ABSTRACT. The International Society of Animal Genetics (ISAG) has chosen nine microsatellites (international marker set) as a standard that should be included in all cattle parentage studies. They are BM1824, BM2113, INRA023, SPS115, TGLA122, TGLA126, TGLA227, ETH10, and ETH225. We decided to ascertain whether this microsatellite set could be used to determine ancestral proportions in individual animals of synthetic breeds produced by crossing zebu and taurine cattle. Since the genotypes of these markers are routinely available, this would constitute a practical and cost-free method to estimate the ancestry of synthetic breed animals. Genotypes of 100 Gir and 100 Holstein animals were examined for this ISAG marker set. As expected, there were very significant allele frequency differences between the two breeds at most loci. We also typed 20 Girolando animals for which there was complete genealogical information. “Structure” software easily distinguished Holstein and Gir animals based on their microsatellite genotypes; it also attributed the genomic proportion of zebu and taurine of each of the 20 Girolando animals. The proportion of Holstein ancestry was then regressed on the genealogical data; there was a highly significant correlation (r = 0.84, P < 0.0001). The nine microsatellites that compose the ISAG international marker set were capable of estimating the ancestral Gir and Holstein genomic proportions in individual Girolando animals within narrow confidence limits. This microsatellite set might also be useful for estimating the proportions of taurine and zebu origins in commercial meat products.

Key words: Microsatellites, Ancestry, Taurine, Zebu, Synthetic breed

INTRODUCTION

Bos taurus taurus (European cattle; taurine) and Bos taurus indicus (Indian cattle; zebu) are the two subspecies of domesticated cattle. Based on the observation of considerable divergence of microsatellite allelic frequencies between them, MacHugh et al. (1997) estimated that these subspecies diverged more than 600,000 years ago. In the first half of the 20th century, several synthetic breeds were developed by crossing zebu and taurine cattle, as exemplified today in Brazil by the breeds Girolando (Gir x Holstein), Brangus (Nelore x Angus) and Simbrasil (Guzerat x Simmental). All of these breeds, which are claimed to have both high resistance to heat and humidity characteristic of Indian cattle and high productivity characteristic of European cattle, have standard proportions of 3/8 zebu and 5/8 taurine.

The International Society of Animal Genetics (ISAG) has chosen nine microsatellites (international marker set) that should be included in all cattle parentage studies to allow record exchange between laboratories. These are BM1824, BM2113, INRA023, SPS115, TGLA122, TGLA126, TGLA227, ETH10, and ETH225. We decided to ascertain whether this microsatellite set could be used to determine ancestral proportions in individual animals produced from mixed taurine and zebu stocks. Since the genotypes of these markers are routinely available, this would constitute a practical and cost-free method to estimate the ancestry of synthetic breed animals.

MATERIAL AND METHODS

Populations studied

DNA samples from 100 Brazilian Gir animals “puros de origem = pure at origin” and 100 Holstein animals imported from Canada or Europe were examined. We also investigated 20 Girolando animals for which there was complete genealogical information.

DNA analysis

DNA from each individual was independently typed for the nine microsatellites of the international marker set of ISAG. The primer sequence and PCR conditions were those described in the webpage of ISAG (http://www.isag.org.uk/journal/comparisonguide.asp). Primers were labeled with one of three fluorescent primers: FAM, TAMRA or HEX. The PCR products were analyzed using a MegaBACE 1000 DNA sequencer (GE Healthcare), according to the manufacturer’s instructions. Analyses of allele sizes were scored using Genetic Profiler (version 2.2) and Fragment Profiler (version 1.2) software (GE Healthcare).

Statistical analysis

We applied a model-based clustering algorithm by using the Structure software, version 2.1 (Pritchard et al., 2000), which uses a Bayesian algorithm to perform k-means clustering. The genotypes of Gir and Holstein animals were used as parentals. Although zebu cattle also have some taurine ancestry (Kumar et al., 2003), it is relatively small and can be ignored for our purposes. An admixture model with correlated allele frequencies was used. Every run consisted of 50,000 burn-in steps, followed by 250,000 Markov Chain Monte Carlo iterations. Regression analysis was performed with the Statistica software.

RESULTS AND DISCUSSION

As expected, there were very significant allele frequency differences between the two breeds at most loci (Table 1).

The Structure software easily distinguished Holstein and Gir animals based on their microsatellite genotypes and also determined the genomic proportion of zebu and taurine for each of the 20 Girolando animals (Figure 1). There was a highly significant correlation between the arc sine of the proportion of Holstein ancestry and the genealogical data (Figure 2, r = 0.84, P < 0.0001).

In conclusion, the nine microsatellites that compose ISAG’s international marker set can be used to estimate, within narrow confidence limits, the ancestral Gir and Holstein genomic proportions in individual Girolando animals. This same widely available microsatellite set might also be useful to estimate the proportions of taurine and zebu origins in commercial meat products.

ACKNOWLEDGMENTS

We are grateful to Leandra D. Soares and Junia A. Viveiros for expert technical help.

REFERENCES

Kumar P, Freeman AR, Loftus RT, Gaillard C, et al. (2003). Admixture analysis of South Asian cattle. Heredity 91: 43-50.

MacHugh DE, Shriver MD, Loftus RT, Cunningham P, et al. (1997). Microsatellite DNA variation and the evolution, domestication and phylogeography of taurine and zebu cattle (Bos taurus and Bos indicus). Genetics 146: 1071-1086.

Pritchard JK, Stephens M and Donnelly P (2000). Inference of population structure using multilocus genotype data. Genetics 155: 945-959.