A tremendous expansion of copy number in crossbred bulls ( × ).

Crossbreeding between cattle () and yak () exhibits significant hybrid advantages in milk yield and meat production. By contrast, cattle-yak F hybrid bulls are sterile. Copy number variations (CNV) of multicopy gene families in male-specific regions of the mammalian Y chromosome (MSY) affect human and animal fertility. The present study investigated CNV of (), (), (), and () in 5 yak breed bulls ( = 63), cattle-yak F ( = 22) and F ( = 2) hybrid bulls, and Chinese Yellow (CY) cattle bulls ( = 10) by quantitative real-time PCR. showed restricted amplification in yak bulls in that the average geometric mean copy number (CN) was estimated to be 4 copies. The most compelling finding is that there is a tremendous expansion of CN in F hybrids (385 copies; 95% confidence interval [CI] = 351-421) and F hybrids (356 copies) compared with the male parent breed CY cattle (142 copies; 95% CI = 95-211). Copy numbers of and were also extensively expanded on the Y chromosome in yak and CY cattle bulls. The geometric mean CN of and were estimated to be 123 (95% CI = 114-132) and 250 copies (95% CI = 233-268) in yak bulls and 71 (95% CI = 61-82) and 133 (95% CI = 107-164) copies in CY cattle, respectively. Yak and CY cattle have 2 copies of the gene on the Y chromosome. Similarly to gene, the F and F hybrid bulls have higher CN of , , and than CY cattle ( < 0.01). These results indicated that the MSY of yak and cattle-yak crossbred hybrids was fundamentally different from cattle MSY in the context of genomic organization. Based on the model of cattle-yak F and F hybrid bull sterility, the CNV of may serve as a potential risk factor for crossbred bull ( × ) infertility. To our knowledge, this is the first study to examine differences in multicopy genes in MSY between yak and cattle-yak bulls.