Germline-restricted chromosome (GRC) is widespread among songbirds

Abstract

An unusual supernumerary chromosome has been reported for two related avian species, the zebra and Bengalese finches. This large, germline-restricted chromosome (GRC) is eliminated from somatic cells and spermatids and transmitted via oocytes only. Its origin, distribution among avian lineages, and function were mostly unknown so far. Using immunolocalization of key meiotic proteins, we found that GRCs of varying size and genetic content are present in all 16 songbird species investigated and absent from germline genomes of all eight examined bird species from other avian orders. Results of fluorescent in situ hybridization of microdissected GRC probes and their sequencing indicate that GRCs show little homology between songbird species and contain a variety of repetitive elements and unique sequences with paralogs in the somatic genome. Our data suggest that the GRC evolved in the common ancestor of all songbirds and underwent significant changes in the extant descendant lineages.

On the origin and evolution of germline chromosomes in songbirds

“…Even within families, both small and large GRCs occurred. Nevertheless, cross-species fluorescent in situ hybridization analysis showed that the GRCs of closely related species share part of their genetic content (11). These results reveal a highly dynamic nature of the songbird GRC, with various gains and losses of genetic material over relatively brief evolutionary time frames within and between passerine families.…”

“…The finding of taxonomically widespread GRCs in songbirds provides strong support for a monophyletic origin of the GRC at the time when the oscines were formed ∼35 million y ago (12). Whether GRCs occur also in suboscine passerines, a clade of over 1,000 species that is the sister group to songbirds, remains to be determined (11). Interestingly, the GRC was found to vary dramatically in size among the 16 songbird species and appeared both as a micro-and a macrochromosome, and there was no phylogenetic clustering of the GRC by size.…”

“…1A). In PNAS, Torgasheva et al (11) now reveal that avian GRCs are not exclusive to finches but are widely occurring in songbirds, a large clade consisting of over 4,000 species.…”

“…Torgasheva et al (11) use comparative cytogenetics to search for GRCs in 16 species representing 9 families of oscine passerines (Passeri; Passeriformes), i.e., songbirds, and in 8 species representing 7 nonpasserine orders. Their results are striking: A GRC was present in all the songbirds examined, but in none of the nonpasserines (Fig.…”

Comprehensive Chromosome End Remodeling During Programmed DNA Elimination

“…In the first form, chromosome loss, one or many chromosomes are eliminated. This form occurs in a few insects, mites, nematodes (Strongyloides ratti), hagfish, frogs, songbirds and marsupials [2][3][4][5], and it is typically associated with sex determination or species that reproduce by hybridogenesis [2,6,7]. In the second form, chromosomes break with retention of some chromosome fragments and the elimination of others.…”