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.
Highlights
An unusual supernumerary chromosome has been reported for two related avian species, the zebra and Bengalese finches
Using antibodies to the core proteinaceous structure of meiotic chromosomes, the synaptonemal complex (SC), we found that germline-restricted chromosome (GRC) are present in all 16 songbird species examined (14 in this study and two in the previous studies)
The macro-GRCs were usually present as a bivalent, containing one or two terminally located recombination sites visualized by antibodies to MLH1, a mismatch repair protein
Summary
An unusual supernumerary chromosome has been reported for two related avian species, the zebra and Bengalese finches. Recent studies revealed that the zebra finch GRC contains multiple copies of genes paralogous to the genes from the somatic genome [7, 8]. Itoh et al [9] found that the zebra finch GRC contains sequences homologous to an interval of chromosome 3 as well as repetitive elements absent from the sequenced somatic genome.
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