Results
Calpastatin
Tenderness is an important quality attribute that pork consumers desire (Moeller, 1999). However, currently there are few direct selection tools that can be utilized to improve this important trait in livestock.
Calpain, a calcium-dependent protease, has been reported to be a contributor to the postmortem tenderization of skeletal muscle (for review see Koohmaraie, 1996). Calpastatin (CAST) is the endogenous inhibitor of the calpains. Levels of calpastatin vary considerably between species (Koohmaraie, 1991; Ouali and Talmant, 1990), breeds (Shackelford et al., 1994) and muscles (Geesink et al., 1992). Many studies in livestock, particularly beef animals, have been completed to determine calpastatin's physiological role in tenderness, as well as the genetic components of the CAST gene (Parr et al., 1999; Boehm et al., 1998; Huff-Lonergan et al., 1996; Lonergan et al., 1995; Killefer and Koohmaraie, 1994). Previous studies have found an inverse relationship between calpastatin levels and the development of tenderness in meat (Sensky et al., 1996; Koohmaraie et al., 1994; Koohmaraie et al., 1991). The relationship between levels of calpastatin and muscle growth has also been investigated in livestock (for review see Goll et al., 1998).
Results of the two CAST PCR-RFLPs: RsaI (CASTRsa) and MspI (CASTMsp) (developed by Ernst et al., 1998) indicate that the markers were polymorphic in all breeds analyzed: Berkshire (n=169), Duroc (n=76), Landrace (n=48), and Hampshire (n=150). The frequency of allele 1 for CASTRsa was 0.75. Allele 1 of CASTRsa was associated with fatter animals in the total population. Genotypic frequencies varied among the breeds, however the CASTRsa 11-animals were the most frequent (0.58) in the total population. Interestingly, the allelic frequencies for the CASTMsp marker were the opposite of the CASTRsa PCR-RFLP with 0.29 for allele 1. Allele 2 of CASTMsp was associated with fatter animals in the total population, and the 22-animals were the most frequent (0.53) in the total population.
Total population analysis reveals effects (P<0.05) of CASTRsa and CASTMsp on last lumbar backfat, 10th rib backfat, and loin muscle area (Table 1). Ultimate loin pH, tenderness score, and Instron tenderness (CASTMsp only) also approached significance for the total population. The results of the individual breed analysis (not shown) revealed differences (P<0.05) between CASTRsa and CASTMsp genotypes for Instron tenderness in Berkshire; loin muscle area in Duroc; and last lumbar backfat, average backfat and loin muscle area in Landrace. Hampshire genotypes differed (P<0.05) for Japanese color and color scores for the CASTMsp locus and tenderness score for the CASTRsa locus. These CAST PCR-RFLPs remain interesting genetic markers for the improvement of carcass composition in pigs.