Genetic Diversity And Evolution Of TLR9 Gene In Chinese House Mice(Mus Musculus)

Both the wild populations and laboratory strains of house mice(Mus musculus)are the most important model organisms in biological and biomedical researches.China is a vast country with diverse natural and geographical conditions,and house mice spread throughout the country.House mice in China contain two subspecies(M.m.musculus in the northern region and M.m.castaneus in the southern region)bounded by the Yangtze River,which can provide abundant wild resources for population genetics research.The TLR9(Toll-like receptor 9)could recognize the unmethylated Cp G DNA of invasive bacteria and activate immune response,so it plays an important role in innate immune response.Up to date,no systematic studies have been conducted on the genetic diversity and evolutionary characteristics of TLR9 in Chinese house mice,though the many researches had been carried on the population genetic structure and evolutionary models of different TLRs genes.In this thesis,the TLR9 gene segments of 332 house mice individuals from 30 regions were amplified and sequenced.The TLR9 gene fragments we obtained were1,030 bp in length,which can code 342 amino acids locating in the conserved domain of disease-related molecular recognition.The genetic diversity,population genetic structure and evolutionary characteristics were analyzed based on the sequence data.The results showed that:1)for the population structure,the haplotype phylogenetic tree revealed that there was no obvious genetic differentiation in TLR9 gene segments of two subspecies of Chinese house mice;the genetic distance between different geographic populations was small(0-0.02),and the Fst values for most populations were small though the values varied greatly(0-0.89);88.19% of genetic diversity was contributed by different individuals within population;the above parameters indicated that the TLR9 gene in Chinese house mice was relatively conservative;2)for genetic diversity,43 haplotypes,75 variant base sites and 34 amino acid variant sites were defined by 332 sequences;the haplotype diversity(Hd: 0.154-0.933)and the nucleotide diversity(?: 0.00016-0.00432)of TLR9 gene segments in 30 geographical populations varied greatly,and the average genetic diversity was relatively low(? =0.00168,Hd = 0.467,K = 1.532),and the results further indicated that TLR9 gene in Chinese house mice was conservative;3)for the evolution characteristics,the CG content(71.1%)of the third codon in the protein-coding region of TLR9 was significantly higher than the AT content(28.8%);the codons encoding Leu,Ser and Asn were more frequently used;the Ka/Ks values of TLR9 gene were significantly smaller than 1(0.0871-0.1419),which indicated that this gene would have subjected to strong purification selection pressure in natural environments;however,in the TLR2 gene segments(another immune gene that has similar function with TLR9),the CG content(33.6%)of the third codon in the protein-coding region was significantly lower than the AT content(66.4%);the Ka/Ks values of TLR2 gene in different geographical populations(0.3621-0.9990)were significantly larger than those of TLR9 gene,which clearly suggested that the selection pressure acted on different immune genes vary greatly,resulting in different evolutionary model,though the gene functions were similar.In summary,we preliminarily analyzed the population genetic diversity and evolution characteristics of TLR9 gene in Chinese house mice on the basis of extensive sampling.Our results can help us to understand the evolutionary mechanism of TLRs immune genes in mammals.And,it would provide important reference for the future application of genetic engineering and codon optimization technique to increase the expression of foreign genes and to develop vaccines.