Genomic and phylogenetic structure of the Sphagnum imbricatum complex (Sphagnaceae) and related allopolyploids

Sphagnum imbricatum sensu lato (Sphagnum, subgenus Sphagnum) is sometimes considered a single widespread and polymorphic species, or up to four separate species. This study was conducted to provide a phylogenetic delimitation of the S. imbricatum complex, assess species delimitation within the complex, and evaluate likely parentage of five allopolyploid species in subgenus Sphagnum that may be related to haploid species in the complex.

RADSeq data were assembled from 192 samples of subgenus Sphagnum plus three outgroup taxa from related Sphagnum subgenera. Parentage of allopolyploids was assessed using STRUCTURE and differentially fixed SNPs among haploid taxa and their distribution within and among the allopolyploids.

The S. imbricatum complex was circumscribed to include S. affine, S. austinii, S. imbricatum, S. portoricense and S. steerei. The traditionally recognized species, S. affine, was resolved to include two morphologically similar clades that are phylogenetically divergent (non-sister groups) within the complex. All the northern allopolyploids were derived from crosses between species in the S. imbricatum and S. magellanicum complexes.

The two clades formerly known as S. affine s.l. will need to be recognized as separate species pending in-progress morphological study. Our results suggest testable hypotheses about specific haploid taxa identified as likely parents of the allopolyploid species. Newly resolved phylogenetic relationships among species in the S. imbricatum complex indicate that the tropical species S. portoricense is nested within a clade otherwise distributed in arctic to temperate regions, empowering this group for research about warm climate adaptation utilizing naturally occurring variation.