The Genetic Variation And Gene Flow On Geographical Populations Of Sitodiplosis Mosellana (Gehin)

The wheat midge, Sitodiplosis mosellana (Gehin), is a serious pest of wheat, outbreaking sporadically and distributing in the main wheat-growing region of China. It outbroke and plagued many times in history and made great losses in the wheat production. Forties and eighties of 20th century were its main outbroke periods. Recently about ten years, it still outbreaks and plagues at certain wheat fields and has a trend to extend their habit, having the potential threat to the wheat production of China. In this paper, the genetic diversity of 12 geographic populations of wheat midges was studied through sequence analysis of mtDNA COⅡ and ND4 gene and the PCR technique with repeat sequence primer, in order to analyze its population variation, dispersal and migration patterns and the level of genetic diversity in molecular level. This investigation will provide some essential information for understanding possible local adaptation and migration patterns of the pest, and might provide useful data for the pest forecasting and control in a large area.The results about mtDNA COⅡ and ND4 sequence analysis showed that: 1) 550bp DNA fragment was amplified from COⅡ gene and only 447bp can be reliably read for all 11 geographic populations. Among those analyzed sequences, there are 7 variable sites including 5 transitions, 2 transversions and no any delete and insert. Mean A%+T% content for the sequences was 78.6%, while mean G%+C% content was 21.4%. Through comparing pairwise differences among sequences, it was found that the maximum of nucleotide substitions was 6 and minimum was 1, defining 7 haplotypes designated as H1-H7. The haplotype diversity (Hd) of the wheat midge was 0.8782±0.0017. 2) 450bp DNA fragment was amplified from ND4 gene and only 327bp can be reliably read for all 43 individuals from 12 geographic populations. Among those analyzed sequences, there were 21 variable sites including 17 transitions and 4 transversions and no any delete and insert. Mean A%+T% content for the sequences was 75.6%, while G%+C% content was 24.4%. Through comparing pairwise differences among sequences, it was found that the maximum of nucleotide substitions was 8 and minimum was 1, defining 22 haplotypes designated as Hap1-Hap22. The sequence differences among haplotypes was 0.31%～3.06% (mean was 1.68%), the haplotype diversity (Hd) was 0.8782±0.0017.The phylogenetic tree of haplotype constructed by mtDNA COⅡ and ND4 gene showed that the phylogeny among haplotypes was closely related with geographical distribution. The haplotypes from the spring wheat region were clustered into one group, and most ones from the winter wheat region were clustered into another group, but the haplotypes in SC population in winter wheat region were found synchronously in each group, so distribution area of SC population become a transition place of haplotype distribution. The main factors that may influence the haplotype distribution were analyzed. It was concluded that the restricted gene flow and distributing pervasion were the main reasons that can effect the haplotype distribution of the wheat midge. The analysis of mismatch distribution of haplotype frequence indicated that the population of the wheat midge had undergone two large population expanding in history.The analysis results of genetic differentiation and gene flow about different geographic populations showed that Fst values within populations in winter wheat region and spring wheat region respectively were less than 0.33, so their genetic differentiations were very small and there existed more gene flow. But Fst values between winter wheat region populations and spring wheat region populations were larger than 0.33. This maybe was caused mainly by genetic drift. The Fst values between SC population and other populations were all less than 0.33. Those imply that there existed more gene flows between SC population and other populations. The gene flow model among different geographic populations of wheat midge should belong to "distance-isolation" model. That means gene f...