Sequencing of α-SNAP in select passerines. bird metagenome

Developmentally programmed genome rearrangements are rare in vertebrates but have been reported in scattered lineages including the bandicoot, hagfish, lamprey, and zebra finch (Taeniopygia guttata). In the finch, a well-studied animal model for neuroendocrinology and vocal learning, one such programmed genome rearrangement involves a Germline-Restricted Chromosome, or GRC, which is found in germlines of both sexes but eliminated from mature sperm. Transmitted only through the oocyte, it displays uniparental female-driven inheritance, and early in embryonic development it is apparently eliminated from all somatic tissue in both sexes. The GRC comprises the longest finch chromosome at over 120 million basepairs and previously, the only known GRC-derived sequence was repetitive and non-coding. Because the zebra finch genome project was sourced from male muscle (somatic) tissue the remaining genomic sequence and protein-coding content of the GRC remain unknown. Here we report the first protein-coding gene from the GRC: a member of the α-Soluble NSF Attachment Protein (α-SNAP) family hitherto missing from zebra finch gene annotations. In addition to the GRC-encoded α-SNAP, we find an additional paralogous α-SNAP residing in the somatic genome (a somatolog)—making zebra finch the first example in which α-SNAP is not a single-copy gene. We show divergent, sex-biased expression for the paralogs and also that positive selection is detectable in the bird α-SNAP lineage, including the two zebra finch genes. This study presents the identification and evolutionary characterization of the first protein-coding GRC gene in any organism.