Gene flow and genetic diversity: a comparison of freshwater bryozoan populations in Europe and North America

We have used microsatellite and mitochondrial sequence data to gain insight into patterns of gene flow and genetic diversity among North American and European populations of the freshwater bryozoan Cristatella mucedo. Mitocho ndrial sequence data reveal numerous, widely distributed, divergent genetic lineages in North America that can be broadly categorized into two groups, one of which is genetically homogenous and relatively similar to the Europ ean haplotypes, the other of which is more diverse. The maximum North Ameri can sequence differentiation translates into a divergence time of approxima tely 1.5 Myr sp. In contrast, European populations contained only three hap lotypes that are all closely related. Microsatellite data reveal higher ove rall levels of genetic diversity in North America than Europe, although lev els of within-population genetic variation are similar on the two continent s. In North America. two of the three microsatellite loci show bimodal dist ributions of allele sizes which are significantly associated between the tw o loci. As a result, two microsatellite lineages are evident, and these are assortatively distributed between the mitochondrial haplotype groupings. T he combined mitochondrial and microsatellite data suggest two distinct gene tic lineages in North America that may represent cryptic species. Hybridiza tion between the two presumptive species or subspecies may have contributed to the high levels of genetic diversity in North America. The overall lowe r levels of genetic diversity in Europe can be attributed to postglacial de rivation of extant populations from a single mitochondrial lineage, and con formation to a metapopulation structure.