Abstract:
BACKGROUND: Positive multi-locus heterozygosity-fitness correlations have been observed in a number of natural
populations. They have been explained by the correlation between heterozygosity and inbreeding, and the
negative effect of inbreeding on fitness (inbreeding depression). Exotic introgression in a locally adapted population
has also been found to reduce fitness (outbreeding depression) through the breaking-up of co-adapted genes, or
the introduction of non-locally adapted gene variants.
In this study we examined the inter-relationships between genome-wide heterozygosity, introgression, and death
or illness as a result of infectious disease in a sample of calves from an indigenous population of East African
Shorthorn Zebu (crossbred Bos taurus x Bos indicus) in western Kenya. These calves were observed from birth to
one year of age as part of the Infectious Disease in East African Livestock (IDEAL) project. Some of the calves were
found to be genetic hybrids, resulting from the recent introgression of European cattle breed(s) into the indigenous
population. European cattle are known to be less well adapted to the infectious diseases present in East Africa. If
death and illness as a result of infectious disease have a genetic basis within the population, we would expect both
a negative association of these outcomes with introgression and a positive association with heterozygosity.
RESULTS: In this indigenous livestock population we observed negative associations between heterozygosity and
both death and illness as a result of infectious disease and a positive association between European taurine
introgression and episodes of clinical illness.
CONCLUSION: We observe the effects of both inbreeding and outbreeding depression in the East African Shorthorn
Zebu, and therefore find evidence of a genetic component to vulnerability to infectious disease. These results
indicate that the significant burden of infectious disease in this population could, in principle, be reduced by
altered breeding practices.
