Effects of Endosymbionts Wolbachia on mtDNA Variation and Evolution in Natural Populations of Tetranychu Urtica Koch (Acari: Tetranychidae) from China
|School||Nanjing Agricultural College|
|Course||Agricultural Entomology and Pest Control|
|Keywords||Wolbachia mtDNA variation Tetranychus urticae Koch Neutrality Test|
The two spotted spider mite Tetranychus urticae Koch, belonging to Arachnida, Acari, Acariformes, Tetranychidae, is a very important mite pest. The mite is also a worldwide economically pest damaging fruits, vegetables and greenhouse crops. Wolbachia is maternal inherited endosymbiotic bacteria that infect arthropods, and can induce various reproductive disorders, such as cytoplasmic incompatibility (CI), feminization, thelytokous parthenogenesis and male killing, which are beneficial to the stable and spread of Wolbachia in hosts.As Wolbachia is transmitted from the mother to her offspring, the mother’s mitochondrion is transmitted along with it. Mitochondrial DNA is a very useful marker, one of recent historical events because of its high evolutionary rate and an effective population size approximately one-quarter that of nuclear markers, and because it is relatively easy to be sequenced. Recent theories suggest that patterns of mtDNA variation within a species may be influenced by Wolbachia. The association between beneficial symbionts and their hosts can last for long periods, as indicated by cocladogenesis of symbionts and hosts. However, some theory predicts that mtDNA variation and evolution may be influenced substantially by endosymbionts which are co-inherited with the mitochondria.We investigated the effects of Wolbachia infection on mtDNA variation in spider mites by sequencing a portion of the mitochondrial cytochrome oxidase I (COI) gene from individuals of known infection status. Four Wolbachia strains were described in the current study, namely wUrtOril, wUrtOri2, wUrtOri3 and wUrtConl. Most of the populations were infected wUrtOril, which was the most common Wolbachia strain. Partial COI sequences were obtained from 198 individuals, comprising 88 infected and 110 uninfected specimens. Among these 198 sequences,69 polymorphic sites were observed and allowed us to distinguish 13 different mitochondrial haplotypes. There were eight haplotypes in infected individuals and 12 haplotypes in uninfected individuals. Of the 13 haplotypes, nine were shared amongst two or more of the populations, and four were found in four populations of T. urticae respectively. Haplol, the most common haplotype, was observed in the populations of five regions. HaploB was detected only in one population, whereas HaploF, the second most common haplotype, was dominant in three populations. As predicted, the haplotype and nucleotide diversity were lower in infected individuals than uninfected individuals. However, these mtDNA haplotype data is not entirely concordant with the wsp sequence data and both infected and uninfected individuals were found in the same haplotype.We used AMOVA to formally assess and test the association between Wolbachia and the mtDNA sequences. An analysis of molecular variance (AMOVA) clearly showed significant differentiation amongst haplotypes from the different geographical regions, suggesting that there is a strong natural barrier to gene flow amongst these populations. The differentiation among individuals within populations is the most important variance, and the percentage of variance is over 80%. AMOVA clearly showed significant genetic differentiation between the uninfected and the infected subgroup except for the wUrtOri3 and wUrtOri2/wUrtOri3 subgroups.Sequence data for the COI gene was used in our analyses of mtDNA molecular evolution. Haplotype and nucleotide diversity estimates calculated from the mtDNA sequence data for various groups of T. urticae. The molecular diversity (Hd,πand k) of the infected groups was significantly lower than the uninfected groups. Although the values of Tajima’s D and Fu & Li’s F were consistently less than zero for most infected groups, the McDonald-Kreitman tests suggested that the patterns of variation were different from those expected under neutrality in the single uninfected group. Thus, the neutrality tests do not show a clear effect of Wolbachia infection on patterns of mtDNA variation and substitution in spider mites.