Summary. The mammalian sex chromosomes evolved from an ordinary pair of autosomes during evolution. Unlike the X chromosome that is highly conserved, the Y chromosome is poorly conserved among mammalian lineages. Several special features set the Y chromosome apart from the rest of genome: male-limited transmission, absence of recombination, abundance of Y-specific repetitive sequences, degeneration of Y-linked genes during evolution, acquisition of autosomal genes, and accumulation and functional cluster of "testis genes" for maleness and reproduction. Since the degeneration process is lineage-dependent, different lineages retain different subsets of genes from the ancestral proto-Y chromosome, resulting in a diverse and lineage-specific Y chromosome gene content. During bovine evolution, a lineage-specific 'autosome-to-Y' transposition event resulted in three bovid-specific Y chromosome gene families, PRAMEY, ZNF280BY and ZNF280AY. Together, the male-specific region (MSY) of the bovine Y chromosome (BTAY) contains ~1200 protein coding genes that can be classified into 12 single copy and 16 multiple copy protein families. The copy number (CN) of these Y-linked gene families varies from 13 for PRAMEY to 236 for ZNF280BY, with significant differences between the taurine and indicine Y lineages. In addition, 367 non-coding RNA families (ncRNAs) were also identified on BTAY. Transcriptome analysis revealed that 95% of the BTAY genes/ncRNAs are expressed predominantly in testis and may be involved in spermatogenesis and male fertility. Though the functional role for the majority of the Y-linked genes needs to be determined, the preliminary data on PRAMEY clearly indicated a role in spermiogenesis. Furthermore, copy number variations (CNVs) of PRAMEY, ZNF280BY, TSPY and HSFY were found to be associated with testis size, sperm quality and fertility in dairy bulls. The authors discuss several challenges that influence male fertility selection associated with the bovine Y chromosome.
© 2014 Society for Reproduction and Fertility