Tandem Duplications in the C-terminal Domain of the Mesotocin Receptor Exclusively Identified Among East Eurasian Thrushes

Mesotocin is a neurohypophyseal hormone found in some non-mammalian vertebrates, including birds, reptiles, and amphibians. In this study, we identified and characterized 18-amino acid duplications in the C-terminal domain of the mesotocin receptor (MTR), specifically found in Turdus thrushes (Aves: Passeriforms: Turdidae). These duplicated elements are located in the distal part of the C-terminal tails of MTR and consist of amino acids that are highly conserved among major vertebrates. Intraspecific polymorphisms in a variable number of tandem duplications are commonly found in East Eurasian Turdus, but not in any other genus of Turdidae. Moreover, the genus Turdus can be further classified into 2 groups according to the presence or absence of a 3-amino acid deletion just adjacent to the putative palmitoylation site in the cytoplasmic C-terminal tail. The phylogeny presented here strongly supports the conspecific group of 4 East Eurasian thrushes (Turdus pallidus, T. chrysolaus, T. obscurus, and T. celaenops). Our findings, therefore, provide a new synapomorphy that can be used for phylogenetic assumptions and shed a light on the history of diversification within Eurasian Turdus clades.