Phylogenetic analyses of nuclear and chloroplast DNA sequences resolve four major clades within the peatmosses, and these lineages correspond to sections that have traditionally been recognized based on morphology. The sectional placement of most species is unambiguous, but four taxa combine the nuclear sequences of one section with the chloroplast sequences of another. A hypothesis of past reticulate evolution is favored over lineage sorting to account for this incongruence, because single genome analyses (nuclear vs. chloroplast) indicate that the species are derived within their respective sections. Sphagnum mendocinum, from the Pacific coast of North America, has the nuclear DNA of section Subsecunda, but the chloroplast DNA of section Cuspidata. Sphagnum cuculliforme, from Ecuador, has the nuclear sequences of section Subsecunda, but the chloroplast sequences of section Sphagnum. Sphagnum falcatulum and S. ehyalinum, both from temperate Southern Hemisphere, have the nuclear sequences of section Cuspidata, but the chloroplast sequences of section Subsecunda. While morphological evidence alone would not be sufficient to hypothesize past reticulations, at least three of the four species are atypical and ambiguous with regard to sectional placement. Sphagnum mendocinum is morphologically intermediate between the Subsecunda and Cuspidata, S. cuculliforme has previously been classified in its own monotypic section because of a unique combination of morphological characters, and S. ehyalinum is highly unusual because of a near absence of differentiated chlorophyllose and hyaline cells in the branch leaves. S. ehyalinum is described as new in this paper. Sphagnum falcatulum is morphologically typical of the section Cuspidata, to which it appears to belong based on nuclear DNA sequences. These inferences of reticulation between widely divergent taxa add to growing evidence of hybridization in the peatmosses.
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Vol. 103 • No. 2