Nineteen populations of Sphagnum affine were included in a study of genetic diversity and structure in fragmented and less fragmented landscapes, and differentiation at intercontinental and three regional levels. Isozyme electrophoresis of eight enzyme systems revealed 12 variable loci, which could be used for haplotype identification. A hierachical analysis of variance (AMOVA) revealed no significant intercontinental differentiation, and very limited differentiation among European regions. A trend of decreasing diversity with increasing latitude was apparent. Gametic phase disequilibria was high, suggesting nonrandom mating and regionally high incidences of inbreeding. The partitioning of genetic variation within and among populations in each region varied among regions, the northernmost populations having 86% of the total variation among populations, the southernmost in Scandinavia having 25% of the variation among populations, whereas the American populations displayed 89% of the variation within populations. Fifteen alleles at eight loci occurred in the U.S.A. which were not encountered in Europe, whereas only three European alleles at one locus in three populations were not encountered in U.S.A. The differences in diversity between North America and Europe may result from loss of genetic diversity caused by founder effects during postglacial recolonization of northern Europe. In Europe, the main mountain ranges extend E-W, posing severe barriers to northwards migration of lowland species, compared to the N-S trend of mountain ranges in North America. The decline in genetic diversity and increase in population differentiation and gametic phase disequilibria towards the north in Scandinavia may be caused by a series of founder effects during postglacial migration. These may have corresponded to minor climatic oscillations that influenced the migration front/leading edge in the suboceanic lowlands of Norway. According to this model random genetic drift will be an increasingly important structuring factor with latitude.

Hybrid zones between female sporophyte-bearing haploid Sphagnum girgensohnii and male allodiploid S. russowii were studied. The existence of hybrids was initially hypothesized based on incidence of conspicuously large sporophytes in S. girgensohnii in the presence of male S. russowii. Measurements of nuclear DNA content, morphology and isozymes provide the first evidence of spontaneous hybridization across ploidal levels in bryophytes. Viable spores from the hybrid capsules yielded triploid protonemata and juvenile gametophores in culture. Isozyme profiles of the triploid gametophores displayed unbalanced heterozygous patterns containing two alleles. As S. russowii is an allodiploid species with S. girgensohnii and S. rubellum as progenitors, isozyme banding patterns of the triploids are interpreted as consisting of a single allele copy from the S. rubellum genome in S. russowii, and two copies of another allele at the same locus inherited from S. girgensohnii. The triploid sporophytes can be considered as F 2 hybrids formed by introgression. The allotriploid gametophores are formed through diplospory or syndiploidy in the hybrid capsules. Comparison of hybrid sporophyte morphology to S. girgensohnii and S. russowii confirmed that hybrid capsules are larger and with an abnormally swollen sporophyte foot. The spore sizes of the hybrid capsules were significantly larger than spores from normal, intraspecific S. girgensohnii capsules. The percentage of germinated spores was ,5% in cultures from hybrid sporophytes, which is much lower than in intraspecific sporophytes. The new knowledge opens the way for re-evaluation of polyploid evolution in bryophytes with emphasis on gametic non-reduction, introgression and two-step models of polyploid formation.

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