Identification of Novel Loci Associated with Gastrointestinal Parasite Resistance in a Red Maasai x Dorper Backcross Population

Abstract

Gastrointestinal (GI) parasitic infection is the main health constraint for small ruminant production, causing loss of weight and/or death. Red Maasai sheep have adapted to a tropical environment where extreme parasite exposure is a constant, especially with highly pathogenic Haemonchus contortus. This breed has been reported to be resistant to gastrointestinal parasite infection, hence it is considered an invaluable resource to study associations between host genetics and resistance. The aim of this study was to identify polymorphisms strongly associated with host resistance in a double backcross population derived from Red Maasai and Dorper sheep using a SNP-based GWAS analysis. The animals that were genotyped represented the most resistant and susceptible individuals based on the tails of phenotypic distribution (10% each) for average faecal egg counts (AVFEC). AVFEC, packed cell volume (AVPCV), and live weight (AVLWT) were adjusted for fixed effects and co-variables, and an association analysis was run using EMMAX. Revised significance levels were calculated using 100,000 permutation tests. The top five significant SNP markers with - log10 p-values >3.794 were observed on five different chromosomes for AVFEC, and BLUPPf90/PostGSf90 results confirmed EMMAX significant regions for this trait. One of these regions included a cluster of significant SNP on chromosome (Chr) 6 not in linkage disequilibrium to each other. This genomic location contains annotated genes involved in cytokine signalling, haemostasis and mucus biosynthesis. Only one association detected on Chr 7 was significant for both AVPCV and AVLWT. The results generated here reveal candidate immune variants for genes involved in differential response to infection and provide additional SNP marker information that has potential to aid selection of resistance to gastrointestinal parasites in sheep of a similar genetic background to the double backcross population.

Determinação de valores de referência séricos para os eletrólitos magnésio, cloretos, cálcio e fósforo em ovinos das raças Dorper e Santa Inês

“…A raça Dorper apresenta alta taxa de desenvolvimento de carcaça com boa conformação, sendo muito utilizada em cruzamentos com ovelhas de raças nativas do Nordeste, tais como a Santa Inês, em virtude do seu porte e da velocidade de crescimento (Carneiro et al 2007). Também já foi descrito que características genéticas da raça Dorper conferem maior resistência a parasitos gastrointestinais (Benavides et al 2015), o que torna essa raça de alto interesse para criação em regiões brasileiras caracterizadas por alta infestação com esses parasitos (Wilmsen et al 2014). Com o objetivo de aumentar a produção de carne ovina, aumentou-se o interesse no monitoramento da sanidade do rebanho, utilizando diversas ferramentas como auxiliares no diagnóstico clínico, tais como os intervalos de referência para metabólitos sanguíneos (componentes hemato-bioquímicos) (Peixoto & Osório 2007).…”

Immunogenomics of gastrointestinal nematode infection in ruminants – breeding for resistance to produce food sustainably and safely

“…Resistance to GINs, like many complex traits, is influenced by many genes and a large proportion cannot be captured using the technologies or sample sizes currently employed in much of the research in this area. Studies which used the Ovine50SNP chip reported that less than 0.5% of the SNP markers were significantly associated with FEC and that, in the case of the most significant effects, there was no known gene in the region . While there is little evidence of any major selective sweeps in the sheep populations studied to date, selection under a polygenic‐control model would result in modest allele frequency changes across the genome, which may be undetectable using current methods, which seems to fit with the current genetic evidence on parasite resistance in ruminants.…”

“…Hence, both Kemper et al . and Benavides et al . are consistent in finding that <5% of the phenotypic variance in FEC arising from H. contortus infection can be accounted for by observed variation in SNP genotype.…”

“…During the subsequent data analysis, a proportion of these SNPs are discarded for a variety of reasons, including SNPs that have a minor allele frequency <1% or that deviate from Hardy‐Weinberg equilibrium. The size of the final data set has varied from 31 686 SNPs to 48 327 SNPs . The type of analysis also affects the number of SNPs available as reported by Riggio et al., who had 44 388 SNPs available for genomewide association (GWA) analysis, but only 42 841 SNPs were available for analysis using regional heritability mapping (RHM), as this methodology only utilizes mapped SNPs.…”

“…In a GWA study of a double‐backcross population involving Red Masai and Dorper sheep and using a 100‐SNP “moving window,” Benavides reported that only 0.33% of SNPs were significantly associated with FEC . These authors reported that the 5 most significant SNPs accounted for 2.33% of the phenotypic variance in FEC.…”