Diversity of Mitochondrial DNA Haplogroups and Their Association with Bovine Antral Follicle Count

Simple Summary Chinese Holstein cows, crossbred cattle in China, have abundant genetic diversity due to the fact that Holstein cows were crossbred with various local breeds. Therefore, to reveal their genetic diversity, we sequenced the complete mitochondrial DNA (mtDNA) D-loop region of 501 Chinese Holstein cows and found abundant genetic resources (including 219 haplotypes, polymorphisms of 260 SNPs, and 32 indels) and two haplogroups in the study population. Meanwhile, the two haplogroups had different maternal origins and corresponding distinguished mutations. Furthermore, the association between haplogroups and antral follicle count was found, and the mtDNA haplogroup HG1 with Bos indicus maternal origin had more antral follicles (diameter >= 8 mm), implying that these individuals of HG1 had higher reproductive potential. Finally, these distinguished mutations between two haplogroups could be considered as important genetic markers for cattle breeding. Maternal origins based on the bovine mitochondrial D-loop region are proven to have two main origins: Bos taurus and Bos indicus. To examine the association between the maternal origins of bovine and reproductive traits, the complete mitochondrial D-loop region sequences from 501 Chinese Holstein cows and 94 individuals of other breeds were analyzed. Based on the results obtained from the haplotype analysis, 260 SNPs (single nucleotide polymorphism), 32 indels (insertion/deletion), and 219 haplotypes were identified. Moreover, the nucleotide diversity (pi) and haplotype diversity (Hd) were 0.024 +/- 0.001 and 0.9794 +/- 0.003, respectively, indicating the abundance of genetic resources in Chinese Holstein cows. The results of the median-joining network analysis showed two haplogroups (HG, including HG1 and HG2) that diverged in genetic distance. Furthermore, the two haplogroups were significantly (p < 0.05) correlated with the antral follicle (diameter >= 8 mm) count, and HG1 individuals had more antral follicles than HG2 individuals, suggesting that these different genetic variants between HG1 and HG2 correlate with reproductive traits. The construction of a neighbor-joining phylogenetic tree and principal component analysis also revealed two main clades (HG1 and HG2) with different maternal origins: Bos indicus and Bos taurus, respectively. Therefore, HG1 originating from the maternal ancestors of Bos indicus may have a greater reproductive performance, and potential genetic variants discovered may promote the breeding process in the cattle industry.