Invisible in plain sight: recurrent double allopolyploidy in the African Sphagnum Xplanifolium (Sphagnaceae)

Microsatellites, nucleotide sequences, and flow cytometry were used to determine if two sympatric African peat mosses (Sphagnum X planifolium and S. X slooveri) had a history of inter-subgeneric hybridization and to assess their phylogenetic relationship. Both species had previously not been considered to be hybrids. Sphagnum X slooveri was found to be gametophytically allodiploid. Its maternal parent was S. recurvum (or a closely related species) of subg. Cuspidata and its paternal parent was an unidentified species from the S. africanum complex of subg. Subsecunda. Sphagnum X planifolium was found to be a cryptic species complex of gametophytic allotriploids, with recurrent double allopolyploidy resulting in at least two evolutionarily distinct lineages. The maternal parent of both lineages was S. X slooveri. The paternal parent of one lineage (S. xplanifolium I) was an unidentified haploid associated with the S. capense complex of subgenus Subsecunda while the paternal parent of the second lineage (S. Xplanifolium II) was S. cuspidatum (or a closely related species) of subg. Cuspidata. Four species having a history of double allopolyploidy are now documented in bryophytes; all are gametophytically allotriploid, all are in Sphagnum, and all had an allodiploid parent having a history of inter-subgeneric hybridization. It is postulated that a high genetic divergence between subgenomes may facilitate double allopolyploidy in Sphagnum. Genetic analyses reveal that S. pulchricoma, S. recurvum and S. sancto-josephense form a complex of non-hybrid and hybrid plants in the Neotropics, with the hybrids having a history of hybridization between S. cuspidatum and S. recurvum. Reticulate evolution needs more attention in bryophyte studies and this requires experimental designs sufficiently robust to detect it.