Genetic mapping of quantitative trait loci for resistance to Haemonchus contortus in sheep

Abstract

This paper presents results from a mapping experiment to detect quantitative trait loci (QTL) for resistance to Haemonchus contortus infestation in merino sheep. The primary trait analysed was faecal worm egg count in response to artificial challenge at 6 months of age. In the first stage of the experiment, whole genome linkage analysis was used for broad-scale mapping. The animal resource used was a designed flock comprising 571 individuals from four half-sib families. The average marker spacing was about 20 cM. For the primary trait, 11 QTL (as chromosomal/family combinations) were significant at the 5% chromosome-wide level, with allelic substitution effects of between 0.19 and 0.38 phenotypic standard deviation units. In general, these QTL did not have a significant effect on faecal worm egg count recorded at 13 months of age. In the second stage of the experiment, three promising regions (located on chromosomes 1, 3 and 4) were fine-mapped. This involved typing more closely spaced markers on individuals from the designed flock as well as an additional 495 individuals selected from a related population with a deeper pedigree. Analysis was performed using a linkage disequilibrium-linkage approach, under additive, dominant and multiple QTL models. Of these, the multiple QTL model resulted in the most refined QTL positions, with resolutions of <10 cM achieved for two regions. Because of the moderate size of effect of the QTL, and the apparent age and/or immune status specificity of the QTL, it is suggested that a panel of QTL will be required for significant genetic gains to be achieved within industry via marker-assisted selection.

Possibilities to breed for resistance to nematode parasite infections in small ruminants in tropical production systems

“…Quantitative trait loci (QTL) studies for nematode resistance, using microsatellite markers, have now been undertaken in many countries (Paterson et al, 1999;Beh et al, 2002;Diez-Tascon et al, 2002;Cockett et al, 2005;Davies et al, 2006;Moreno et al, 2006;Gutié rrez-Gil et al, 2009;Marshall et al, 2009). A conclusion from these published results is that most significant QTL effects tend to be scattered throughout the genome.…”

“…Most reported genetic marker studies for nematode resistance have detected and explored microsatellite-based QTL (Paterson et al, 1999;Beh et al, 2002;Diez-Tascon et al, 2002;Cockett et al, 2005;Davies et al, 2006;Moreno et al, 2006;Gutié rrez-Gil et al, 2009;Marshall et al, 2009). However, results from such studies are often difficult to utilise in breeding programmes for a number of reasons.…”

The distribution of SNP marker effects for faecal worm egg count in sheep, and the feasibility of using these markers to predict genetic merit for resistance to worm infections

“…19-0 . 36 phenotypic standard deviations, Marshall et al, 2009) QTL effects for WEC. Differences between our estimates and those of linkage studies can be partly attributed to the overestimation of effects in linkage studies (Xu, 2003) and also to the possibility of multiple linked QTL contributing to linkage peaks.…”

Helminth parasites in pigs: New challenges in pig production and current research highlights

“…Several studies have identified QTLs with influence on helminth infections in sheep (e.g. Marshall et al, 2009;Dominik et al, 2010) and recently a DNA test has been commercialized (Worm STAR, Pfizer Animal Genetics Ltd.) as a supplementary tool in breeding for helminth resistance in sheep. If similar DNA based tests can be identified in pigs with respect to helminth worm load, this will allow us to produce the 'right animal for the right purpose'.…”