New Insights into Mammalian Sex Chromosome Structure and Evolution from High-quality Bovine X and Y Chromosome Sequences

Abstract Background Mammalian X chromosomes are mainly euchromatic with a similar size and structure among species whereas Y chromosomes are smaller, have undergone substantial evolutionary changes and accumulated male specific genes and genes involved in sex determination. The PAR is conserved on the X and Y and pair during meiosis. The structure, evolution and function of mammalian sex chromosomes is still poorly understood because few species have high quality sex chromosome assemblies. Results Here we report the first bovine sex chromosome assemblies that include the complete pseudoautosomal region (PAR) spanning 6.84 Mb and three Y chromosome X-degenerate (X-d) regions. We show that the ruminant PAR comprises 31 genes and is similar to the PAR of pig and dog but extends further than those of human and horse. Differences in the pseudoautosomal boundaries are consistent with evolutionary divergence times. Conclusions A bovidae-specific expansion of members of the lipocalin gene family in the PAR may reflect immune-modulation and anti-inflammatory responses that contribute to parasite resistance in ruminants. Comparison of the X-d regions of Y chromosomes across species reveal five conserved X-Y gametologs, which are global regulators of gene activity, and may have a fundamental role in mammalian sexual dimorphism.