Multigene Phylogeny of Epichloe Species, Fungal Symbionts of Grasses

Epichloe species are fungal symbionts (endophytes) of grasses, many of which are benign or mutualistic and have a balance of horizontal (contagious) and vertical (seed-borne) transmission, whereas others mainly transmit horizontally and are more antagonistic. Over the past eight years several Epichloe species have been described based largely on the biological species concept. We conducted a multi-gene phylogenetic analysis to evaluate these endophytes as phylogenetic species, and thereby assess the relationship of phylogenetic and biological species. Variation mainly in introns of genes encoding β-tubulin (tub2), translation elongation factor 1-α (tef1), and actin (act1) provided robust phylogenetic signal distinguishing the described Epichloe species. Outgroup rooting split the genus into two major groups. One group included most species with balanced transmission strategy, and in this group the phylogenetic and biological species concepts corresponded well. In contrast, these species concepts poorly corresponded for the other group, the Epichloe typhina complex, with predominantly antagonistic, horizontally transmitted endophytes. We suggest that the balance of vertical and horizontal transmission may promote ecological (host) specialization and subsequent genetic isolation as mechanisms promoting speciation; whereas strict horizontal transmission may select for broader host ranges, slow the development of genetically isolated species, and thereby increase lineage sorting effects that cause conflicts between phylogenetic and biological species.