Abstract
BackgroundPatterns of mtDNA variation within a species reflect long-term population structure, but may also be influenced by maternally inherited endosymbionts, such as Wolbachia. These bacteria often alter host reproductive biology and can drive particular mtDNA haplotypes through populations. We investigated the impacts of Wolbachia infection and geography on mtDNA variation in the diamondback moth, a major global pest whose geographic distribution reflects both natural processes and transport via human agricultural activities.ResultsThe mtDNA phylogeny of 95 individuals sampled from 10 countries on four continents revealed two major clades. One contained only Wolbachia-infected individuals from Malaysia and Kenya, while the other contained only uninfected individuals, from all countries including Malaysia and Kenya. Within the uninfected group was a further clade containing all individuals from Australasia and displaying very limited sequence variation. In contrast, a biparental nuclear gene phylogeny did not have infected and uninfected clades, supporting the notion that maternally-inherited Wolbachia are responsible for the mtDNA pattern. Only about 5% (15/306) of our global sample of individuals was infected with the plutWB1 isolate and even within infected local populations, many insects were uninfected. Comparisons of infected and uninfected isofemale lines revealed that plutWB1 is associated with sex ratio distortion. Uninfected lines have a 1:1 sex ratio, while infected ones show a 2:1 female bias.ConclusionThe main correlate of mtDNA variation in P. xylostella is presence or absence of the plutWB1 infection. This is associated with substantial sex ratio distortion and the underlying mechanisms deserve further study. In contrast, geographic origin is a poor predictor of moth mtDNA sequences, reflecting human activity in moving the insects around the globe. The exception is a clade of Australasian individuals, which may reflect a bottleneck during their recent introduction to this region.
Highlights
Patterns of mtDNA variation within a species reflect long-term population structure, but may be influenced by maternally inherited endosymbionts, such as Wolbachia
Patterns of within-species variation in animal mtDNA are influenced by various factors, including mutation, selection, demography and geography, and analysis of haplotype diversity patterns can provide information on population structure and gene flow
Maternally-inherited intracellular Wolbachia bacteria are of particular concern, because they have been detected in hundreds of species [3] and are estimated to infect about 2/3 of all insect species [4]
Summary
Patterns of mtDNA variation within a species reflect long-term population structure, but may be influenced by maternally inherited endosymbionts, such as Wolbachia. These bacteria often alter host reproductive biology and can drive particular mtDNA haplotypes through populations. A single infection can be responsible for causing more than one phenotype, e.g. CI and male-killing [7] Wolbachia enhance their own spread through the host population by means of these phenotypes and, in so doing, drive the associated host mitochondrial haplotypes to high frequencies, causing dramatic changes in host mtDNA patterns [8]. Analysis of variation in insect mtDNA should take account of possible endosymbiont effects whilst attempting to uncover and explain patterns in terms of host ecology [2,8]
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