Expression of Rad51 and the histo-morphological evaluation of testis of the sterile male cattle-yak.

Meiotic recombination is key to the repair of DNA double-strand break damage, provide a link between homologs for proper chromosome segregation as well as ensure genetic diversity in organisms. Defects in recombination often lead to sterility. The ubiquitously expressed Rad51 and the meiosis-specific DMC1 are two closely related recombinases that catalyze the key strand invasion and exchange step of meiotic recombination. This study cloned and sequenced the coding region of cattle-yak Rad51 and determined its mRNA and protein expression levels, evaluated its molecular and evolutionary relationship as well as evaluated the histo-morphological structure of testes in the yellow cattle, yak and the sterile cattle-yak hybrid. The Rad51 gene was amplified using PCR, cloned and sequenced using testicular cDNA from yak and cattle-yak. Real-time PCR was used to examine the expression levels of Rad51/DMC1 mRNA in the cattle, yak and cattle-yak testis while western blotting, immunofluorescence and immunohistochemistry were used to assess the protein expression and localization of Rad51/DMC1 protein in the testicular tissue sections. The results revealed that the mRNA and protein expression of Rad51 and DMC1 are extremely low in the male cattle-yak testis with a corresponding higher incidence of germ cell apoptosis. There was also thinning of the germinal epithelium possibly due to the depletion of the germ cells leading to the widening of the lumen area of the cattle-yak seminiferous tubule. Our findings provide support for the hypothesis that the low expression of Rad51 and DMC1 may contribute to the male hybrid sterility in the cattle-yak.