Phylogenomics reveals extreme gene tree discordance in a lineage of dominant trees: hybridization, introgression, and incomplete lineage sorting blur deep evolutionary relationships despite clear species groupings in Eucalyptus subgenus Eudesmia

Abstract

Eucalypts are a large and ecologically important group of plants on the Australian continent, and understanding their evolution is important in understanding evolution of the unique Australian flora. Previous phylogenies using plastome DNA, nuclear-ribosomal DNA, or random genome-wide SNPs, have been confounded by limited genetic sampling or by idiosyncratic biological features of the eucalypts, including widespread plastome introgression. Here we present phylogenetic analyses of Eucalyptus subgenus Eudesmia (22 species from western, northern, central and eastern Australia), in the first study to apply a target-capture sequencing approach using custom, eucalypt-specific baits (of 568 genes) to a lineage of Eucalyptus. Multiple accessions of all species were included, and target-capture data were supplemented by separate analyses of plastome genes (average of 63 genes per sample). Analyses revealed a complex evolutionary history likely shaped by incomplete lineage sorting and hybridization. Gene tree discordance generally increased with phylogenetic depth. Species, or groups of species, toward the tips of the tree are mostly supported, and three major clades are identified, but the branching order of these clades cannot be confirmed with confidence. Multiple approaches to filtering the nuclear dataset, by removing genes or samples, could not reduce gene tree conflict or resolve these relationships. Despite inherent complexities in eucalypt evolution, the custom bait kit devised for this research will be a powerful tool for investigating the evolutionary history of eucalypts more broadly.

Keywords: Discordance; Eucalyptus; Hybridization; Incomplete lineage sorting; Phylogeny; Target capture sequencing.