GENETIC DIFFERENTIATION IN NORTH-AMERICAN AND EUROPEAN POPULATIONS OFTHE CLADOCERAN BYTHOTREPHES

Bythotrephes cederstroemi, a Palearctic cladoceran (Crustacea) zooplan kter, 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 int rapopulation genetic variation, with 1-12 multiple-locus genotypes per population. Genotype frequencies were often different from Hardy-Wein berg expectations for North American populations, primarily due to het erozygote excesses. Interpopulation variation was low within drainage basins but high among basins. Great Lakes populations were most simila r to Finnish populations, while Swedish and German populations formed a separate group. North American populations appear to have been found ed from a single European population, probably located in the northeas tern Baltic region. Sympatric populations of B. cederstroemi and B. lo ngimanus showed a greater degree of similarity than allopatric conspec ifics, leading to the conclusion that these may represent morphologica lly 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 amon g populations of Bythotrephes. Genetic variation among populations wit hin drainage basins was lower than across basins, regardless of geogra phic distance between sites. Thus, river basins act as pathways of dis persal, leading to significant migration between populations within a basin but little migration between basins.