Low genetic structure and diversity of Red-billed Tropicbirds in the Mexican Pacific

Understanding genetic structure and gene flow can elucidate the mechanisms of diversification and adaptation in seabirds and help define conservation and management units. From 2012 to 2016, we collected blood and feather samples from 156 Red-billed Tropicbirds (Phaethon aethereus) from seven colonies distributed along the Gulf of California and Mexican tropical Pacific to estimate genetic diversity, genetic structure, and gene flow using microsatellite markers and mitochondrial DNA (mtDNA; control region) sequences. Nuclear and mtDNA data revealed relatively low or null levels of genetic diversity, respectively, possibly the result of a founder effect in the eastern Pacific followed by a subsequent population expansion. Nuclear data revealed significant genetic structure among the colonies, but the differences were not associated with regional grouping (i.e., Gulf of California vs. Tropical Pacific). Greater gene flow was observed from the tropical Pacific toward the Gulf of California, possibly related to shared dispersal patterns during the non-breeding season (individuals traveling north to reach warm currents with abundant prey). With the exception of one colony in the Mexican tropical Pacific, we found no evidence of recent bottleneck events. Nonetheless, the overall reduced genetic diversity suggests a high intrinsic vulnerability and risk of extinction for this species.