Wolbachia in butterflies and moths: geographic structure in infection frequency.

INTRODUCTION: Butterflies and moths (Lepidoptera) constitute one of the most diverse insect orders, and play an important role in ecosystem function. However, little is known in terms of their bacterial communities. Wolbachia, perhaps the most common and widespread intracellular bacterium on Earth, can manipulate the physiology and reproduction of its hosts, and is transmitted vertically from mother to offspring, or sometimes horizontally between species. While its role in some hosts has been studied extensively, its incidence across Lepidoptera is poorly understood. A recent analysis using a beta-binomial model to infer the between-species distribution of prevalence estimated that approximately 40 % of arthropod species are infected with Wolbachia, but particular taxonomic groups and ecological niches seem to display substantially higher or lower incidences. In this study, we took an initial step and applied a similar, maximum likelihood approach to 300 species of Lepidoptera (7604 individuals from 660 populations) belonging to 17 families and 10 superfamilies, and sampled from 36 countries, representing all continents excluding Antarctica. RESULTS: Approximately a quarter to a third of individuals appear to be infected with Wolbachia, and around 80 % of Lepidoptera species are infected at a non-negligible frequency. This incidence estimate is very high compared to arthropods in general. Wolbachia infection in Lepidoptera is shown to vary between families, but there is no evidence for closely related groups to show similar infection levels. True butterflies (Papilionoidea) are overrepresented in our data, however, our estimates show this group can be taken as a representative for the other major lepidopteran superfamilies. We also show substantial variation in infection level according to geography - closer locations tend to show similar infection levels. We further show that variation in geography is due to a latitudinal gradient in Wolbachia infection, with lower frequencies towards higher latitudes. CONCLUSIONS: Our comprehensive survey of Wolbachia infection in Lepidoptera suggests that infection incidence is very high, and provides evidence that climate and geography are strong predictors of infection frequency.We thank the McGuire Center for Lepidoptera and Biodiversity, Florida Museum of Natural History for their continued support. This study was supported by the University of Florida Research Opportunity Seed Fund (ROSF) and the National Science Foundation grant number DEB-1354585 to AYK.This is the final published version. It first appeared at http://link.springer.com/article/10.1186%2Fs12983-015-0107-z

Rickettsia ‘In’ and ‘Out’: Two Different Localization Patterns of a Bacterial Symbiont in the Same Insect Species

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Horizontal transmission of the insect symbiont <i>Rickettsia</i> is plant-mediated

Bacteria in the genus Rickettsia , best known as vertebrate pathogens vectored by blood-feeding arthropods, can also be found in phytophagous insects. The presence of closely related bacterial symbionts in evolutionarily distant arthropod hosts presupposes a means of horizontal transmission, but no mechanism for this transmission has been described. Using a combination of experiments with live insects, molecular analyses and microscopy, we found that Rickettsia were transferred from an insect host (the whitefly Bemisia tabaci ) to a plant, moved inside the phloem, and could be acquired by other whiteflies. In one experiment, Rickettsia was transferred from the whitefly host to leaves of cotton, basil and black nightshade, where the bacteria were restricted to the phloem cells of the plant. In another experiment, Rickettsia -free adult whiteflies, physically segregated but sharing a cotton leaf with Rickettsia -plus individuals, acquired the Rickettsia at a high rate. Plants can serve as a reservoir for horizontal transmission of Rickettsia , a mechanism which may explain the occurrence of phylogenetically similar symbionts among unrelated phytophagous insect species. This plant-mediated transmission route may also exist in other insect–symbiont systems and, since symbionts may play a critical role in the ecology and evolution of their hosts, serve as an immediate and powerful tool for accelerated evolution. </jats:p

Means (± SE) of the bacterial/host gene copy ratio and statistical tests^a.

a<p>Values correspond to the mean number of symbiont gene copies ± SE (<i>16S rDNA</i> for <i>Portiera</i>, <i>dnaK</i> for <i>Hamiltonella</i> and <i>gltA</i> for <i>Rickettsia</i>) divided by the number of nuclear gene copies (<i>actin</i> gene). Kruskal-Wallis and Mann-Whitney nonparametric tests were performed with a probability level of significance of 0.05. Means marked with the same letter are not significantly different (Mann-Whitney test). For Kruskal-Wallis tests, p-values are indicated. Bold typeface indicates significant effects. <i>n</i> -number of individuals used for the quantifications.</p

Mean relative <i>Rickettsia</i> densities (±SE) (number of copies of the symbiont gene divided by number of copies of the host gene) were evaluated in terms of <i>gltA</i> copy number per number of <i>Bemisia actin</i> gene copies.

<p>Values correspond to the average of 13 to 20 individuals per line. Bars marked with the same letters are not significantly different (Mann-Whitney test, p = 0.05).</p