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.

Centre for Molecular Biodiversity Research Leibniz Institute for the Analysis of Biodiversity Change Adenauerallee 127 53113 Bonn Germany

Department of Organismal Biology Systematic Biology Evolutionary Biology Centre Science for Life Laboratory Uppsala University Norbyvägen 18D 752 36 Uppsala Sweden

Department of Zoology Faculty of Science Charles University Prague Czech Republic

Department of Zoology Faculty of Science Palacky University Olomouc Czech Republic

Institute for Environmental Studies Faculty of Science Charles University Prague Czech Republic

Institute of Evolutionary Biology and Ecology University of Bonn An der Immenburg 1 53121 Bonn Germany

Institute of Molecular Genetics Czech Academy of Sciences Prague Czech Republic

Institute of Vertebrate Biology Czech Academy of Sciences Brno Czech Republic

Military Health Institute Military Medical Agency Tychonova 1 160 01 Prague 6 San Antonio Czech Republic

School of Biological Sciences University of East Anglia Norwich UK

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The genetic mechanism of B chromosome drive in rye illuminated by chromosome-scale assembly

Micro Germline-Restricted Chromosome in Blue Tits: Evidence for Meiotic Functions