Extensive Genome-Wide Phylogenetic Discordance Is Due to Incomplete Lineage Sorting and Not Ongoing Introgression in a Rapidly Radiated Bryophyte Genus

TLDR

The role of introgression in diversification remains debated, and distinguishing it from incomplete lineage sorting is difficult because gene flow leaves genomic signatures similar to ILS and requires accurate phylogenies. We used whole nuclear, plastid, and organellar genomes from 12 Sphagnum species to reconstruct the species phylogeny and quantify introgression with a suite of phylogenomic methods. Extensive phylogenetic discordance across nuclear and organellar data is best explained by rapid radiation and ILS rather than post‑speciation introgression; ancient introgression among ancestral lineages is supported, but recent gene flow is limited despite widespread hybridization, illustrating how diverse phylogenomic methods can disentangle complex patterns of ILS and introgression.

Abstract

The relative importance of introgression for diversification has long been a highly disputed topic in speciation research and remains an open question despite the great attention it has received over the past decade. Gene flow leaves traces in the genome similar to those created by incomplete lineage sorting (ILS), and identification and quantification of gene flow in the presence of ILS is challenging and requires knowledge about the true phylogenetic relationship among the species. We use whole nuclear, plastid, and organellar genomes from 12 species in the rapidly radiated, ecologically diverse, actively hybridizing genus of peatmoss (Sphagnum) to reconstruct the species phylogeny and quantify introgression using a suite of phylogenomic methods. We found extensive phylogenetic discordance among nuclear and organellar phylogenies, as well as across the nuclear genome and the nodes in the species tree, best explained by extensive ILS following the rapid radiation of the genus rather than by postspeciation introgression. Our analyses support the idea of ancient introgression among the ancestral lineages followed by ILS, whereas recent gene flow among the species is highly restricted despite widespread interspecific hybridization known in the group. Our results contribute to phylogenomic understanding of how speciation proceeds in rapidly radiated, actively hybridizing species groups, and demonstrate that employing a combination of diverse phylogenomic methods can facilitate untangling complex phylogenetic patterns created by ILS and introgression.