Genetic susceptibility to infectious disease in East African Shorthorn Zebu: a genome-wide analysis of the effect of heterozygosity and exotic introgression

Abstract

BackgroundPositive 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.ResultsIn 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.ConclusionWe 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.

Review: Sustainability of crossbreeding in developing countries; definitely not like crossing a meadow…

“…However, in developing countries, pedigree and performance recording remain a bottleneck for the implementation of selection schemes . More generally, genomics provides effective tools to assess and monitor the level of introgression within local population (Leroy et al, 2012;Murray et al, 2013). Genome-wide analysis of crossbred and improved livestock under tropical conditions may also reveal the genomic regions submitted to recent selection and therefore linked to adaptation, as investigated by Kim and Rothschild (2014) in Kenyan dairy cattle.…”

“…Molecular approaches may also provide valuable information on the level of crossbreeding with local populations. In Kenya, Murray et al (2013) performed genomic analysis on a sample of 500 East African Shorthorn Zebu and reported average introgression levels around 2%, with 20% of calves showing levels of introgression consistent with crossing with European breeds ⩽5 generations ago. A study of the same data revealed a greater rate of European introgression in closer proximity to animal markets (Mbole-Kariuki et al, 2014).…”

“…For instance, the meta-analysis performed by Galukande et al (2013) on 23 studies (Murray et al, 2013). …”

The efficacy of alcelaphine herpesvirus-1 (AlHV-1) immunization with the adjuvants Emulsigen ® and the monomeric TLR5 ligand FliC in zebu cattle against AlHV-1 malignant catarrhal fever induced by experimental virus challenge

“…It is also possible that, following repeated annual exposure to migrating wildebeest, the African breed is more resistant to AlHV-1. This view is supported by recent genetic studies on the closely-related East African Shorthorn Zebu, which provide evidence of selection for traits related to survival in the African environment, including factors such as resistance to endemic pathogens (Bahbahani et al, 2015, Murray et al, 2013). Additionally the increased survival of SZC seen could be the result of natural adaptive immunity following previous exposure.…”

A putative genomic map for resistance ofBos indicuscattle in Cameroon to bovine tuberculosis

“…Cattle were defined as inbred using a method adapted from Murray et al (2013), which we described here. Non-ET introgressed cattle were categorised as inbred if they had an inbreeding coefficient value of greater than 0.184 (more than 0.10 above the mean for this group).…”