The relationship between mitochondrial DNA haplotype and litter size in commercial pigs

The mitochondrial genome (mtDNA) is associated with a number of traits, which include tolerance to heat (Wallace et al. 2003), growth and physical performance (Nagao et al. 1998), meat and milk quality (Brown et al. 1989; Mannen et al. 2003), and fertility (Sutarno et al. 2002). A key region of mtDNA is the D loop, which is widely used to determine maternal lineages. Maternal lineages cluster into mtDNA haplotypes that have evolved over billions of years (Ruiz-Pesini et al. 2004). In this study, we aimed to determine if: pig fertility is directly related to the sow’s mtDNA haplotype; sows with mtDNA haplotypes favourable to increased litter size produce more developmentally competent oocytes; and their developmentally competent oocytes have higher mtDNA copy number, which according to Spikings et al. (2007) is associated with successful fertilisation.

The D-loop region for 368 sows from four Australian commercial breeders was sequenced to determine their maternal lineages. Litter size was determined for each haplotype. Developmentally competent cumulus-oocyte-complexes (COCs) were selected using the dye, brilliant cresyl blue (BCB), to determine the ratio of developmentally competent (BCB⁺) to incompetent (BCB^–) COCs. Oocyte quality was also assessed by quantifying mtDNA copy number. Developmental potential of BCB⁺ COCs was assessed by in vitro maturation, fertilisation and embryo culture. Statistical differences were determined using ordinary one-way ANOVA followed by parametric multiple comparison post-hoc tests.

In this study, we identified five mtDNA haplotypes (A to E) in the commercial pig breeding population in Australia. Haplotypes C, D and E had significantly larger litter sizes than haplotype A but when live births were assessed only C and E were significantly larger. In addition, fewer sows from haplotype A produced ≥15 piglets per pregnancy than C (P < 0.05), D (P < 0.01) and E (P < 0.05). The ratio of BCB⁺ to BCB^– COCs per ovary was similar for each haplotype. However, mtDNA copy number for BCB⁺ oocytes was higher for haplotype D oocytes than for haplotypes B (P < 0.01) and E (P < 0.001). The proportion of oocytes progressing to metaphase II following in vitro maturation was lower for haplotype C oocytes compared with haplotypes A (P < 0.001), B (P < 0.01) and E (P < 0.05). Following insemination of BCB⁺ oocytes and culture to the blastocyst stage, fewer oocytes from haplotype C fertilised and cleaved than A (P < 0.05), B (P < 0.01) and E (P < 0.01). However, there was no difference (P > 0.05) in blastocyst development rates amongst the haplotypes. Although haplotype C produced proportionally fewer developmentally competent oocytes, the resultant embryos had the same potential to develop to blastocyst as embryos from other haplotypes. This highlights a more pronounced selection process during gametogenesis for haplotype C.

The results demonstrated that haplotypes C and E produced significantly larger litter sizes. However, each haplotype had different rates of oocyte maturation and fertilisation. This suggested that each haplotype has very different mechanisms for generating their respective litter sizes. These findings could lead to a simple genotyping test for the selection of sows with better reproductive capacity, which would enhance economic breeding values.