Is interspecific gene flow and speciation in peatmosses (Sphagnum) constrained by phylogenetic relationship and life-history traits?

Peatmosses are interesting for studies of speciation processes not only because of their frequent hybridization and recent diversification, but also their phenotypic diversity, ecological importance and ancient history. Diverse and widespread hybridization has been widely documented in the genus, but little is known about what factors underlie this phenomenon. We hypothesize that these factors include phylogenetic distance and variation in life-history traits of parental species. We summarize current knowledge about the occurrence of hybridization in peatmosses and explore how it is associated with phylogenetic distance and life-history trait variation of parental species. Possibly as much as one out of five (or more) peatmoss species hybridize, mostly producing allopolyploid hybrids. Parents of admixed haploids are more closely related to each other than parents of allopolyploids. Hybridization seems to be most frequent in 1) monoicous and polyoicous species exhibiting 2) relatively high sporulation frequency, 3) producing relatively small spores, as well as 4) growing in poor habitats. Surprisingly, neither phylogenetic proximity nor life-history trait variation explain patterns of hybridization in peatmosses, and other likely explanations for patterns observed are discussed.