On the search for markers of tick resistance in bovines.

Genetic differences in susceptibility to tick (Riphicephalus (Boophilus) microplus) are considerable in bovine. In general, Bos taurus is more susceptible to ticks than Bos indicus. On the search for the molecular basis of tick resistance we used mapping, association and gene expression approaches. A B. taurus x B. indicus F2 population was developed by Embrapa and 300 individuals were measured for parasite load and genotype for microsatellite markers covering chromosomes 5, 7 and 14. A suggestive chromosome-wide QTL for tick load (P < 0.05) was mapped on chromosome 5 and two suggestive (P < 0.10) QTL on chromosome 7 and 14. 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 in seven to ten tick Counts. Microsatellite markers close to the genes for interleukin 2 (IL2), interleukin 4 (IL4) and interferongamma (lFNG) were analysed. Tick counts were associated to the marker for interleukin 4 (P < 0.05) in NE, CN and AN genetic groups. We also investigated if there were differences in cytokine mRNA of naive versus infested Nelore as well as of resistant versus susceptible cows from NE, CN and AN genetic groups. Relative quantification of cytokines from infested compared to naives showed down regulation of IL2. When resistant cows from the from genetic groups were. compared to susceptible animals, IL8 was down regulated but no differences were found for IL4 expression