Genetic consequences of the metapopulation biology of a facultatively sexual freshwater invertebrate

Theoretical models of the impact of a metapopulation structure on the genet ics of a species have resulted in multiple predictions that have seldom bee n empirically evaluated. Here we present microsatellite data from 14 popula tions of a freshwater bryozoan, Cristatella mucedo, collected along a water fowl migratory route in north-western Europe. C. mucedo is facultatively se xual and has the unusual tactic of dispersing via asexually generated propa gules. These propagules are likely to be dispersed by waterfowl and therefo re the populations that we sampled were expected to maintain some degree of connectivity. Our data illustrate a metapopulation comprising well-differe ntiated populations connected by low levels of ongoing gene flow, patterns that agree with predictions based on theoretical work. However, contrary to expectations of a metapopulation, particularly one in which asexual reprod uction predominates, genetic variation within populations was often high. T his diversity seems to be at least partially attributable to the gene flow that results from ongoing dispersal.