Germline-restricted chromosomes (GRCs) are accessory chromosomes that occur only in germ cells. They are eliminated from somatic cells through programmed DNA elimination during embryo development. GRCs have been observed in several unrelated animal taxa and show peculiar modes of non-Mendelian inheritance and within-individual elimination. Recent cytogenetic and phylogenomic evidence suggests that a GRC is present across the species-rich songbirds, but absent in non-passerine birds, implying that over half of all 10,500 bird species have extensive germline/soma genome differences. Here, we review recent insights gained from genomic, transcriptomic, and cytogenetic approaches with regard to the genetic content, phylogenetic distribution, and inheritance of the songbird GRC. While many questions remain unsolved in terms of GRC inheritance, elimination, and function, we discuss plausible scenarios and future directions for understanding this widespread form of programmed DNA elimination.

Department of Behavioural Ecology and Evolutionary Genetics Max Planck Institute for Ornithology Eberhard Gwinner Straße 82319 Seewiesen Germany

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

Department of Zoology Faculty of Science Charles University Viničná 7 128 44 Prague Czech Republic

School of Biological Sciences University of East Anglia Norwich Research Park Norwich NR4 7TU UK

Siberian Department Russian Academy of Sciences Institute of Cytology and Genetics Prospekt Akademika Lavrent'yeva 10 630090 Novosibirsk Russia

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

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