| To determine whether the two sheep populations found in Sichuan, namely the Butuo group, Jialuo group, are new genetic resources, we collected tissue samples of six groups, including the Butuo group, Liangshan semi-fine wool sheep, Liangshan local sheep group, Jialuo group, Hongyuan local sheep group, and Suffolk and Jialuo hybrid group, from Sichuan Liangshan and Aba. By employing the polymorphism of sheep mitochondrial DNA D-Loop region, this study analyzed the origin and genetic diversity of these six populations. At the same time, combined studies were assessed by downloading 68 domestic sheep mitochondrial DNA D-Loop region sequences from GenBank to construct a phylogenetic tree of the entire sequence. In the end, haplotype network analysis, population expansion analysis, nucleotide mismatch distribution analysis, and neutrality test were performed on the six sheep groups. The results are as follows:1. The mtDNA D-loop region sequence length of 286 individual sheep in the six groups that were divided eight types, which are 1106bp?1179bp?1180bp?181 bp?1182bp? 1183bp?1184bp and 1256 bp, among which 1181 bp accounts for the biggest proportion (79.6%). No 1106bp length was found in Butuo group, and there were only three lengths of mtDNA D-loop regions of Butuo group, which are 1180bp,1181bp, and 1182bp. This indicates less variations of D-loop length in Butuo group. Three 1256bp D-loop were found in Jialuo group, while among the rest five groups, only one 11256bp D-loop was found in Suffolk and Jialuo hybrid group. This indicates that 1256bp could be exclusive in Jialuo group.286 Sheep mtDNA D-loop sequences were obtained in this study; 210 variable sites were found, in which 43 were single polymorphic locus, and 167 were parsimony informative sites. In terms of base composition, A+T content (62.693%) was significantly higher than the G+C content (37.324%), and in addition to deletions and insertions sites, transformation occurred 192 times, and transversions occurred 18 times, including transitions and transversions occurred simultaneously at 7 loci. Variations are mainly conversions, which indicated that sheep mtDNA D-loop region sequence has a conversion bias. In the six sheep groups, Butuo group has the lowest G+C content, which is 37.108%, and Jialuo group has the highest A+T content of 62.902%; no significant difference was observed in base composition. A and T content accounting for a large proportion suggests that sheep mtDNA D-loop is A and T rich region.2.133 Haplotypes were found in six groups, of which 51 haplotypes are intergroup and intragroup sharing, and the remaining 82 are group-specific haplotypes. Only 11 haplotypes in Butuo group makes it the group that has the fewest haplotype. Nucleotide diversity of 286 sequences is an average of 0.01724; haplotype diversity is an average of 0.9843. The range of nucleotides diversity of six groups is 0.01099-0.02114; and the range of haplotype diversity is 0.780-0.995. The lowest and highest haplotype diversity groups are Butuo group and Jialuo group respectively. The results suggested that genetic diversity of Butuo groups is relatively poor, so reasonable protection are needed for their germplasm resources; Genetic diversity of Jialuo group is rich, which reveals a good prospect for cultivating Jialuo sheep from the point of view of genetic diversity.3. The intergroup genetic distance of six sheep groups was 0.014-0.023. The genetic distance between Suffolk and Jialuo hybrid group and Liangshan semi-fine wool sheep as well as between Suffolk and Jialuo hybrid group and Jialuo group is relatively small, which is 0.014. The genetic distance between Hongyuan local sheep group and Butuo group is the largest, which is 0.023. The clustering diagram of six sheep groups showed that genetic distance of Butuo group is larger than other five groups, forming a single cluster, which indicates that Butuo group is a separate local group. The genetic distance between Jialuo group and Hongyuan local sheep group in Aba is 0.018, and the genetic differentiation of these two groups is relatively distinct, therefore, Jialuo group can be regarded as a relatively separate group. In short, there is no significant geographical genetic structure between Butuo group and Jialuo group and other groups.4. Phylogenetic tree of 286 sequences was constructed by MEGA6.0, and the result showed that these sequences are divided into three branches. At the same time,6 wild sheep sequences downloaded from GenBank were compared with 62 domestic sheep sequences, and the results demonstrated that Mouflon cluster in branch B, Butuo group presents in branch A and B, and Jialuo group appears in all three branches. Therefore, it is speculated that Butuo group was evolved from two different ancestors, and Jialuo group was evolved from three different ancestors, and that Mouflon is the common ancestor of Butuo group and Jialuo group. Ibex and argali were clustered separately, so it is impossible that they are the maternal origin of Butuo group and Jialuo group.5. Analysis of population expansionIn this study, mismatch distribution and neutral test were employed to determine whether expansion happened in a population. The mismatch distribution diagrams of the two groups were both multimodal, and Tajima's D and Fu's Fs neutral test were bot not significant. Therefore, no expansion occurs in these two groups. |
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