Association between mitochondrial DNA copy number and production traits in pigs.

Mitochondria are essential organelles in the regulation of cellular energetic metabolism. Mitochondrial DNA copy number (mtDNA_CN) can be used as a proxy for mitochondria number, size, and activity. The aims of our study are to evaluate the effect of mtDNA_CN and mitochondrial haploblocks on production traits in pigs, and to identify the genetic background of this cellular phenotype. We collected performance data of 234 pigs and extracted DNA from skeletal muscle. Whole-genome sequencing data was used to determine mtDNA_CN. We found positive correlations of muscle mtDNA_CN with backfat thickness at 207 d (+0.14; p-value = 0.07) and negative correlations with carcase loin thickness (-0.14; p-value = 0.03). Pigs with mtDNA_CN values below the lower quartile had greater loin thickness (+4.1 mm; p-value = 0.01) and lower backfat thickness (-1.1 mm; p-value = 0.08), which resulted in greater carcase lean percentage (+2.4%; p-value = 0.04), than pigs with mtDNA_CN values above the upper quartile. These results support the hypothesis that a reduction of mitochondrial activity is associated with greater feed efficiency. Higher mtDNA_CN was also positively correlated with higher meat ultimate pH (+0.19; p-value <0.01) but we did not observe significant difference for meat ultimate pH between the two groups with extreme mtDNA_CN. We found no association of the most frequent mitochondrial haploblocks with mtDNA_CN or the production traits, but several genomic regions that harbour potential candidate genes with functions related to mitochondrial biogenesis and homeostasis were associated with mtDNA_CN. These regions provide new insights into the genetic background of this cellular phenotype but it is still uncertain if such associations translate into noticeable effects on the production traits.

A novel QTL region for pH and meat color in Duroc pigs.

One of the most important processes that occur during the transformation of muscle to meat is the pH decline as a consequence of the post-mortem metabolism of muscle tissue. Abnormal pH declines lead to pork defects such as pale, soft, and exudative meat. There is genetic variance for ultimate pH and the role of some genes on this phenotype is well established. After conducting a genome-wide association study on ultimate pH using 526 purebred Duroc pigs, we identified associated regions on Sus scrofa chromosomes (SSC) 3, 8, and 15. Functional candidate genes in these regions included PRKAG3 and PHKG1. The SSC8 region, at 71.6 Mb, was novel and, although no candidate causative gene could be identified, it may have regulatory effects. Subsequent analysis on 828 pigs from the same population confirmed the impact of the three associated regions on pH and meat color. We detected no interaction between the three regions. Further investigations are necessary to unravel the functional significance of the novel genomic region at SSC8. These variants could be used as markers in marker-assisted selection for improving meat quality.

Identification of a missense variant in the porcine AGPAT gene family associated with intramuscular fat content through whole-genome sequencing.

The 1-acylglycerol-3-phosphate O-acyltransferases (AGPATs) are enzymes that catalyze the conversion of lysophosphatidic acid to phosphatidic acid, which is a precursor of triacylglycerol, the main fat reservoir in mammals. We used whole-genome sequencing of 205 pigs to identify 6639 genetic variants in the porcine AGPAT gene family. Of these, 166 common variants in the AGPAT5 gene had significant associations with fat content and composition traits. We preselected a missense single nucleotide polymorphism in exon 6 of AGPAT5 (rs196952262, A>G) for validation of its associations in 1034 pigs from the same Duroc line. The A allele showed a positive additive effect for intramuscular fat content (+1.12% ± 0.21, p < 0.001, for gluteus medius and +0.89% ± 0.33, p < 0.01, for longissimus). We also observed significant associations with fatty acid composition that were, at least in part, independent of the increased intramuscular fat. The A allele resulted in more monounsaturated fatty acids (+0.34% ± 0.15, p < 0.05, for longissimus) and a greater monounsaturated/polyunsaturated fatty acids ratio (+0.11 ± 0.04, p < 0.01, for gluteus medius and +0.13 ± 0.05, p < 0.05, for longissimus). The effect of the AGPAT5 variant on intramuscular fat was more noticeable in fatter pigs, and AGPAT5 interacts with other genes that affect overall fatness such as LEPR. AGPAT5 was the most expressed gene of the AGPAT family in pig skeletal muscle. This variant can be used as a marker in assisted selection for modulating pig fat deposition and fatty acid content.