Germline-Restricted Chromosome (GRC) is Widespread among Songbirds

The genome of flying birds, the smallest among amniotes, reflects overweight of the extensive DNA loss over the unrestricted proliferation of selfish genetic elements, resulted in a shortage of repeated sequences and lack of B-chromosomes. The only exception of this rule has been described in zebra finch, which possesses a large germ-line restricted chromosome (GRC), transmitted via oocytes, eliminated from male postmeiotic cells and absent in somatic cell. It is considered as a rarity and its origin, content and function remain unclear. We discovered that all songbirds possess GRC: in various size and genetic content it is present in all fifteen songbird species investigated and absent from germ-line genomes of all eight species of other bird orders examined. Our data based on fluorescent in situ hybridization of DNA probes derived from GRCs of four different Passeri species and their sequencing indicate that the GRCs show low homology between avian species. They contain fragments of the somatic genomes, which include various unique and repetitive sequences. We propose that the GRC has formed in the common ancestor of the extant songbirds and undergone subsequent divergence. GRC presence in the germ line of every songbird studied indicate that it could contain genetic element(s) indispensable for gametogenesis, which are yet to be discovered.