Population Genetic Structure Of Sesamia Inferens (Lepidoptera:Noctuoidae)

The pink borer, Sesamia inferens (Walker) (Lepidoptera:Noctuidae), is a major rice pest in Asia. According to the past researches, the north limit of S. inferens distribution is roughly in the north latitude 34 ° in our country. With the global warming, S. inferens was further dispersed to northern district. At the beginning of 21st century, S. inferens has become one of major rice pests with its damages increasing. In this study, we investigated population genetic structure of S. inferens from China using both microsatellite and mitochondrial DNA markers. By this, we analyzed genetic relationships of different populations and investigate the origin of S. inferens. We also investigated prevalence and diversity of Wolbachia infection in S. inferens populations. Through comparing the diversity of mtDNA from infected and uninfected S. inferens individuals, we would like to study that whether Wolbachia affect S. inferens mtDNA variation and evolution or not. The main results are as follows:1. Development and Characterization of polymorphic microsatellites from S. inferensThirty polymorphic microsatellite loci were developed and characterized by FIASCO (Fast Isolation by AFLP of Sequences Containing Repeats). Our results from two geographic locations (Yangzhou, Jiangsu & Guiyang, Guizhou) showed that the polymorphic information content (PIC) ranged from 0.181 to 0.947, with an average of 0.633, exhibiting high level of polymorphism). Moreover, the number of alleles per locus ranged from 2 to 31 with expected heterozygosities from 0.196-0.962 and observed heterozygosities from 0.043-1.000; meanwhile, 14 loci showed no significant departure from Hardy-Weinberg equilibrium after sequential Bonferroni’s correction at least in one population (P< 0.01); furthermore, loci CA194 & GT106, GA13 & TG52, GA13 & TG51, CA31 & CA43, GT16 & GT206 showed significant linkage disequilibrium. A preliminary analysis of mutation patterns in microsatellites was undertaken, and three types of microsatellite mutation patterns were described.2. Population genetic structure of S. inferens based on microsatellitesWe found high levels of genetic diversity in S. inferens from China. There were average 38.6667 alleles for nine microsatellite loci. Moderate population genetic diversity existed in S. inferens populations (0.05< FST< 0.15); most populations deviated significantly from the Hardy-Weinberg equilibrium due to heterozygote deficiency. Molecular variance analysis revealed that more than 85% of variation was among samples within populations. UPGMA dendrogram revealed that the entire S. inferens populations in China could be separated into two main clusters, populations from southern China cluster into one group; and other populations from eastern, central, western and northern China cluster into another group, which is in accordance with STRUCTURE analyses. However, topological structure of the phylogenic tree demonstrated no geographic clustering was observed, which suggested that there is no association between genetic diversity and geographic distance. Bottleneck effect showed that, under the IAM model, few populations had experienced a recent population bottleneck. However, the entire S. inferens populations are steady and more likely to have a tendency to increase.3. Population genetic structure of S. inferens based on mtDNAWe found high levels of genetic diversity in S. inferens from China. Moderate population genetic diversity existed in S. inferens populations (0.05< FST< 0.15). But several populations such as HBTS, SCCD, SCDZ and GXNN exhibited high genetic diversity (FST=1.00000). Moreover, BI phylogenetic tree of haplotype and NJ phylogenetic tree of all S. inferens individuals also suggested that the entire S. inferens populations in China could be separated into two main clusters, which is in accordance with above microsatellites analysis. Five population groups or regions could be identified (NC, CEC, CCW, SWC and SC) based on SAMOVA analysis. Analysis of molecular variance (AMOVA) results revealed that most of the genetic variation was within populations and within five groups; less variation was detected among populations, which suggested obvious genetic differentiation among these groups.Mismatch distribution analysis showed multimodal in S. inferens populations. In addition, results of Tajima’s D and Fu’s Fs neutrality tests (not significant, P> 0.05) implied that there might not be a population expansion in recent time and the S. inferens populations was at demographic equilibrium. Our study also showed that the divergence time of S. inferens was 93-97 million years ago.The distribution of the largest shared haplotype, the star-like haplotype networks of combined sequences, the trend of asymmetrical gene flow, approximate Bayesian computation approach, niche modelling analysis and phylogeographic variation of the borer all indirectly suggest that the borer may originate from reaches of the Yangtze River Basin.4. Detection and analysis of symbiotic bacteria, Wolbachia, in S. inferens and its effects on mtDNA variation and evolution of S. inferensOur study made a survey of the Wolbachia infection patterns of the pink stem borer, S. inferens (Walker), an important rice pest in China, based on the surface protein gene (wsp) and cell division protein gene (ftsZ). We found infections to be infrequent (8.2% of hosts) but genetically diverse (six strains), with mainly prevails in borer populations in southern China. Besides, we identified two new strains (wCam and wInf). Nucleotide indexes (haplotype diversity [Hd], nucleotide diversity [π] and number of variable sites [S]) in infected populations were not significantly different from that in uninfected group, and there was no association between Wolbachia infection strains and phylogeny of mtDNA haplotype. An analysis of molecular variance clearly showed little differentiation between infected or uninfected groups. The COI and Cytb genes did not deviate significantly from neutral evolution (Tajima’s D and Fu’s F values were not significant, P>0.05). All these results indicated that there was no effects of Wolbachia on host mtDNA variation and evolution. Furthermore, phylogenetic analysis revealed evidence of horizontal transmission of Wolbachia strains among insects within the rice-field community.