Population structure and interspecific differentiation of the peat moss sister species Sphagnum rubellum and S-capillifolium (Sphagnaceae) in northern Europe

Isozyme electrophoresis was used to study the morphologically similar sister species Sphagnum rubellum and S. capillifolium from a sample of 1313 plants representing 37 populations from Scandinavia, Great Britain and S Germany. The mean pairwise genetic identities (I) among conspecific populations were 0.976 for S. rubellum and 0.969 for S. capillifolium, versus 0.627 between populations of the two species. Interspecific gene flow was indicated by the observation of occasional plants in sympatric populations with alleles otherwise unique to the other species. Populations of bisexual S. capillifolium were significantly more variable than populations of unisexual S. rubellum. Alpine populations of S. rubellum and S. capillifolium were dominated by few genotypes, and differentiation among populations was pronounced, indicating a low level of sexual recombination. In S. rubellum, maximum variability was found in western areas with high annual precipitation. Distribution of alleles in S. rubellum indicated restricted gene flow between Great Britain and Scandinavia. Postglacial migration from separate refugia may explain large-scale variation in S. rubellum.