Climate niches structure a regional hybrid zone in Sphagnum (peatmoss, Bryophyta)

Premise: Hybridization is an important evolutionary process across all groups of embryophyte land plants, but relatively little is known about hybridization and introgression in plants with a dominant gametophyte life cycle stage. This paper focuses on hybridization between four closely related species of the moss genus Sphagnum. Methods: Analyses utilized three types of molecular data: restriction-site-associated DNA sequencing (RADseq), RADseq-like data derived from in silico digestion of genome sequences, and species-specific barcode markers developed previously for this group. Sampling included 582 gametophytes from 79 collecting sites from 27° to 56°N. A range of analytical methods were employed: phylogeny reconstruction, genetic analyses using the program structure, demographic modeling, and comparative genomics. Results: Gene flow was detected among all pairwise combinations of extant species and between ancestral lineages and those species. Hybridization between S. diabolicum and S. magniae was especially pronounced and plants in a regional zone from North Carolina to New Jersey were genetically admixed. Demographic analyses indicated that this admixture reflects hybridization. Introgressed SNPs were detected across all chromosomes, but introgressed SNPs fixed in genetically pure samples of the two species were concentrated on four autosomes: 2, 7, 14, and 19. Patterns of genomic admixture/introgression were significantly correlated with climate variation across collection sites within the hybrid zone. Conclusions: The genomic structure of plants in a regional hybrid zone between S. magniae and S. diabolicum was structured by climate adaptation and strengthens the value of this group for learning more about speciation and climate adaptation.