Effects on resistance to Salmonella carrier-state to two candidate gene in fowls

As significant heritabilities were estimated for resistance to Salmonella carrier-state (defined as the persistency of the bacteria four weeks after inoculation) selection could reduce the risk for the consumer of food toxi-infections but identifying the genes underlying resistance would avoid expensive and extensive measurements. Two genes, Nramp1 (Natural Resistance Associated Macrophage Protein 1) renamed SIc11a1 for Solute carrier family 11, member 1 and TIr4 (Toll-like Receptor 4) have recently been identified in a mouse model of Salmonella infection and the implications of the regions carrying these genes in the resistance of 1 day-old chicks demonstrated. The goal of this work was thus to test the hypothesis of an implication of SLC11A1 and TLR4 in resistance to Salmonella carrier-state. A total of 345 hens issued of 20 sires and 51 dams were therefore orally inoculated at peak of lay with 1,5 x 109 c.f.u. of a PT4 strain of Salmonella enteritidis and slaughtered 4 weeks later. Colonisation of spleen, liver, caeca and ovary was investigated, both with and without enrichment, to distinguish between low and high levels of contamination. Resistance was assessed by five all-or-none traits, discriminating between contaminated (whether after enrichment or not) and uncontaminated animals for each organ and for all of them (discriminating between animals whose all four organs were found free from Salmonella versus those with at least one contaminated organ). For the analysis of data with extreme incidences (close to 0 or 100%), that is ovary and animal contamination, data of family with no positive organs, i.e. no information, were discarded. Five microsatellites from each region were used for genotyping. Association between the markers and the resistance to carrier-state, was analysed with the QTLMAP program using the principles of interval mapping and maximum likelihood. The model of analysis took into account the structure of the population, i.e. a mixture of full and half sib families. The significance thresholds were estimated from the 5% empirical quantiles of the test statistic distribution. A significant effect on chromosome 7 could be observed on the rate of spleen contamination (P<0.05) at 45 cM from the origin of the linkage group, in the vicinity of ScI11a41, suggesting an effect of this gene. Testing SNP within this gene will allow to disentangle the influence of SLC11A1 from the effect of the rest of the chromosome. If the effect of Scl11A1 is confirmed, it will be interesting to investigate at which stage of infection the gene is involved. No significant effect was obtained for the other organs, a result that could, at least partly, be due to contamination values further from the ideal 50%, most favourable for an all-or-none trait analysis, No significant effect of chromosome 17 could be observed but further investigations (including test of SNP thanks to the cloning of the gene) are needed before definitely concluding to the absence of effect of the TLR4 gene. (Texte intégral