Discovery of Candidate Genes for Stallion Fertility from the Horse Y Chromosome

The genetic component of mammalian male fertility is complex and involves thousands of genes. The majority of these genes are distributed on autosomes and the X chromosome, while a small number are located on the Y chromosome. Human and mouse studies demonstrate that the most critical Y-linked male fertility genes are present in multiple copies, show testis-specific expression and are different between species. In the equine industry, where stallions are selected according to pedigrees and athletic abilities but not for reproductive performance, reduced fertility of many breeding stallions is a recognized problem. Therefore, the aim of the present research was to acquire comprehensive information about the organization of the horse Y chromosome (ECAY), identify Y-linked genes and investigate potential candidate genes regulating stallion fertility. To achieve theses goals, a direct cDNA (complementary DNA) selection procedure was used to isolate Y-linked genes from horse testes and 29 Y-specific genes were identified. All 29 genes were mapped to ECAY and their sequences were used to further expand the existing map. Copy number analysis identified 15 multicopy genes of which 9 were novel transcripts. Gene expression analysis on a panel of selected body tissues showed that some ECAY genes are expressed exclusively in testes while others show ubiquitous or intermediate expression. Quantitative Real-Time PCR using primers for 9 testis-specific multicopy genes revealed 5 genes with statistically significant differential expression in testis of normal fertile stallions and stallions with impaired fertility. Gene copy number analysis showed that the average copy number of 4 such genes was decreased in subfertile/infertile stallions compared to normal animals. Taken together, this research generated the first comprehensive physical gene map for the horse Y chromosome and identified a number of candidate genes for stallion fertility. The findings essentially expand our knowledge about Y chromosome genes in horses, open a new avenue for investigating the potential role of ECAY genes in stallion fertility which contribute to the development of molecular tools for the assessment of fertility in stallions.