Abstract:
African animal trypanosomosis (AAT) or Nagana is a transboundary disease with immense
negative impact in Agricultural development across 37 African countries, with 60M livestock
at risk. Utilization of trypanotolerant breeds as a form of control may offer a viable and
sustainable option following recent identification of nine trypanotolerant quantitative trait
loci (QTL) with positive effects in an F2 cattle population ofN'DamaJ Kenyan Beran, under
a challenge of 1180 clone of Tcongolence.
The current study was to confirm whether these identified QTLs would be expressed
under natural tsetse and trypanosomosis challenge. To achieve this, a QTL challenge and
mapping experiment was designed involving 192 backcross of (N'Dama x Kenyan-Boran)
Kenyan-Boran, Bel, 13 FI (N'Dama x Kenyan-Boran) sires and 8 founders of 4 Kenyan-
Boran females and 4 N'Dama males. All animals were screened using 35 polymorphic
microsatellite markers spanning through BTA2, BTA4, BTA7, BTA16 and BTA17 target
chromosomes. Thirty seven F IS and 23 Kenyan-Boran were used as controls for the field
challenge.
All cattle types were exposed to natural tsetse and trypanosomosis challenge in Narok
district, Kenya and monitored for one year against 46 defined traits. Intra-genetic-types (FI,
Bel, and K-Boran) means and standard deviations of the defined traits were computed and
used as a measure oftolerance index for comparison. Overall mean number ofN'Dama
alleles CaNAS) and specific 'Dama allele score components, inherited by the Bel from the
N'Dama grandsire through the heterozygous FI sire were computed. Based on this ONAS
score, BCl were divided into 30% upper and lower N'Dama alleles inherited and individual
traits in each group were ranked in terms of trypanotolerance. In QTL analysis, a single trait
QTL model, within the framework of multiple interval mapping (MIM) of MultiQTL
software was implemented with 46 trait scores and 35 microsatellite markers span over the
five target chromosomes.
Results revealed FJ as the most trypanotolerant, K-Boran the most susceptible with BCl
intermediate but more towards the K-Boran, an indication of some recessives. The upper
30% N'Dama allele group was more trypanotolerant at individual and overall
trypanotolerance trait levels than the lower N'Dama allele group. Despite the expected
increased error variance for measurement of traits under field conditions, QTLs were
detected in all five chromosomes at false discovery rate of 15%, deriving positive alleles
from N'Dama breed. It is concluded that QTLs detected under controlled experiment are
expressed under field conditions thus setting a stage for practical application of the results
through marker assisted selection and marker assisted introgression programmes. This will
benefit farmers in tsetse endemic areas by enhancing disease resistance of the K-Boran
susceptible breed while retaining its desired productivity characteristics.
