As a cost and time-effective design, pooling DNA has been used in many kinds of research, such as, detecting SNP [16], estimating allele frequencies [17], QTL mapping [18], as well as genome association scan [19]. In the present study, the pooled DNA was chosen for evaluating SNP heterozygosity, and finally, 59 markers were determined and selected from 269 candidates. According to the results of genotyping data in individuals, it demonstrated that the pooling DNA was a successful and effective way to identify markers with high information for parentage testing in a given population.
The method of MALDI-TOF MS was used to obtain the genotypes of 59 SNPs for individuals in the current study. For assessing the accuracy of this platform, 10 samples were repeated once and 936 data were gathered, out of which only one pair of a duplicate sample in one SNP data was not identical (data not shown), showing a genotyping error rate of 0.002 in MALDI-TOF MS genotyping method, the similar SNP genotyping error was reported by Heaton et al. [20] in sheep. In addition, according to Cooper et al. [21], the call rate of the animal was also related to genotyping accuracy, the individual call rate ranged from 96–99% in our study. All the above results revealed that the genotyping data got in the present study was validated, and the method of MALDI-TOF MS could be a stable platform for genotyping those SNPs [22, 23].
Given the effects of SNPs of call rate and Hardy-Weinberg equilibrium on parentage analysis in our population, 9 markers with a call rate lower than 85% or departed from Hardy-Weinberg equilibrium were detected and deleted in further analysis. Finally, we selected the panel of 50 informative SNPs with an average MAF value of 0.43, HO value of 0.497, HE value of 0.484, and PIC value of 0.366, which indicated that these SNPs have strong power for assignment of paternity in this crossbred cattle population. Generally, the cumulative probability of exclusion (CPE) for one-parent exclusion case and both-parent exclusion case was estimated to be 0.997974 and 0.999999, that was consistent with the study of Zhang et al. [24] who reported 50 highly informative SNP markers for paternity testing in pure Simmental cattle population and suggested the CPE of 35–50 SNPs could > 99% if selecting those highly heterozygous. In other earlier research, Herráeza et al. [6] documented that the exclusion power of 43 SNPs could exceed 98% in Galloway cattle. The International Committee for Animal Recording (ICAR) has also recommended a cattle consensus panel of 100 core SNPs for genetic identification and parentage analysis [25]. However, few SNPs used in the present study were same as the above studies. The number one reason why those SNPs for paternity testing significantly vary between different populations is because core markers closely linked with heterozygosity and call rate of SNPs in the test population. Generally, more and more findings have reported the SNP panels for parentage verification in different cattle population, for instance, the Red Sindhi cattle in Brazil [26], the Brahman cattle in Costa Rica [27], the Angus Beef Cattle in United States of America [8], as well as the Holstein population in Mexico [28], so it is necessary to develop an SNP panel with sufficient power to identify individuals and their parents in certain populations [7]. Therefore, our work in the current study made up for the shortcomings of non-specific paternity identification markers for the crossbred population of Simmental and Holstein in China.
The set of 50 SNP markers was further used for verification of parentage testing in the population of 168 individuals. In the present study, 7 calves with a confidence level lower than 85% were considered as error paternity records. The reason for this was that their real mother or father was not sampled in this experiment, and there was a close relationship between two Simmental bulls i.e. one sire was the uncle of the other. What’s more, 18 calves whose inferred parents were incompatible with the putative ones. Parental information of these 18 individuals was reconfirmed according to the on-farm records. The reasons for paternity mistakes were analyzed using both birth and calving data, as well as insemination records. Six calves had incorrect ear tags due to which their parent tracing was puzzled. In this farm, ear tagging of freshly born calves is practiced once a week not immediately after birth which caused paternity error in some calves. Three individuals were misidentified due to incorrect recording the semen or the cow label by AI technicians, and incorrect paternity recording for the other 8 calves were due to multiple inseminations using different sires. In Israel, multiple inseminations could explain at most 20% of the rejected paternity [1], however, in the present population, more than half of the paternity mistakes were due to this reason. Thence, the comparison results confirmed that pedigree inferred from the developed SNP-panel was correct, which absolutely showed the effective and powerful identification of this SNP paternity testing system. At the same time, combining the on-farm and genotypic data for paternity analysis is an effective option [29].
In the current study, according to the on-farm data, it can be confirmed that there were paternity mistakes of sire-offspring in 18.5% and mother-offspring in 8.6%, which were in line with the reports by Chu et al. [30] and Guo et al. [31] who found the higher paternity mistakes in paternity inference than that of in maternity inference in Chinese Holstein population, and the reason of the multiple inseminations using different bulls may account for those results. Many countries had also reported the paternity error in cattle. In the United Kingdom (UK), Visscher et al. [32] reported a total paternity error rate of 10% in the dairy population. Similarly, 7% paternity error was found in the Angeln dairy cattle population of German [2]. In Kenya, the sire misidentification rate was even over 50% in Boran cattle [33], which was much higher than the results of the present study. Those results suggest that the validation for parenting information is completely required in the cattle industry, efforts should be made to improve the accuracy of pedigree records. Therefore, parentage testing as an essential tool for correcting pedigrees are extremely important for both breeding and practice. Overall, our results showed that the set of 50 SNPs could be a practical tool for correcting pedigrees in the crossbred population of Simmental and Holstein in China.