|Van Tassell, Curtis - Curt|
Submitted to: Animal Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2011
Publication Date: 2/1/2012
Citation: Silva, M.V., Sonstegard, T.S., Hanotte, O., Mugambi, J., Garcia, J.F., Nagda, S., Gibson, J., Iraqi, F., Mcclintock, A.E., Kemp, S., Boettcher, P., Malek, M., Van Tassell, C.P., Baker, R.L. 2012. Identification of quantitative trait loci affecting resistance to gastro-intestinal parasites in a double backcross population of Red Maasai and Dorper sheep. Animal Genetics. 43(1):63-71. Interpretive Summary: Understanding the genetic variation underlying host control of gastro-intestinal parasite infection is a first step in developing management strategies that are not completely reliant on antibiotic drug control of infections. For small ruminants, this is knowledge is even more critical especially in countries where access to drugs is often limited or cost-prohibitive for subsistence farming. In this study, we identified regions of the sheep genome containing genetic variation affecting resistance to parasites that are common to pastures of East Africa. This is the first genome mapping study of parasite indicator traits derived from a sheep breed that is native to East Africa. The results of this study will be used to refine regions of the genome conferring the parasite resistance phenotype inherent in Red Masaii sheep, and ultimately will help development of DNA-based strategies for increasing production in East Africa.
Technical Abstract: A genome-wide scan for quantitative trait loci (QTL) affecting gastrointestinal (GI) nematode resistance was completed using a double backcross sheep population derived from Red Maasai and Dorper ewes bred to F1 rams. These breeds were chosen, because Red Maasai sheep are known to be more tolerant than Dorper sheep to infection and disease caused by Haemonchus contortus and Trichostrongylus spp., the predominant parasite species of East African rangeland. Parasite indicator phenotypes (blood packed cell volume-PCV and fecal egg count-FEC) and live weight-LWT were collected on a weekly basis from 1,064 lambs during both a single 3 mo. post-weaning grazing challenge on infected pastures. The averages of last measurements for FEC (AVFEC) and PCV (AVPCV) along with starting PCV (PCVST) and decline in PCV (PCVD) from start to end of the grazing challenge were determined. A subset of lambs (N=371) representing the tails (10% threshold) of phenotype distributions for AVFEC, AVPCV, and PCVD were chosen for selective genotyping. Marker genotypes for these lambs, the F1 sires and their grandparents were scored across 172 microsatellite loci covering 25 autosomes (chromosome 24 not included) with a total map distance of 1560.7 cM. QTL detection was completed using QxPak with Genoprob corrected genotypes, Box-Cox transformed AVFEC, and arcsine transformed PCV statistics. Transformation effectively reduced the coefficients of asymmetry and kurtosis existing in the phenotype variables to improve estimates of variance components for QTL detection. Highly significant QTL were identified on 4 chromosomes (Chr 3, 6, 14, and 22) for AVFEC, PCVST and PCVD. The most significant QTL overlapped the same marker interval on Chr 22 (40 cM) suggesting the potential for a single causative mutation affecting all three traits. On Chr 6, QTL confidence intervals contained a region of the genome known to influence immune response in ruminants. The results for Chr 3 support previous reports of QTL near the interferon gamma locus (IFNG). The eventual identification of markers in linkage disequilibrium with these QTL will be useful for accelerating genetic improvement of parasite tolerant sheep breeds raised in these marginal production environments.