Local and Temporal Distribution of Different Genotypes of Pond-Dwelling Stentor coeruleus

DNA fingerprints of 721 individuals of the freshwater ciliate Stentor coeruleus revealed a heterogeneous local distribution of four genotypes of this species. Different genotypes dominated in seven ponds located 0.1-400 km apart; none of the detected genotypes were detected in all of the ponds. Identities of the relative genotype frequencies in the pond populations varied from 0.00 to 0.99. These values did not correlate to the geographical distance between ponds. Several comparisons of populations showed very high genetic identity (0.99) associated with the largest geographic distance (400 km). The data suggest no strong isolation of populations, but rather relatively high levels of gene flow among several populations. In one pond, significant temporal changes in the relative genotype frequencies accompanied a decrease in the Stentor abundance during the summer of 1996. For one genotype, the changes were reversed until spring of 1997, in the others they were not. Divergent seasonal selection factors may cause such reversible shifts, but chance effects may underlie non-reversed frequency changes.