Out of Africa: genetic characterization and diversity of Mashona cattle in the United States.

Abstract

Mashona is a Sanga breed believed to be an ancient Bos taurus and Bos indicus composite. Since importation to the United States producers with relatively small herds (<70 cows per herd) and located in hot-humid regions have found them useful in crossbreeding programs due to purported resistance to pests and heat stress. This study evaluated the genomic composition of U.S. Mashona to better understand their population history and determine the level of genetic diversity. Twenty-four Mashona from Tennessee State University were sampled and genotyped. A reference population of B. taurus (Angus, Hereford, Wagyu, Romosinuano, and Florida Cracker) and Bos indicus (Brahman) breeds, as well as from an additional Sanga breed (Tuli) was obtained from the USDA-ARS gene bank. All individuals were genotyped using the 777k Illumina BovineHD panel. Genomic diversity across breeds was evaluated using measures of genetic distance, allelic frequency, inbreeding, and admixture analyses. The principal component analysis results revealed the first 4 principal components to explain 15.5%, 4.3%, 3.1%, and 2.4% of the genetic variation in the genotyped animals. Mashona and Tuli did not overlap but tended to cluster near one another for all principal components, particularly along the 4th principal component (variation along the European-African axis). The ADMIXTURE analysis revealed that the average proportion of B. taurus genetics in Mashona was 0.81, with individual proportions ranging from 0.77 to 0.84. Additionally, the unsupervised ADMIXTURE analysis indicated that Mashona and Tuli form a distinct ancestry group, suggesting that Sanga breeds possess unique genetic diversity compared to the other breeds evaluated in this study. The evaluation of regions with high autozygosity or differentiated from other breeds revealed several selection signatures in the Mashona population. A run of homozygosity (ROH) region on chromosome 6 contained genes associated with horn fly resistance. Additional ROH regions contained genes and quantitative trait loci associated with calving ease, reproduction, and maternal ability. Historical trends in Mashona's effective population size (Ne = 28) align with known past demographic events and indicate a narrowing of its genetic base. With insights into Mashona's unique level of genetic diversity but its relatively small population size, breeders will need to balance existing genetic diversity and selection for important traits.