Effects Of Host Plant On Population Genetic Differentiation Of Panonychus Citri (McGregor) Inferred From ITS1 And Cox1 Sequences

The citrus red mite, Panonychus citri (McGregor) (Acari:Tetranychidae), has a worldwide distribution and is one of the most important citrus pests in many countries such as China, Japan, and USA. Apart from citrus, P. citri can also damage many crops and fruit trees, and seriously affect yield and quality of fruits. Owing to high reproduction capacities and rapid developments, the mites are easy to establish new populations in new environments. In this study, the intraspecitic variations of the ribosomal internal transcribed spacer 1 (ribosomal ITS 1) and mitochondrial coxl sequences of Panonychus citri collected from different host plants. The study was to know the population genetic structure and distribution mechanism of citrus red mite in association with its hosts and clarifythe role of host plants in genetic differentiation and population gene flow of citrus red mite. The study was supported by the Grants-in-Aid from the Ministry of Agriculture (nyhyzx07-057), Chongqing Natural Science Funds for Distinguished Young Scholars (CSTC,2009BA1042) and the Earmarked Fund for Modern Agro-Industry (Citrus) Technology Research System of China. The main results are summarized as follows:1 The collection of P. Citri populationsTo elucidate diversity of the citrus red mite, samples were collected in the Citrus Research Institute of Southwest University and Jiangjin District, Chongqing, China. Eight populations were collected from 8 representative host plants in citrus orchard and greenbelt. In the two localities, several different host plants were available for P. citri, denoted as STY, RC, YLKNM, WZMG, MLGSC, ZK, HJ and GH were collected from family Rutaceae genus citus (Citrus maxima, C. reticulate, C. Limon, C. unshiu, C. aurantium), Fructus Citri and Zanthoxylum bungeanum and family Osmanthus fragrans. Samples were collected and stored in -80℃or 95% ethanol until DNA extraction.2 Population genetic diversity of P. citri from different host plants based on ITS1 sequencesThe ribosomal internal transcribed spacer (ITS) is widely used in phylogenetic and genetic differentiation; especially ITS1 is used for analyzing the relationship among intraspecific, interspecies and different populations. In this study, more than 10 sequences for each population were sequenced by random.The length of the ITS1 sequences after alignment and trimming varied from 498 to 501 bp. The ITS1 sequences of P. citri were characterized by very high levels of diversity. Totally,100 individuals present 86 polymorphic sites,59 haplotypes. Haplotypes H50 is shared by YXK and GH population, meaning H50 may be interpreted as an ancestral or more ancient haplotypes than others.The nucleotide diversity (π) and haplotypes diversity (h) of YXK population (including ZK, HJ, YLKNM, WZMG, RC, MLGS and STY) is higher than GH population. The genetic diversity of YXK population is the highest among these populations, indicating that the rate of evolution of YXK population is higher than GH population.The genetic distance (p-distance) of the 8 populations ranged from 0.005 to 0.008. There is no significance among these populations. The results exhibiting these 8 populations close contact with each others. Arlequin 3.1 was used to calculate pairwise FST, gene flow (Nm), Mismatch distribution and neutrality test, in order to analyzing the host-plants influence over P. citri genetic structure. Populations of P. citri displayed genetic differentiation and low levels of gene flow (Nm< 1) between the other populations. AMOVA analysis suggests that the differentiation is no relevance to host-plants.3 The sequencing and comparison of P. citri coxl among different populationsThere is no differentiation among 7 populations of P. citri (collected from Rutaceae host plant) by ITS1 analysis. Thus, a total of 34 coxl sequences from 3 populations were sequenced. There are only 9 mutation sites in coxl sequence and these mutations are synonymous; 34 individuals present 2 haplotypes and haplotype H1 is shared by ZK and HJ populations. There is no gene flow (Nm=0) between other two populations, further more GH population differentiate from other populations (FST= 1,P< 0.05). The genetic differentiation is caused by genetic drift.4 Phylogenetic relationshipsCombining these two molecular markers, we found that both coxl and ITS1 sequences have a higher A and T content. There is no significantly genetic diversity of P. citri from Rutaceae host plants. Though the GH population has differentiated from other populations, all populations of P. citri were clustered in one clade. Pairwise p-distance suggests that colonization in the Osmanthus fragrans cannot be separated into a new species.