the northeastern Baltic region. Sympatric populations of B. cederstroemi and B. longimanus showed a greater degree of similarity than allopatric conspecifics, leading to the conclusion that these may represent morphologically distinct forms of a single species. Our results indicate that, as with other cladoceran species, restricted gene flow and founder events may be important in determining patterns of geographic variation among populations of Bythotrephes. Genetic variation among populations within drainage basins was lower than across basins, regardless of geographic distance between sites. Thus, river basins act as pathways of dispersal, leading to significant migration between populations within a basin but little migration between basins.