Potential merger of ancient lineages in a passerine bird discovered based on evidence from host‐specific ectoparasites

The merger of formerly isolated lineages is hypothesized to occur in vertebrates under certain conditions. However, despite many demonstrated instances of introgression between taxa in secondary contact, examples of lineage mergers are rare. Preliminary mt DNA sequencing of a Malagasy passerine, Xanthomixis zosterops (Passeriformes: Bernieridae), indicated a possible instance of merging lineages. We tested the hypothesis that X . zosterops lineages are merging by comparing mt DNA sequence and microsatellite data, as well as mt DNA sequence data from host‐specific feather lice in the genus Myrsidea (Phthiraptera: Menoponidae). Xanthomixis zosterops comprises four deeply divergent, broadly sympatric, cryptic mt DNA clades that likely began diverging approximately 3.6 million years ago. Despite this level of divergence, the microsatellite data indicate that the X . zosterops mt DNA clades are virtually panmictic. Three major phylogroups of Myrsidea were found, supporting previous allopatry of the X . zosterops clades. In combination, the datasets from X . zosterops and its Myrsidea document a potential merger of previously allopatric lineages that likely date to the Pliocene. This represents the first report of sympatric apparent hybridization among more than two terrestrial vertebrate lineages. Further, the mt DNA phylogeographic pattern of X . zosterops , namely the syntopy of more than two deeply divergent cryptic clades, appears to be a novel scenario among vertebrates. We highlight the value of gathering multiple types of data in phylogeographic studies to contribute to the study of vertebrate speciation.