Association analysis of mitochondrial DNA polymorphisms with oocyte number in pigs

In pigs, correlations between mitochondrial (mt) DNA polymorphisms and economic traits have been widely reported across and within swine breeds. In fecundity studies, the number of oocytes within ovaries was highly correlated with litter size. However, the effect of mitochondrial polymorphisms on porcine oocyte number remained unclear. In this study, 181 porcine ovaries were collected to analyse the relationship between oocyte number and mtDNA polymorphisms. There were considerable differences in oocyte numbers among different ovaries from commercial pig breeds, ranging from 2.7 × 105 to 1.3 × 106. Mitochondrial D-loop sequencing discovered 53 polymorphic sites. Association analysis revealed that 13 variations were associated with the number of oocytes (P < 0.05). A C323T polymorphism showed the largest value between the C and T carriers, which differed at 105 oocytes (P < 0.05). The 53 polymorphic sites generated 45 haplotypes, which clustered into two haplogroups, A and B. Haplogroup A had a higher number of oocytes than Haplogroup B (P < 0.05), whereas Haplotype H6 in Haplogroup A had the highest number of oocytes (~7.5 × 105) of all haplotypes studied (P < 0.05). The results of this study highlight a correlation between mtDNA polymorphisms and oocyte number, and suggest the potential application of mtDNA polymorphism analyses in pig selection and breeding practices.