Comparative cytogenetic analysis of the germline-restricted chromosome in Fringillidae species (Passeriformes, Aves)

An additional germline-restricted chromosome (GRC) has been found in the germline cells of all studied passerine bird species. It is eliminated from somatic cells during early embryogenesis and from spermatocytes after the first or second division of male meiosis. The GRC is transmitted across generations predominantly via the maternal line. It contains amplified and rearranged copies of genomic regions from the standard chromosome set. Some of these genes are expressed in the gonads of both males and females. However, the function and evolutionary dynamics of the GRC remain unknown. We conducted a comparative cytogenetic analysis of the GRC in five closely related finch species - the Eurasian bullfinch Pyrrhula pyrrhula, the common greenfinch Chloris chloris, the European goldfinch Carduelis carduelis, the common redpoll Acanthis flammea, and the pine grosbeak Pinicola enucleator - using fluorescent in situ hybridization (FISH) with a whole-chromosome DNA probe derived from the bullfinch GRC on spread spermatocytes of these species and immunolocalization of synaptonemal complex (SC) and centromere proteins. We described for the first time the SC karyotype of the pine grosbeak (2n = 82 + GRC). The standard chromosome set consists of nine submetacentric bivalents (seven macro- and two microbivalents) and 32 acrocentric microbivalents. All acrocentric microbivalents contain centromeres composed of multiple centromeric domains (metapolycentromeres). The grosbeak GRC is a large acrocentric macrounivalent. Cross-species in situ hybridization of the bullfinch GRC DNA probe showed only weak signals on the GRC of the grosbeak and redpoll, whereas no signal was detected on the greenfinch and goldfinch GRCs. These data are consistent with published results for two other representatives of this family and indicate rapid divergence and high species specificity of GRC sequences within the family Fringillidae. We also detected interspecies differences in the localization of sequences homologous to the bullfinch GRC on the bivalents of the standard set of these species. Thus, our data indicate rapid evolution of the GRC's genetic composition and reveal species-specific dynamics of increase and decrease in the copy number of detected sequences in the standard chromosome set during the evolution of songbird species.