Mitochondrial Genome Analyses Reveal A Significant Genetic Differentiation Between Northern And Southern Populations Of Chinese Bombyx Mandarina

The domestication of the domestic silkworm(Bombyx mori)for silk production is a great invention in ancient China,which has made a great contribution to human civilization.After nearly 200 years of scientific exploration,it has been concluded that the domestication event occurred in China and the wild ancestor of the domestic silkworm is the Chinese wild silkworm(Chinese Bombyx mandarina).However,the place where the domestication event occurred in China is still controversial,especially the lack of genetic evidence.The mitochondrial genome is an ideal marker to study the origin and differentiation of the domestic animals.Therefore,this paper performs a preliminary study on the genetic differentiation between northern and southern populations of Chinese B.mandarina based on the mitochondrial genome,and the origin of the domestic silkworm is also discussed.In the paper,the mitochondrial genome of B.mandarina Shenyang in northern China was determined,and the mitochondrial genome sequences of 58 silkworms available to date were comparatively analyzed.The phylogenetic relationship between northern Chinese B.mandarina(3 sequences),southern Chinese B.mandarina,Japanese B.mandarina and B.mori was reconstructed.The degree of the genetic differentiation between northern and southern populations of Chinese B.mandarina was also studied based on the CytB gene sequence.The main results are as follows:1.In the paper,the mitochondrial genome of B.mandarina Shenyang was reported.The mitochondrial genome of B.mandarina Shenyang was 15,682 bp in size,which was composed of 13 protein coding genes,22 tRNA genes,2 rRNA genes and an A+T-rich region(491 bp).The gene composition and order were consistent with those of other known mitochondrial genomes of silkworms.This mitogenome is the second mitochondrial genome of northern Chinese B.mandarina following B.mandarina Qingzhou.2.Mitochondrial genomes comparison revealed a distinctive differentiation between northern and southern populations of Chinese B.mandarina.The alignment result of 58 silkworm mitochondrial genome sequences clearly displayed four haplotype clades,corresponding to B.mori clade(41 sequences),Japanese B.mandarina clade(2 sequences),southern Chinese B.mandarina clade(12 sequences),and northern Chinese B.mandarina clade(3 sequences),respectively.The four haplotype clades of silkworm in the mitochondrial genome sequence showed distinctive sequence divergence,but highly sequence similarities within clade.3.Phylogenomics provided a solid evidence that the direct wild ancestor of the domestic silkworm is northern Chinese B.mandarina population.Phylogenetic analysis based on mitochondrial genome unveiled the nearest relationship between B.mori and northern Chinese B.mandarina.This result revealed that the direct wild ancestor of the domestic silkworm is northern Chinese B.mandarina,rather than well-studied southern Chinese B.mandarina.4.Comparison of the CytB gene sequences confirmed a significant genetic differentiation between northern and southern populations of Chinese B.mandarina.A total of 19 haplotypes were identified in 22 CytB gene sequences of Chinese B.mandarina(10 for northern population and 12 for southern population).The genetic variation among populations(84.95%)was significantly greater than that within populations(15.05%).The results of phylogeny and haplotype Network analysis both showed a significant geographical genetic structure between northern Chinese B.mandarina and southern Chinese B.mandarina.Our data indicated that a significant genetic differentiation has occurred between northern and southern populations of Chinese B.mandarina,and that the direct wild ancestor of the domestic silkworm is northern Chinese B.mandarina,which provide new insights into the origin and differentiation of the domestic silkworm.