Systematics of the Sphagnum recurvum Complex: Morphological Variation, Hybridization and the Delineation of Intermediate Taxa

Abstract

The delineation of closely related plant species is difficult, as finding a discrete and distinct set of characters can be problematic in taxa that exhibit extreme morphological variability. Such difficulties arise in the genus Sphagnum because of its diversity and variability, as well as its propensity to hybridize. This study asks (1) do the five Norwegian morphospecies of the Sphagnum recurvum complex, a group of closely related and taxonomically controversial boreal species, correspond to genetically distinct entities? And (2) are morphologically intermediate plants a result of phenotypic plasticity or interspecific hybridization? Using “next generation” RADseq-based phylogenetic analyses, three highly distinct clades emerged, corresponding to S. angustifolium, S. flexuosum, and a clade containing S. fallax, S. isoviitae and S. brevifolium. This result suggests the boundaries of S. fallax should be expanded. The lack of genetic differentiation among the members of the S. fallax clade and the clear separation of the three clades was supported by DAPC multivariate clustering and a novel analysis comparing the lineages revealed by individual loci. These analyses also tested for genetic admixture within plant samples. However, there were no intermediate samples between the genetic clusters discerned from any of the analyses, and the phylogenetic tree demonstrates strong clade cohesion. This lack of genetic intermediates suggests that the morphological variation observed in these populations is likely to be due to intraspecific genetic variation or phenotypic plasticity. A revised key and diagnoses for Norwegian species in the Sphagnum recurvum complex is provided to facilitate identification by collectors.