Analysis of B. taurus and B. indicus admixture in Uganda as revealed by the Illumina BovineSNP50 Genotyping BeadChip

The NextGen project investigates disease resistance in indigenous Ugandan cattle. Since population structure and stratification may produce biased results, We have investigated the genomic structure of sampled animals genotyped with the BovineSNP50 Genotyping Beadchip. A total of 788 animals from 9 populations belonging to Ankole (crossbred between B. indicus and B. taurus), Zebu and Ankole-Zebu crosses have been sampled in 52 grid cells throughout the country (Table 1). We merged this data whit other 400 Italian Holstein Cattle, genotyped in the framework of SELMOL project to seek for a likely introgression of European B. taurus. The data were filtered with the following exclusion criteria: MAF < 0.01, genotype call rate (SNPs) < 0.95, genotype call rate (Animals) < 0.95. The resulting working dataset were composed of 43494 SNPs and 1188 animals. Hidden genetic structures were investigated by a Bayesian clustering approach with the ADMIXTURE software (Novembre et al. 2010). The software Admixture identified four ancestral genomic components. Three of them likely correspond to European taurine, African indicine and African taurine components (Figure 2). The fourth has a still unidentified origin (Yellow, Figure 2d). Most Ugandan individuals investigated have a remarkable level of admixture. Overall, about 20% of the Zebu genome is of African taurine origin, confirming previous data on the foundation of African Zebu. The European taurine (Blue, Figure 2) is a minor component of African genomes, rare in Zebu and evenly distributed in Ankole, other taurine subgroups and Ankole- Zebu crosses. Indicine and taurine components show a clear geographical structure, the former being predominant in north-eastern Uganda, and the latter in the south-west. Holstein Fresian introgression is spread mostly in south-western Uganda, while the fourth component is located in restricted geographical area in the East (Figure 3). The Ugandan cattle population is a complex admixture of African taurine (green in Figure 2) and zebuine (red) genomes, with a minor component of European origin (blue) and a rare but relevant contribution (yellow) from a still unidentified source. This complexity is to be accounted for in the following GWAS and selection signatures analyses planned within the NextGen project.