Evolutionary history of chloridoid grasses estimated from 122 nuclear loci

Abstract

Chloridoideae (chloridoid grasses) are a subfamily of ca. 1700 species with high diversity in arid habitats. Until now, their evolutionary relationships have primarily been studied with DNA sequences from the chloroplast, a maternally inherited organelle. Next-generation sequencing is able to efficiently recover large numbers of nuclear loci that can then be used to estimate the species phylogeny based upon bi-parentally inherited data. We sought to test our chloroplast-based hypotheses of relationships among chloridoid species with 122 nuclear loci generated through targeted-enrichment next-generation sequencing, sometimes referred to as hyb-seq. We targeted putative single-copy housekeeping genes, as well as genes that have been implicated in traits characteristic of, or particularly labile in, chloridoids: e.g., drought and salt tolerance. We recovered ca. 70% of the targeted loci (122 of 177 loci) in all 47 species sequenced using hyb-seq. We then analyzed the nuclear loci with Bayesian and coalescent methods and the resulting phylogeny resolves relationships between the four chloridoid tribes. Several novel findings with this data were: the sister lineage to Chloridoideae is unresolved; Centropodia+Ellisochloa are excluded from Chloridoideae in phylogenetic estimates using a coalescent model; Sporobolus subtilis is more closely related to Eragrostis than to other species of Sporobolus; and Tragus is more closely related to Chloris and relatives than to a lineage of mainly New World species. Relationships in Cynodonteae in the nuclear phylogeny are quite different from chloroplast estimates, but were not robust to changes in the method of phylogenetic analysis. We tested the data signal with several partition schemes, a concatenation analysis, and tests of alternative hypotheses to assess our confidence in this new, nuclear estimate of evolutionary relationships. Our work provides markers and a framework for additional phylogenetic studies that sample more densely within chloridoid tribes. These results represent progress towards a robust classification of this important subfamily of grasses, as well as proof-of-concept for hyb-seq next-generation sequencing as a method to generate sequences for phylogenetic analyses in grasses and other plant families.

Keywords: Chloridoideae; Grass phylogeny; Hyb-seq targeted sequencing; Phylogenomics; Poaceae.