Genetic variation in resistance to mixed, predominantly Teladorsagia circumcincta nematode infections of sheep: from heritabilities to gene identification

Abstract

In cool temperate areas, such as Scotland, sheep are infected by a variety of nematodes but the dominant nematode is Teladorsagia circumcincta. Resistant animals have one or more of the following features: fewer adult nematodes, more inhibited larvae, shorter adult nematodes and decreased production of nematode eggs. In lambs at the end of the first grazing season, the heritability of adult worm length is very strong, whereas the heritability of egg production is moderate. The heritability of worm number is low while there is no detectable genetic variation in the number of inhibited larvae. The major mechanisms underlying resistance to T. circumcincta appear to be the IgA mediated suppression of worm growth and the mast cell mediated regulation of worm number. Mast cell responses are slow to develop, possibly because they are responsible for protein loss and reduced growth of the host. Two genes have been repeatedly associated with resistance to T. Circumcincta: the MHC class II DRB1 locus on chromosome 20 and the interferon-gamma locus on chromosome 3. Although the causative mutations are still unknown both genes are plausible candidates.

Genome-wide association and regional heritability mapping to identify loci underlying variation in nematode resistance and body weight in Scottish Blackface lambs

“…Moreover, we identified two SNPs (although below the suggestive significance level) on OAR 3 associated with IgA, which were also significant for FEC traits when analysed in ASReml. Although no causative mutation has been identified for these OAR3 QTL, the effect is often presumed to be mediated through INFG in sheep (Stear et al, 2009). …”

Confirmation of two quantitative trait loci regions for nematode resistance in commercial British terminal sire breeds

“…Crawford et al, 2006) have failed to find any evidence on chromosome 3. Although no causative mutation has been identified for the FEC OAR3 QTL, the effect is often presumed to be mediated through INFG in sheep (Stear et al, 2009). Two QTL studies have reported an OAR14 FEC QTL, one in Blackface sheep (Davies et al, 2006) and the other in lactating Spanish milk sheep (Gutierrez-Gil et al, 2009), but the QTL positions were different in these two studies and neither study suggested any possible candidate genes underlying the QTL.…”

Which is the best phenotypic trait for use in a targeted selective treatment strategy for growing lambs in temperate climates?

“…Random selection was achieved using a pseudorandom number generator to identify lamb IDs to be drenched. FEC measurements were assumed to be taken 5 days prior to treatment to allow time for the samples to be processed and analyzed, and included a 20% random sampling error (Bishop et al, 1996;Stear et al, 2009). LW was assumed to be measured the day prior to treatment.…”