Expression Of Tryp Anotolerant Quantitative Trait Loci In A Boran .•R~Sed Backcross Under Natural Tsetse Challenge

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) x

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

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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.