Origin, taxonomy and population structure of the allopolyploid peat moss Sphagnum majus

The polyploid peat moss Sphagnum majus shows considerable phenotypic plasticity along ecological gradients in mires. It is considered taxonomically heterogeneous, and two subspecies have been described. Isozyme analyses were carried out on populations of S. majus from Central Norway and from eastern coast of North America in order to assess the origin, taxonomy and population structure of this species. High levels of fixed heterozygosity in the populations demonstrate that S. majus is a genetic allopolyploid. At all 'loci' screened, extant populations of S. cuspidatum shared enzyme bands with S. majus. The other most likely progenitor based on morphology, S, annulatum, was fixed for enzyme bands not found in S. majus. The progenitor genotype of S. annulatum? may have been missed because of inadequate sampling or extinction. Alternatively, another extinct or undetected taxon may constitute the second progenitor. The observed patterns of genetic variation and linkage disequilibria were uncorrelated with the previously proposed subspecific classification of S. majus. Lack of genetic divergence between continents suggests that the origins of S. majus in Europe and North America were not independent. Low mutation rates and large effective population sizes may be important causing populations to diverge slowly, and may explain the observed patterns without hypothesising frequent long-distance dispersal.