Genetic diversity of North American populations of Cristatella mucedo, inferred from microsatellite and mitochondrial DNA

Research over the past 20 years has shown, with the help of molecular marke rs, that the population genetics and distribution patterns of freshwater in vertebrates in North America are often more complex than was previously bel ieved. Were we extend this research to an, as yet, unstudied but widespread and common group, the freshwater bryozoans. Colonies of the bryozoan Crist atella mucedo were collected from a number of lakes across central North Am erica, and were characterized genetically by analysis of microsatellite loc i and mitochondrial DNA (mtDNA) cytochrome b sequences. The microsatellites illustrate a pattern of generally diverse and highly differentiated popula tions that contain little evidence of recent gene flow. The mtDNA sequences yielded highly variable levels of divergence, ranging from 0.0 to 8.8% wit hin populations, and 0.0 to 9.8% among populations. The multiple divergent mtDNA lineages within populations provide evidence for repeated colonizatio n events. The lack of clustering of haplotypes by site suggests that there has been widespread dispersal of multiple genetic lineages since the last i ce age. While some of the haplotype lineages may have evolved in disjunct g lacial refugia, the maximum levels of divergence predate the time since the last glacial-interglacial cycles. It is likely that multiple factors inclu ding vicariance events, patterns of dispersal, localized extinction, and an unusual life history, explain the unique phylogeographic patterns evident today in populations of C. mucedo.