Migration patterns and hybridization within the Asian stonechat complex in response to a major geographical barrier

Facing geographical barriers, phenotypic responses of migratory landbirds may differ even between closely related taxa: some may bypass barriers while others cross them. Such phenotypic variation can arise from different genetic backgrounds and lead to different adaptive processes. A classic example is migratory divide, i.e., breeding ranges where different migration-route phenotypes co-exist or meet. Such variation can facilitate an understanding of the evolutionary background of migration and responses to barriers. In Asia, a migratory divide has been suggested in response to the major landscape, the Qinghai-Tibet Plateau (QTP) for 2 stonechat taxa (Siberian stonechat Saxicola maurus maurus and Amur stonechat S. stejnegeri). Detouring along either side of the QTP, these taxa are believed to disfavor crossing over the highland. However, the Tibetan stonechat (S. m. przewalskii), breeding on the QTP, may differ in its barrier response due to possible adaptation to high elevation. To investigate migration patterns and potentially associated genetic differences, we studied migration routes and population genetics of populations around the assumed migratory divide in Russia and Mongolia and of a Chinese population from the QTP. Our results confirmed the existence of a migratory divide between maurus and stejnegeri, albeit with extensive hybridization over generations. Of the 3 genotyped individuals employing intermediate routes across the QTP, 2 were clear hybrids. Meanwhile, przewalskii consistently followed a highland-crossing route and was genetically differentiated from maurus and stejnegeri. The diverse migration routes indicate that Asian stonechats respond differently to the geographical barrier. The partly viable intermediate route may be associated with hybridization and potentially facilitating gene flow between maurus and stejnegeri. The Asian stonechat complex thus offers great opportunities for research, with its specific evolutionary background associated with inhabiting and crossing the QTP, offering novel perspectives on the genetics and evolution of migration.