Genetic changes within freshwater bryozoan populations suggest temporal gene flow from statoblast banks

Temporal changes in the genetic structure of populations of the freshwater bryozoan Cristatella mucedo were investigated by genotyping colonies at five microsatellite loci. Colonies were collected from five different locations over time periods ranging from 1 month to 5 yr. Our findings identified substantial within-site genetic changes, both within and between years, that were not consistent with variation produced by sexual reproduction. There was a general trend toward a decrease in genetic diversity throughout the growing season, possibly due to clonal selection. In addition, 18 out of 21 within-site comparisons showed a significant difference in the allele frequencies at one or more loci. Two of the populations revealed identical clones in >1 yr. At one site, an apparent population crash occurred in 1993; nevertheless, numerous clones were present in 1994 and 1995, including two multilocus genotypes that were present at the same site in 1992. Since the populations from before and after the bottleneck remained genetically similar, the most plausible source of these clones is reintroduction from a statoblast bank rather than through introductions from other genetically distinct populations. These findings, in conjunction with earlier studies, indicate that propagule banks influence the population dynamics of a wide range of freshwater invertebrates in a number of ways, including the enhancement of within-population genetic diversity and a decreased likelihood of local extinctions.