Genetic variation in a freshwater bryozoan: II: The effect of increasing spatial scale

In order to clarify the relationship between genetic differentiation of the bryozoan Cristatella mucedo and geographic distance in the British Isles, we characterised populations of this species in Scotland, Ireland, and Devon and from one site in North America by analysis of RAPD markers and mtDNA sequences. We then compared the genetic relationships between populations collected from these varying spatial scales with C. mucedo from the Thames Valley region of England. Three independent analyses provided evidence that genetic differentiation amongst C. mucedo populations increases at increased spatial scales across the British Isles. This is the first time that a relationship between genetic and geographic distance has been demonstrated in C. mucedo populations. The presence or absence of such a relationship appears to be explained by the likelihood of dispersal via migratory or local waterfowl. Analysis of mtDNA sequences contribute to recent evidence that populations of C. mucedo in Europe and the British Isles have remained separate from those in North America during prolonged periods of ice cover.