| 其他摘要 | Based on the classical definition of domestication, loss of genetic diversity in domesticated species relative to their wild ancestors would be expected ascribed to founder effects and artificial selection, which is seen as a common feature of domestication and has been proved in crops. However, our understanding of this feature in livestock is very limited because of the scarcity of existing data and the lack of the wild ancestor samples. Wild boar, the ancestor of domestic pig, is widely distributed in the world, which affords us an excellent chance to study the impacts of domestication on livestock genetic diversity. We chose growth hormone gene as study object, obtained totally 690 pGH gene sequences from 115 wild boars and 230 domestic pigs by PCR, cloning and sequencing, and identified 124 nucleotide polymorphisms, which defined 117 distinct haplotypes. Moreover, the sequences of Malaya wild boars, warthog and red river hog were obtained to be used as outgroups in phylogeny analysis. The results showed that in both domestic pigs and wild boars, pGH gene did not show a significant phylogeographic structure consistent to that of mtDNA, and there was no obvious differentiation between Malaya wild boars and Eurasian pigs. This suggests that pGH gene was highly conserved and evolved more slowly than mtDNA, so it could not behave like mtDNA to produce genetic differentiation among the different geographical subpopulations and to have a significant phylogeographic structure. As a result, pGH gene is unable to be used to study the origin of domestic pig just like mtDNA. To understand the impacts of founder effects and artificial selection on livestock genomes during domestication, we performed population genetic analyses on pGH gene and borrowed the data of mtDNA D-loop and GPIP from other works in our lab to compare synthetically the genetic diversity between wild boars and domestic pigs. The results showed that there was no significant reduction in the genetic diversity of pGH gene in domestic pig relative to wild boar, which was different from the results of mtDNA D-loop and GPIP. Since pGH gene was proved to be associated with some economic traits and might be acted by artificial selection, we presumed that the high genetic level of this gene in domestic pigs should be formed by artificial selection. The results of neutrality tests suggested that natural selection acting on pGH gene should be purifying/negative selection and artificial selection acting on this gene should be positive directional selection. To elucidate the connection between genetic diversity and artificial selection on pGH gene, we analyzed and compared the haplotype frequency spectrums of this gene between wild boars and domestic pigs. The results showed that there was a slight increase in the proportion of haplotypes with relatively high frequency in domestic pig relative to wild boar, which implied that artificial selection acting on this gene was composed by several positive directional selections. From the synthetical effects, this situation was very similar to that of balancing selection. And whether as several directional selections or synthetically as balancing selection, artificial selection is the main force to preserve the genetic level of domestic pig at the pGH locus by increasing the frequencies of multiple favored alleles. Although there was no reduction of genetic diversity at pGH in domestic pigs, there were other evidences such as the loss of low-frequency alleles, the change of the allele frequency spectrum and the increase of LD to support the existence of founder effects on this locus. Moreover, the data of neutral markers (mtDNA D-loop and GPIP) indicated that the above mentioned feature of domestication should also be fit to mtDNA genomes and nuclear genomes of livestock like crops. |
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