Submitted to: Developments in Biologicals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2008
Publication Date: 12/31/2008
Citation: Regitano, L.C., Ibelli, A.M., Gasparin, G., Miyata, M., Azevedo, A.L., Coutinho, L.L., Teodoro, R.L., Machado, M.A., Silva, M.V., Nakata, L.C., Zaros, L.G., Sonstegard, T.S., Silva, A.M., Alencar, M.M., Oliveira, M.C. 2008. On the search for markers of tick resistance in bovines. Developments in Biologicals. 132:225-230. Interpretive Summary: Although the existence of genetic differences of host response to tick infestation has been validated in cattle, the regions of the genome conferring these differences are still unknown. This manuscript is one of the first reports describing the putative locations of these differences through QTL mapping of a crossbred dairy resource population developed by Embrapa in Brazil. Also considered in this manuscript were several candidate genes thought to be critical to the innate immune response in cattle. In these experiments, marker genotypes for DNA markers located in close proximity to three critical cytokine response genes were tested for association to tick indicator phenotypes. The results show the marker near IL4 was significantly associated with tick resistance in all three breed groups. Overall, the results show the need to generate more comprehensive genome-based approaches to dissect the phenotypic differences of tick resistance segregating in the various cattle breeds of importance.
Technical Abstract: Genetic differences in susceptibility to ticks (Rhipicephalus (Boophilus) microplus) are considerable in bovines. Here, mapping, association and gene expression approaches were employed to further advance our understanding of the molecular basis of tick resistance. A B. taurus x B. indicus F2 population was developed by Embrapa and 382 individuals were measured for parasitic load. Scanning of all chromosomes is in progress. Quantitative trait loci (QTL) for tick load were mapped to chromosomes 4, 5, 7, 10, 14, 18 and 23 out of the 20 chromosomes scanned and were dependent on the season in which the phenotype was scored. In the candidate gene approach, females from the genetic groups Nelore (NE - 184), Canchim x Nelore (CN - 153), Aberdeen Angus x Nelore (AN - 123) and Simmental x Nelore (SN - 120) were evaluated under natural infestation. Microsatellite markers close to the genes for interleukin2 (IL2), interleukin 4 (IL4) and interferon gamma (IFNG) were analyzed. Tick counts were associated with the marker for interleukin 4 (P<0.05) in three genetic groups. Differences in cytokine mRNA levels of naïve versus infested Nelore calves as well as between resistant versus susceptible cows from NE, CN and AN genetic groups were also investigated. Comparison of cytokines from infested and naïve animals showed down regulation of IL2.When resistant cows were compared to susceptible animals, IL8 was down regulated. These results reinforce the multi-loci nature of tick resistance and the need to consider QTL and environment interactions.