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. Mitochondrial 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 homogeneous and relatively similar to the European haplotypes, the other of which is more diverse. The maximum North American sequence differentiation translates into a divergence time of approximately 1.5 Myr BP. In contrast, European populations contained only three haplotypes that are all closely related. Microsatellite data reveal higher overall levels of genetic diversity in North America than Europe, although levels of within-population genetic variation are similar on the two continents. In North America, two of the three microsatellite loci show bimodal distributions of allele sizes which are significantly associated between the two loci. As a result, two microsatellite lineages are evident, and these are assortatively distributed between the mitochondrial haplotype groupings. The combined mitochondrial and microsatellite data suggest two distinct genetic lineages in North America that may represent cryptic species. Hybridization between the two presumptive species or subspecies may have contributed to the high levels of genetic diversity in North America. The overall lower levels of genetic diversity in Europe can be attributed to postglacial derivation of extant populations from a single mitochondrial lineage, and conformation to a metapopulation structure.