Sphagnum troendelagicum is only known from five localities in central Norway. Three populations were analyzed to assess its genetic affinities with other Sphagnum species. Isozyme and cytological data indicate that the species is allopolyploid. Morphological, RAPD, and isozyme data further indicate that it has originated from hybridization between Sphagnum tenellum (sect. Mollusca) and S. balticum (sect. Cuspidata). Sphagnum troendelagicum is morphologically distinct, with unique combinations of character states found in sect. Cuspidata and sect. Mollusca. Sphagnum tenellum is genetically similar to S. balticum, although these species are morphologically divergent. Our data support the inclusion of S. tenellum within sect. Cuspidata. Sphagnum troendelagicum appears to have originated recurrently; thus its restricted distribution cannot be ascribed to a single origin. The progenitor species are widespread in the Northern Hemisphere and often grow sympatrically in boreal areas. Hypotheses that may account for the restricted distribution of S. troendelagicum, including ecological tolerance of the allopolyploid and opportunities for hybridization between the progenitors, are discussed.

Communicating Editor: Kathleen A. Kron