Genetic Diversity Of Wheat Mite And Biological Characteristics Of Dominant Species In Major Winter Wheat In China

Under the circumstances of increasing population and rigid food demand,the slow crop yield improvement,and even continual natural disasters each year,how to further increase grain production,improve land use efficiency,and achieve sustainable development of agriculture was a major national requirements for food security.It was also a hot issue in the scientific research.Wheat was one of the three major food crops in the world,and it was a main food crop in China.As a worldwide pest mite,wheat mites had a wide range of distribution and many host plants.They were the major pest mites on winter wheat in China.Seriously affected China’s wheat yield and quality.In such a severe situation,clarifying the distribution and dominant populations of wheat mites in the main winter wheat regions of China are,analyzing the genetic structure and genetic differentiation of wheat mites in China,and the study of the occurrence patterns and biological characteristics of different species has theoretical value and practical significance in improving wheat yield.In this thesis,we focused on the three major winter wheat areas in China as research objects.Based on the mitochondrial COI gene and ribosomal ITS1 gene,we studied the distribution of specific species of wheat mites in three winter wheat region and the genetic variation of geographical populations.We used weather data to analyze the occurrence of wheat mites in Santai of the Sichuan and established their indoor populations to study their biology Characteristics,in order to analyze the ecological process and molecular mechanism of adaptation,expansion and catastrophe of wheat and wheat mite populations in China.The main conclusions were as follows:1.Based on sequence comparison of mitochondrial COI gene and ribosomal ITS1 gene and population phylogenetic analysis.The results showed that the mitochondrial COI fragment length was 657～658 bp of 577 wheat mite samples from 23 geographic populations;sequence alignment and phylogenetic tree analysis revealed that the winter wheat mite species divided into two species.P.major had reported in China for a long time,but P.tectus,which was first discovered in China in this study.P.major was the dominant population in China’s winter wheat area.It distributed in 21geographic populations in the Southwestern China（SW）,the Middle and Lower Yangtze Valleys（YV）and the Yellow and Huai Valleys（YH）,accounting for 75.91%of the total sample.P.tectus distributed in 11 geographical populations in three winter wheat regions,but they were mainly distributed in Rongxian（RX-SC）and Nanchong（NC-SC）in the Southwestern China（SW）,and Dangyang（DY-HB）and Luzhou（BZ-AH）and Changfeng（CF-AH）in the Yellow and Huai Valleys（YH）accounted for 80.74%of the 135 samples in 5 geographic populations.In addition,there are single species and compound species in different geographical population samples.The single species of P.major appears in 12 geographical populations,accounting for 57.45%of the total number of samples.A single sample of P.tectus only appeared in two geographic populations of Rongxian（RX-SC）and Ruicheng（RC-SX）,accounting for 5.55%of the total sample.Among the composite species populations,P.major accounted for 52.44 of the total samples and P.tectus accounted for 47.56 of the total samples.The ribosomal ITS1 gene sequence study found that the ITS1 fragment length of 382 wheat mite samples from 19 geographical populations was791～954 bp;phylogenetic tree analysis was consistent with the COI gene,which revealed that there are two species of wheat mite in China.P.major distributed in 17 geographical populations,accounting for 85.60%of the total samples.P.tectus distributed in 7 geographic populations,accounting for 14.40%of the total sample.In summary,this study collected from 23 geographical populations of 11 provinces（municipalities）in the winter wheat.P.major is the dominant population of 3 winter wheat regions in China.It is widely distributed in the Southwestern China（SW）,the Middle and Lower Yangtze Valleys（YV）and the Yellow and Huai Valleys（YH）.P.tectus is mainly dominated in Rongxian（RX-SC）,Nanchong（NC-SC）,Dangyang（DY-HB）,Luzhou（BZ-AH）and Changfeng（CF-AH）.2.In order to reveal the level of genetic diversity and genetic structure of wheat mites,the genetic structure of P.major and P.tectus geographical populations were compared by using the mitochondrial COI gene sequence and ribosomal ITS1 gene sequence.（1）Based on the COI gene sequence,a total of 438 P.major and 139 P.tectus COI gene fragments were obtained by sequencing,and 9 and 5 haplotypes were obtained,respectively.Multi-level molecular variation（AMOVA）analysis showed that P.major and P.tectus did not have significant genetic structure among the three geographic groups.P.major was between geographic populations（55.74%of the population within the population）and within the population（42.20%in the population）with a significant genetic variation.The intraspecific variation of P.tectus was88.43%,but its genetic variation was not significant.Neutral test and Mismatch curve analysis show that the wheat spider had not experienced population expansion in the near future.The degree of genetic differentiation was different between P.major and P.tectus paired populations.Most of the fixed index F_ST values of P.major paired populations are greater than 0.25,and some F_ST values are 1,indicating a high degree of genetic differentiation.For P.tectus,most of the F_ST values of the fixed index among the populations were less than 0.1,and the gene flow between the paired populations was mostly greater than 4,indicating that the inter-population communication is sufficient.This may relate to the small population of P.tectus and the low number of samples.（2）Based on the ITS1 gene sequence,P.major and P.tectus had 12 and 7 haplotypes.AMOVA analysis showed that there was no significant genetic structure between the two geographic populations of wheat mites.P.major had a significant genetic structure among the populations in the population,with a mutation rate of 73.67%.The mutation rate among the P.tectus populations is as high as 92.13,but its genetic variation was not significant,and it be caused by different sampling distributions and sample sizes.The results of the Neutral test and Mismatch curve analysis were consistent with the COI gene sequence,and the two wheat mites did not experience population expansion events in the near future.The fixed index F_ST value between P.major and P.tectus paired populations was only a few greater than 0.25,and the gene flow between most of the populations was greater than 4,indicating that the degree of genetic differentiation was low and communication was sufficient between the paired populations.Based on two molecular approaches,the fixed index F_ST values of P.major pairs were different,and the difference may relate to the rate of gene evolution and the number of samples.（3）Comprehensive analysis of two sequence fragments of COI and ITS1 showed that the two parthenogenetic wheat mite had higher genetic diversity.However,the genetic diversity of the dominant population P.major in each wheat region of China was higher than P.tectus.This research result provided genetic evidence for revealing that P.major was widely distributed in China and had strong adaptability to the environment.The two wheat mites had significant genetic differentiation between some populations,and the gene flow between some populations was relatively high.Analysis based on two molecular means showed that the level of genetic differentiation between P.major pair populations was higher than P.tectus,suggesting that control measures for the two wheat mites may be different.There was no significant genetic structure among geographical groups and geographical distance was not the main reason for genetic differentiation.Therefore,the genetic differentiation of the wheat mite population may relate to the variability and size of the niche,the genetic diversity is at the center of the species distribution.3.We used a field site survey to monitor the wheat mite population by three representative plots in the Santai.Based on the meteorological data,we analyzed the appearance period,peak period and recession period.Based on the mitochondrial COI gene sequence,we analyzed the dominant populations of wheat mites in Santai districts from 2019 to 2020.We conclude that four generations of wheat mites occurred in Santai Sichuan from 2019 to 2020.At the end of October,the first generation of the egg began to hatch.At the beginning of November wheat growed to the 1-2 leaf stage,the wheat mites climbed up the seedling and harmed them.The peak of the second generation was at the beginning of January of the following year,the average amount of mites of 100 wheat plants were about 239;the third generation peak was at the end of January;at the peak of fourth generation in early March,the average mites of 100 wheat plants in the three plots was about 712and the total mite of 100 wheat plants in plot 3 was as high as 1286.From March 10 to April 10,there were 15 days where the maximum temperature is higher than 20℃,from April 5th to April 8th,the temperature was higher than 25℃for 4 consecutive days,up to 27℃,29℃,30℃and 30℃,subsequent investigations found that the wheat mites in the field declined sharply after the high temperature event,so it was speculated that sustained high temperature stress was the key factor in the decline of the wheat mites population in the field.Mitochondrial COI gene sequence studies indicated that the dominant population of wheat mites was P.major in Santai,among the 69 samples tested,the proportion of P.major was 98.55%,the number of haplotypes in 68 samples of P.major was as high as 5,the haplotype diversity and nucleotide diversity were at a high level.This indicated that P.major had strong adaptability and high resistance to stress in the Santai.4.By establishing indoor populations of P.major,based on 5 different temperature gradient,it was found that P.major could normally develop and reproduce at 12℃,15℃,18℃and 21℃,but eggs could not hatch at 21℃.At 12℃P.major developmental duration of the longest,most fertility,average total fecundity 44.82;with an average total egg production of 15.15.The hatching rate of eggs at 12℃was 93.40%,and the survival rate of adult mites was 23.90%;the hatching rate of eggs at 21℃was 77.73%,and the survival rate of adult mites was 11.63%.P.major had a significant pre-molting interval,and the shortest period before the moulting was 1.17 days at 21℃,and the minimum period before the moulting was 3.57 days at 12℃.P.major intrinsic rate of increase and the finite rate at 15℃were the largest,at 12℃it had highest net reproductive rate.