Rapid gene content turnover on the germline-restricted chromosome in songbirds

Songbirds have an extra chromosome with unknown function found only in their germline. This study assembles and compares this chromosome in two closely related nightingale species, finding large differences in genetic content and only one conserved gene with probable essential function. The germline-restricted chromosome (GRC) of songbirds represents a taxonomically widespread example of programmed DNA elimination. Despite its apparent indispensability, we still know very little about the GRC's genetic composition, function, and evolutionary significance. Here we assemble the GRC in two closely related species, the common and thrush nightingale. In total we identify 192 genes across the two GRCs, with many of them present in multiple copies. Interestingly, the GRC appears to be under little selective pressure, with the genetic content differing dramatically between the two species and many GRC genes appearing to be pseudogenized fragments. Only one gene, cpeb1, has a complete coding region in all examined individuals of the two species and shows no copy number variation. The acquisition of this gene by the GRC corresponds with the earliest estimates of the GRC origin, making it a good candidate for the functional indispensability of the GRC in songbirds.

Automated summary

The germline-restricted chromosome (GRC) in songbirds is an extra chromosome with unknown function found only in their germline. This study examines and compares the GRC in two closely related nightingale species, revealing significant genetic differences and only one conserved gene with probable essential function. Despite its apparent indispensability, the genetic composition, function, and evolutionary significance of the GRC are still poorly understood.