Investigating the relationship between the prion protein locus and udder morphology traits and milk yield in Sardinian sheep1,2

Abstract

Different approaches were applied to investigate prion protein (PrP)-encoding gene effects on udder morphology and milk yield in Sardinian sheep. The PrP genotype of 23,077 animals (10,029 males) was determined. The direct effect of the PrP or a closely linked gene was analyzed at the population-wide level using 2 animal models, based on records from genotyped animals, including only the PrP genotype as a fixed effect. In the female model, the dependent variable was animal performance deviation, calculated as the sum of the individual random effects. The male model was based on daughter yield deviations. Both dependent variables were obtained from the national genetic evaluations of 2005. The significance of pairwise comparisons between genotypes was assessed by using the Tukey-Kramer multiple-comparison procedure. Within-family analyses were performed on sires heterozygous for the PrP gene to detect those genes that affect the traits of interest and are not in linkage disequilibrium with the PrP locus at the population-wide level. The overall results led us to exclude either a direct or a linkage gene effect of the PrP locus on udder morphology or milk yield in Sardinian sheep. A further analysis of males that neglected the relationship matrix was carried out to evaluate the effect on the loss of genetic gain of the different selection pressures applied on resistant and susceptible genotype classes. Significant differences between genotypes were detected for milk yield. These were due to the different selection pressures applied to the PrP genotype classes. Finally, no negative correlated genetic response on the selection traits is expected from the selection for scrapie resistance in the Sardinian breed. However, a loss of genetic gain for milk yield is likely to occur in the future due to the different selection pressures on resistant and susceptible males.

Effect of PrP genotype on milk traits in Bovška sheep

“…As such a selection programme can potentially have negative effects on certain production traits in particular breeds, we aimed here to determine the frequencies of PrP alleles and genotypes in Bovška sheep and evaluate their possible associations with milk production and composition traits. The frequency of the most frequent allele ARQ (57.14 %) is comparable with the frequencies found in other continental breeds (SALARIS et al 2007, VAN KAAM et al 2005, CASELLAS et al 2006, WISNIEWSKA et al 2006, although higher proportion of ARQ allele was found in the neighbouring Italian province Veneto (68.0 %, DE MARCHI et al 2005), in the Spanish Churra sheep (75.9 %, ALVAREZ et al 2006) and certain Northern German breeds (DRÖGEMÜLLER et al 2001). The proportions of the desired ARR allele found in other continental breeds were mostly higher than in Bovška sheep (17.12 %), from 18.2 % in the Spanish Churra sheep (ALVAREZ et al 2006) to 42.5 % in the Sardinian sheep (SALARIS et al 2007).…”

“…On the other hand, PrP genotype did not significantly affect any milk trait in the Spanish Churra (ALVAREZ et al 2006) and in the East Friesian dairy sheep (DE VRIES et al 2005). However, the ARR homozygotes in Sardinian sheep produced lower milk yields (P<0.05) than ARR heterozygotes or other genotypes (SALARIS et al 2007). Thus, selection for the resistant ARR alleles could result in negative effects on milk productivity in Sardinian sheep.…”

“…The ARQ is the most prevalent allele in most continental breeds ranging between 50.4 % and 75.9 % followed by the ARR allele ranging between18.2 % and 42.5 % (SALARIS et al 2007, VAN KAAM et al 2005, CASELLAS et al 2006, BRANDSMA et al 2004, BRANDSMA et al 2005, DE MARCHI et al 2005, ALVAREZ et al 2006, WISNIEWSKA et al 2006. However, in 7 of 16 breeds of Northern Germany, the ARQ allele was detected at a frequency below 41 % while frequency of the ARR allele was higher than 50 % in 5 of 16 breeds (DRÖGEMÜLLER et al 2001).…”

“…Frequencies of the most sensitive VRQ allele are generally low in continental breeds ranging between 0.1 % in Sardinian sheep (SALARIS et al 2007) and up to 8.0 % in the experimental Texel breed (BRANDSMA et al 2005), with some outliers such as Cameroon sheep reaching a high 95 % (DRÖGEMÜLLER et al 2001). In comparison with continental breed, British populations differ widely in frequencies at the PrP locus.…”

The evidence of associations between prion protein genotype and production, reproduction, and health traits in sheep

“…There was no indication in any of the populations studied that there was an association between PrP genotype and milk yield, percentage fat, or percentage protein. Similarly, there was no evidence for an association between PrP genotype and the udder morphology traits: teat position, degree of udder suspension, udder depth, and degree of separation of two udder halves [72].…”