| Belonging to the Arachinida, Acari, Acariformes, Tetranychidae and Tetranychus, both the two-spotted spider mite Tetranychus urticae Koch and the carmine spider mite T. cinnabarinus (Boisduval) are widely distributed worldwide. The two spider mites have wide range of host plants, including fruit trees, flowers, vegetables, crops and weeds. They live aggregately on the undersurfaces of leaves to suck plant juice, causing damage to host plant and great loss to agriculture. Owing to their rapid development and high reproductive capacities, new geographical populations of these mites are established easily in other areas. In this study, we used the molecular marker of microsatellite to analyze the population genetic structure of T. urticae Koch and T. cinnabarinus (Boisduval) from China. This work will be helpful for understanding their invading approach and outbreak mechanism, laying a solid foundation for controlling their dispersal.1. The collection and rearing of experimental materialSamples of T. urticae Koch and T. cinnabarinus (Boisduval) used in this study were collected from seven and six geographical localities, respectively. Their collections cover an extensive geographical scale representing different regions and environments in China. Mites were then reared separately in the laboratory on leaves of Phaseolus vulgaris L., placed on a water-saturated tissue in a glass dish. All populations were kept in an illumination culture tank at 25±1℃, 60% R.H. and under L16:D8 conditions.2. The application of microsatellite markers in the studies of population genetic structure.Microsatellite, as a molecular marker, is an important tool in the study of population genetic structure. It has a lot of advantages such as extensive in eukaryotic genome, richness of polymorphism information content, codominant genetic marker, easily to be investigated. Because of many advantages, it has often been used in the fields of molecular ecology, population genetics, and so on. Microsatellite has been used in the population genetic structure of the mite by more and more researchers after successfully exploiting microsatellite locus in Tetranychus urticae Koch by Navajas in 1998 and 2002.The method of this experiment was as follows: (1) to amplify known loci successfully; (2) to investigate alleles and allelic genotype at every locus in every sample; (3) PCR products were separated on polyacrylamide gel and silver staining to showing bands; (4) different softwares were used to analyze parameters of population genetic diversity such as allelic frequency, genotypic frequency, allelic richness, heterozygosity, genotypic diversity, and so on. The study of population genetic differentiation includes genetic differentiation index, gene flow, genetic distance, and so on. Finally, the phylogenetic tree based on genetic distances between populations was established. Cluster analysis was performed using the UPGMA algorithm.3. The population genetic structure of Tetranychus urticae Koch from ChinaThe three microsatellite loci selected in this research were amplified successfully. Amplification results showed that a mean number 3.7 of alleles per locus, the average polymorphism information content (PIC) 0.5247. When the value of PIC is bigger than 0.5, the site is highly polymorphic. So these three loci as the genetic marker for the study of T. urticae Koch population genetic structure are practicable. Parameters standing for population genetic diversity is generally low in various geographical populations, these results illustrate the low level genetic diversity of T. urticae Koch populations. The rates of private allele, special genotype and monomorphic locus detected were 27.3%, 54.5% and 33%, respectively. The genotypic diversity of total samples was much higher than respective population. Average pairwise FST was 0.5695, when FST>0.25, population was divided extremely. The above results show that extreme division has happened to T. urticae Koch. Genetic differentiation between populations had a direct bearing on the geographical distance.The result of population genetic structure of T. urticae Koch showed two distinct characteristics: the lower level of population genetic diversity and extreme differentiation among populations. These results caused by intrinsic biological feature (such as mating system), distribution (such as habitat fragmentation, geographical isolation etc), natural selection, migration (gene flow), genetic drift, etc. Therefore, both inbreeding within populations and genetic drift of alleles have probably had a decisive effect on the population genetic structure of T. urticae Koch.4. The population genetic structure of T. cinnabarinus (Boisduval) from ChinaThe population genetic structure of T. cinnabarinus (Boisduval) displayed many similarities to T. urticae Koch. The overall sample study showed significant polymorphism, but all the geographical populations had generally lower levels of genetic diversity. There existed an extreme differentiation between populations (FST>> 0.25). However, the genetic differentiations among populations were disproportionate with geographical distance. The correlation analysis between the genetic differentiation and the geographical distance from the point view of statistics showed a low correlation coefficient between the geographic distance and the population division (R2 = 0.277; R2 = 0.2594), and a significant correlation (P<0.05). Therefore, the population genetic structure of T. cinnabarinus (Boisduval) also showed a lower level of genetic diversity and the extreme polarization characteristics between populations.5. The differences of population genetic structure between Tetranychus urticae Koch and T. cinnabarinus (Boisduval) from China.By comparing the population genetic structures of T. urticae Koch and T. cinnabarinus (Boisduval) from China, we found significant differences between them. Firstly, they had different levels of genetic diversity of populations in these two species. The overall level of genetic diversity of T. cinnabarinus (Boisduval) was significantly higher than that of T. urticae Koch, but the various geographical levels of genetic diversity in T. cinnabarinus (Boisduval) were oppositely lower than that of T. urticae Koch. Secondly, the degree of population differentiation between the two spider species was different. Average pairwise FST between geographical populations of T. cinnabarinus (Boisduval) was far higher than that of T. urticae Koch. This result demonstrated the level of population genetic differentiation of T. cinnabarinus (Boisduval) was much higher than that of T. urticae Koch. Thirdly, by comparing T. cinnabarinus (Boisduval) and T. urticae Koch collected in the same area; we could not find the reduction of the level of genetic differentiation due to the shortening geographical distances between different populations. Geographic isolation between populations wasn't the main reason for genetic differentiation; reproductive isolation could also result in differentiation among populations in the same area. Finally, phylogenetic analysis revealed that seven geographic populations of T. urticae Koch and six geographic populations of T. cinnabarinus (Boisduval) were gathered in different branches.In conclusion, by using microsatellite marker, we revealed population genetic stricture of T. urticae Koch and T. cinnabarinus (Boisduval) and differences between them. It will help us understand their ways of invasion and outbreak mechanism, and lay foundations for effectively controlling them. Additionally, these results will also be helpful for understanding their classification status, and offering the scientific evidence to homogeneous or heterogeneous theory.
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