Studies On Genetic Differentiation And Gene Flow Of Ostrinia Furnacalis Populations Based On Multiple Molecular Markers And Wolbachia Infection Detection

Asian corn borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae) is one of the dominant pests in maize, Zea mays (L.) in China, which injures many other host plants except for maize, and its spatial distribution covers vast geographic distances throughout the country. There exist significant natural and geographical environment discrepancies among different corn regions. Due to high variety of postdiapause development period, voltinism diversity in the Chinese O. furnacalis populations from north to south is present from 1 to 7 generations per year. Based on these properties, it is assumed that the genetic divergency has already happened among different O. furnacalis geographic populations in China.Investigations of the O. furnacalis population genetic structure from China and North Korea using both microsatellite and mitochondrial DNA markers were performed in this research. Genetic differentiation and gene flow among geographic populations were detected. In this study we also investigated prevalence and diversity of Wolbachia infection in O. furnacalis populations and two larval parasitoids: Macrocentrus cingulum Brischke and Lydella grisescens Robineau-Desvoidy to explore the potential horizontal transimissions of Wolbachia between Asian corn borer and its parasitoids. The primary achievements of this dissertation are listed as follows:1. Four mitochondrial genes including COI, COII, ND1 and ND5 full length or fragments were amplified and analyzed with the nucleotide sequences. The results showed high genetic diversity in mtDNA genes of O. furnacalis populations. 108, 53, 72 and 37 haplotypes were observed in COI, COII, ND1 and ND5 respectively.2. Practical microsatellite system for O. furnacalis population studies was developed based on microsatellite loci reported in European corn borer, O. nubilalis. Six polymorphic SSR loci were estimated by GeneScan sequencing technology. Two loci showed deviation from Hardy-Weinberg equilibrium (P-value < 0.05). The genotypic data showed no significant linkage disequilibrium between pairs of loci (P > 0.05). The number of alleles ranged from 12 to 26 at different loci across each population. The mean Shannon's information index was 1.566, and the average observed (HO) and expected (HE) heterozygosity in all populations was 0.572 and 0.734 respectively. The mean allelic richness (AR) was 7.929 and the mean polymorphism information content (PIC) was 0.692. 3. Phylogenic trees reconstructed from the microsatellite data indicated that the Pyongyangpopulation from North Korea was significantly differentiated from the 31 geographic populations collected in China. Topological structure of the phylogenic tree demonstrated no geographic clustering was observed. There was low correlation between Fst / (1 - Fst) and geographical distance (r = 0.135, P = 0.0025).4. Analysis of molecular variance (AMOVA) results revealed that most of the genetic variation was within populations; less variation was detected among populations within each corn region and even lesser variation among the five corn regions, which suggested no obvious genetic differentiation among these corn regions. The degree of genetic divergence and gene flow varied within different corn regions. The relatively lowest Fst and highest degree of gene flow (Nm) was among populations in Huanghuaihai Summer Corn Region, while there detected significant genetic differentiation and relatively low Nm among populations in both North Spring Corn Region and Southeast Hilly Corn Region.5. Mismatch distribution analysis showed multimodal in O. furnacalis 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 Asian corn borer populations was at demographic equilibrium.6. Infection frequency of Wolbachia in populations of O. furnacalis ranged from 0~71.1% with an average infection rate at 10.5%. Nine strains of Wolbachia (wFur1-wFur9) were identified based on wsp sequences isolated in this insect species (GU166588-GU166596). Among these variants, wFur1-wFur7 belonged to Group-A and wFur8-wFur9 belonged to Group-B. wFur1, wFur2, wFur7 and wFur9 were prevalent types, and the rest strains were uncommon among O. furnacalis populations. Superinfection commonly appeared in individuals of O. furnacalis and the coinfection patterns were very complex. There was no obvious geographical pattern for the prevalence and distribution of the nine Wolbachia strains.7. Wolbachia infection rate was extremely low in M. cingulum populations (average infection frequency = 2.2%). Two variants isolated from M. cingulum were designated as wCin1 and wCin2 separately (GU166602 & GU166603). Whereas infection frequencies of Wolbachia in L. grisescens were remarkably high (average infection frequency = 95.7%). Three distinct strains were identified in L. grisescens, named as wGri1, wGri2 and wGri3 (GU166604-GU166606), separately. Superinfection was also a common infection pattern observed in the two parasitoid species.8. Phylogenetic evidences based on wsp gene strongly suggested that horizontal transmission events occurred between O. furnacalis and its larval parasitoids.