Germline-Restricted Chromosome (GRC) in Diploid and Polyploid Spermatocytes of the Eurasian Bullfinch, Pyrrhula pyrrhula (Fringillidae, Passeriformes, Aves)

The germline-restricted chromosome (GRC) is a unique and enigmatic element found exclusively in the germ cells of passerine birds, with its function and evolutionary dynamics still largely unresolved. This study utilizes cytogenetic analysis of the Eurasian bullfinch (Pyrrhula pyrrhula) to explore the meiotic behavior of the GRC. We report the novel discovery of naturally occurring tetraploid and octoploid spermatocytes in this species. Remarkably, in these polyploid cells, the GRC exhibited normal meiotic processes, including full synapsis and recombination. Recombination was restricted to the H3K9me3-negative proximal half of the GRC bivalent, implicating a chromatin-based regulation mechanism. The standard chromosome set in the polyploid cells showed orderly chromosome synapsis. The number of recombination nodules in tetraploid and octoploid nuclei was approximately equal to the standard value for diploids multiplied by 2 and 4, respectively. These findings suggest that polyploidy does not completely hinder meiotic progression in birds and offer new insights into GRC regulation during meiosis.