title: Expression of trypanotolerance in N'Dama x Boran crosses under field challenge in relation to N'Dama genome content.
abstract: BACKGROUND
  Animal 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.
  
  
  METHODS
  192 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  N'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.
authors.affiliation: Department of Genetics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel. soller@vms.huji.ac.il.
authors.forename: Caleb
authors.initials: C
authors.lastname: Orenge
authors.forename: Leonard
authors.initials: L
authors.lastname: Munga
authors.forename: Charles
authors.initials: C
authors.lastname: Kimwele
authors.forename: Steve
authors.initials: S
authors.lastname: Kemp
authors.forename: Abraham
authors.initials: A
authors.lastname: Korol
authors.forename: John
authors.initials: J
authors.lastname: Gibson
authors.forename: Olivier
authors.initials: O
authors.lastname: Hanotte
authors.forename: Morris
authors.initials: M
authors.lastname: Soller