Interploidal gene flow and introgression in bryophytes: Sphagnum girgensohnii × S. russowii, a case of spontaneous neotriploidy

Abstract

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 F2 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.