Expression of trypanotolerance in N’Dama x Boran crosses under field challenge in relation to N’Dama genome content

Abstract

BackgroundAnimal trypanosomosis in sub-Saharan Africa is a major obstacle to livestock based agriculture. Control relies on drugs with increasing incidence of multiple-drug resistance. A previous mapping experiment in an F2 population derived from the indigenous trypanotolerant N’Dama cattle crossed to susceptible (Kenya)-Boran cattle under controlled challenge, uncovered a number of trypanotolerance QTL (T-QTL). The present study was to determine expression of N’Dama trypanotolerance in a backcross to the Boran under conditions of field challenge, and whether chromosomal regions associated with trypanotolerance in the F2 experiment showed similar effects in the BC population.Methods192 backcross animals to the Boran were produced in six batches from June 2001 to December 2006. At one year of age animals were moved to the field and exposed to natural challenge over about one year in Southwest Kenya (Narok). The animals were individually recorded weekly for body weight, packed cell volume, parasitaemia score, and drug treatments, and were genotyped using 35 microsatellite markers spanning 5 chromosomes found in the F2 study to harbour T-QTL.ResultsThe F1 were most trypanotolerant, Boran least, and BC intermediate. Females showed distinctly higher trypanotolerance than males. There was a positive correlation in the BC population between trypanotolerance and number of N’Dama origin marker alleles. QTL mapping revealed T-QTL distributed among all five targeted chromosomes, corresponding in part to the results obtained in the F2 experiment.ConclusionsN’Dama origin trypanotolerance is expressed in a BC population under field conditions in proportion to N’Dama origin marker alleles. Consequently, marker assisted selection in such populations may be a means of increasing trypanotolerance, while retaining the desirable productive qualities of the recurrent parent.

Trypanotolerance in N’Dama x Boran crosses under natural trypanosome challenge: effect of test-year environment, gender, and breed composition

“…Hence T. vivax was the primary pathogen. This is consistent with previous studies [12] that also implicated this parasite as the main pathogen in these humid and sub-humid tsetse infested regions of Africa. The mean parasitemic score for parasitemic detections (MPAR) was 2.27, equivalent to 10 2 -10 3 trypanosome parasites per μl of blood per positive sample.…”

“…The finding of greater trypanotolerance of the female gender is in agreement with previous work reported [12,13], the latter in Galana ranch involving Orma Boran cattle where in both cases, it was shown that females required fewer treatments than their male counterparts. Also, male vs. female weight-gain difference across the challenge period was less than the difference between male and female when reared for a comparable period in the absence of trypanosomiasis challenge.…”

“…This shows a strong additive component to trypanotolerance loci, validating the use of the BCB as a mapping population (the results of the QTL mapping component of the study will be reported separately), and as a platform for development of a synthetic breed combining trypanotolerance of the ND with body size of the Boran. In addition to the comparison of genetic types, the results demonstrate clearly superior trypanotolerance of the female gender as compared to the male within all three genetic types (KB, BCB and F1), again supporting our previous study [12] even more as in the present case many more traits were analyzed. Thus, female BCB animals may be superior cow-mothers under field challenge.…”

“…Moreover, control of parasitemia and control of anemia were shown to be separate traits. All three defense strategies are exemplified in the F1 to KB comparisons of the present study and also in our previous study [12]. The F1 animals had better ability than KB to control parasitemia, as shown by a lower percentage of positive parasitemia scores (%PAR) and lower total parasitemia score (TPS), exemplifying resistance.…”

“…Table 3 shows a more elaborate analysis for gender effects (female trait value as a deviation from the corresponding male trait value) and their significance, than our previous study [12]. Also shown is whether the direction of effect is in the direction of greater tolerance (R) or greater susceptibility (S).…”