Gene Trees Reveal Repeated Instances of Mitochondrial DNA Introgression in Orangethroat Darters (Percidae: Etheostoma)

Phylogenies of closely related animal species are often inferred using mitochondrial DNA (mtDNA) gene sequences. The accuracy of mtDNA gene trees is compromised through hybridization that leads to introgression of mitochondrial genomes. Using DNA sequences from 6 single-copy nuclear genes and 2 regions of the mitochondrial genome, we investigated the temporal and geographic signature of mitochondrial and nuclear introgression in the Etheostoma spectabile darter clade. Phylogenetic analyses of the nuclear genes result in the monophyly of the E. spectabile clade; however, with respect to sampled specimens of 5 species (Etheostoma fragi, Etheostoma uniporum, Etheostoma pulchellum, Etheostoma burri, and E. spectabile), the mitochondrial phylogeny is inconsistent with E. spectabile clade monophyly. Etheostoma uniporum and E. fragi are both fixed for heterospecific mitochondrial genomes. Limited nuclear introgression is restricted to E. uniporum. Our analyses show that the pattern of introgression is consistently asymmetric, with movement of heterospecific mitochondrial haplotypes and nuclear alleles into E. spectabile clade species; introgressive hybridization spans broad temporal scales; and introgression is restricted to species and populations in the Ozarks. The introgressed mitochondrial genome observed in E. fragi has an obscure phylogenetic placement among darters, an ancient age, and is possibly a mitochondrial fossil from an Etheostoma species that has subsequently gone extinct. These results indicate that introgression, both ancient and more contemporaneous, characterizes the history of diversification in the E. spectabile species clade and may be relatively common among clades comprising the species-rich North American freshwater fauna.