The genotyping results of the three yellow cattle populations in this study are shown in Table 4 and Appendix. The classification results are clear, which ensures the reliability of the follow-up research results. For example, the Angus and Simmental contained two genotypes AA and GA and the GG genotypes were only found in the Droughtmaster atthe locus of CDC37-6 (A16154247/G). Similarly, there were two genotypes CC and GC on Angus and Simmental at locus CDC37-7 (C16157867/G) and GG were found in Droughtmaster. In addition, the Angus and Simmental breeds had only one genotype TT at AHSA1-9 (T89718093/A) and STIP1-11(T43118737/C) and Droughtmaster had three genotypes. Although the Angus and Simmental breeds also contain three genotypes at the AHSA1-8 (C89716722/T) and AHSA1-10 (G89722567/A) loci, they have a low frequency of genotypes TT and AA and have no significant differences. On the whole, the Droughtmaster breed has three genotypes at these loci. For the Hardy-Weinberg equilibrium (Table 4), it was found that the Droughtmaster deviated from the Hardy-Weinberg equilibrium at the CDC37-7 (C16157867/G) and STIP1-11 (T43118737/C) and Simmental is also the same at the AHSA1-8 (C89716722/T) locus. A total of three loci in the 3 populations deviated from the Hardy-Weinberg equilibrium, including two sites in Droughtmaster (CDC37-7(C16157867/G), STIP1-11 (T43118737/C)) and one site in Simmental ((AHSA1-8 (C89716722/T)). For a single locus, there is one population deviation from Hardy-Weinberg equilibrium at the CDC37-7 (C16157867/G), STIP1-11 (T43118737/C) and AHSA1-8 (C89716722/T). Aside from these, no populations deviate at other loci.
F
ST is an inter-group genetic differentiation index that can be used to judge the genetic differentiation between populations. The F
ST values of the three cattle populations and each point were calculated based on the genomic markers (Table 1 and Table 5). The F
ST values ranged from 0 to 0.5 between populations and the range of F
ST was larger. The F
ST was 0.10986 between the Angus and Droughtmaster breeds and the F
ST was 0.10073 between Angus and Simmental, indicating moderate genetic differentiation between populations. The F
ST was 0.31454 between the Droughtmaster and Simmental breeds, indicating a high degree of genetic differentiation between populations, with the largest genetic distance being between the Droughtmaster and Simmental breeds. For the F
ST value of a single locus, it was found that the Droughtmaster, Angus and Simmental breeds showed significant differences in the genotype frequencies of all the tested sites (P<0.05), except in the STIP1-11 (T-43118737/C) locus, where the difference between the Droughtmaster cattle and the Angus cattle was not significant (P>0.05).
The Droughtmaster cattle breed was selectively bred in the north Queensland of Australia in the early 20
th century through the cross-breeding Brahman cattle and short-horned cattle. This combined the ability of Brahman cattle to adapt to tropical climates with the high meat yield of short horn cattle, creating a new breed that did well in the heat but still produced a fair amount of meat. The Droughtmaster cattle breed was first introduced into Chongqing, China in the late 1970s. Judging from the production results in recent years, the Droughtmaster has not only kept its original features of fast growth, high meat yield, strong fertility and ability to eat crude feed, but can also adapt to the high temperatures in Chongqing due to its high tolerance of heat and resistance to heat stress. However, for the Angus and Simmental breeds originating in Europe, who were not selectively bred for heat stress resistance, they are highly prone to heat stress in high temperatures and experience irreversible damage to their bodies in such conditions (
Hammond et al., 1996;
Wang et al., 2018).
At present, there are many ways to alleviate heat stress, such as lowering the ambient temperature
(Guo et al., 2007) and improving the ratio of feed (
Aréchiga et al., 1998;
Huber et al., 1994; Brijesh et al., 2018), However, the most effective method is to find genes or genetic marker sites associated with heat tolerance and to breed individuals with genetic resistance to heat stress through molecular breeding techniques
(West et al., 2003; Hansen et al., 2001).
In this study, six SNPs from three heat-tolerant candidate genes (CDC37, AHSA1 and STIP1) were genotyped and genotypic frequencies were statistically analyzed. The Droughtmaster breed has a unique genotype GG compared to the Angus and Simmental breeds in CDC37. It is well known that heat stress proteins (Hsp90) play an important role in reducing thermal damage and enhancing heat resistance
(Bharati et al., 2017; Lalrengpuii et al., 2016). However, it has been reported that CDC37 can be involved in the regulation of protein kinases as a molecular chaperone of Hsp90
(Scholz et al., 2000; Calderwood et al., 2015). In addition,
Ota et al., (2011) also found that Cdc37 / Hsp90 protein mediates the stress response of the endoplasmic reticulum of ins-1 cells and helps regulate the activity of IRE1 protein in insulin synthesis under physiological stimulation
(Ota et al., 2011). Meanwhile, insulin can directly modulate hypothalamic neurons that regulate thermogenesis and CBT and this indicates that insulin plays an important role in coupling metabolism and thermoregulation at the level of the anterior hypothalamus (
Sanchez-Alavez et al., 2010).
Secondly, the latest study found that CHU_1110 may be involved in the stress response of bacteria to heavy metal ions, while CHU_1110 contains three α-helices and an anti-parallel β-sheet, forming a large cavity in the protein center, which is consistent with the structural features of the AHSA1 protein family, so the researchers speculated that AHSA1 and CHU_1110 have the same function during the stress response process (Liang et al., 2018). This study found that two of the three SNPs of the AHSA1 gene (AHSA1-8 (C89716722/T), AHSA1-10 (G89722567/A)) have no species-specific genotypes, but the genotypes frequency of the two SNPs was significantly different (P<0.05) in the Droughtmaster compared to the Angus and Simmental populations, implying that the gene frequency of the locus was altered due to artificial selection. From the STIP11 (T43118737/C) locus genotype frequency of STIP1 gene, it was found that there were 2 unique genotypes CC and CT in the Droughtmaster, while the other population only carried TT type. Almost all studies on the STIP1 gene have focused on cancer and found that STIP1 promotes the growth and migration of cancer cells
(Luo et al., 2018; Chen et al., 2017). Therefore, although there have been no reports on the direct relationship between the STIP1 gene and heat stress in related studies, from the difference in genotype frequency of this locus in the cattle population in this study, it can be inferred that this locus may be having an important role concerning the heat-tolerant potential of the Droughtmaster breed.