Background: The male-specific region of the mouse Y chromosome long arm (MSYq) contains three known highly multi-copy X-Y homologous gene families, Ssty1/2, Sly and Asty. Deletions on MSYq lead to teratozoospermia and subfertility or infertility, with a sex ratio skew in the offspring of subfertile MSYqdel males
Results: We report the highly unusual genomic structure of a novel MSYq locus, Orly, and a diverse set of spermatid-specific transcripts arising from copies of this locus. Orly is composed of partial copies of Ssty1, Asty and Sly arranged in sequence. The Ssty1- and Sly-derived segments are in antisense orientation relative to each other, leading to bi-directional transcription of Orly. Genome search and phylogenetic tree analysis is used to determine the order of events in mouse Yq evolution. We find that Orly is the most recent gene to arise on Yq, and that subsequently there was massive expansion in copy number of all Yq-linked genes.
Conclusion: Orly has an unprecedented chimeric structure, and generates both "forward" (Orly) and "reverse" (Orlyos) transcripts arising from the promoters at each end of the locus. The region of overlap of known Orly and Orlyos transcripts is homologous to Sly intron 2. We propose that Orly may be involved in an intragenomic conflict between mouse X and Y chromosomes, and that this process underlies the massive expansion in copy number of the genes on MSYq and their X homologues.