Intracellular endosymbiotic bacteria are found in many terrestrial arthropods and have a profound influence on host biology. A basic question about these symbionts is why they infect the hosts that they do, but estimating symbiont incidence (the proportion of potential host species that are actually infected) is complicated by dynamic or low prevalence infections. We develop a maximum-likelihood approach to estimating incidence, and testing hypotheses about its variation. We apply our method to a database of screens for bacterial symbionts, containing more than 3600 distinct arthropod species and more than 150 000 individual arthropods. After accounting for sampling bias, we estimate that 52% (CIs: 48-57) of arthropod species are infected with Wolbachia, 24% (CIs: 20-42) with Rickettsia and 13% (CIs: 13-55) with Cardinium. We then show that these differences stem from the significantly reduced incidence of Rickettsia and Cardinium in most hexapod orders, which might be explained by evolutionary differences in the arthropod immune response. Finally, we test the prediction that symbiont incidence should be higher in speciose host clades. But while some groups do show a trend for more infection in species-rich families, the correlations are generally weak and inconsistent. These results argue against a major role for parasitic symbionts in driving arthropod diversification.
IntroductionButterflies 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.ResultsApproximately 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.ConclusionsOur 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-015-0107-z) contains supplementary material, which is available to authorized users.
Genetic studies have long suggested the important role of the DRD2 gene in psychiatric disorders and behavior. Further research has shown a conjoined effect of genes in the Chr11q22–23 region, which includes the NCAM1, TTC12, ANKK1 and DRD2 genes, or NTAD cluster. Despite a growing need to unravel the role of this cluster, few studies have taken into account interspecies and evolutionary approaches. This study shows that behaviorally relevant SNPs from the NTAD cluster, such as rs1800497 (Taq1A) and rs6277, are ancient polymorphisms that date back to the common ancestor between modern humans and Neanderthals/Denisovans. Conserved synteny and neighborhood indicate the NTAD cluster seems to have been established at least 400 million years ago, when the first Sarcopterygians emerged. The NTAD genes are apparently co-regulated and this could be attributed to adaptive functional properties, including those that emerged when the central nervous system became more complex. Finally, our findings indicate that NTAD genes, which are related to neurogenesis and dopaminergic neurotransmission, should be approached as a unit in behavioral and psychiatric genetic studies.
Bacterial secondary symbionts are very common in terrestrial arthropods, but infection levels vary widely among populations. Experiments and within-species comparisons both suggest that environmental temperature might be important in explaining this variation. To investigate the importance of temperature, at broad geographical and taxonomic scales, we extended a global database of terrestrial arthropods screened for Wolbachia and Cardinium. Our final data set contained data from 114,297 arthropods (>2,500 species) screened for Wolbachia and 17,011 arthropods (>800 species) screened for Cardinium, including population samples from 137 different countries, and with mean temperatures varying from -6.5 to 29.2°C. In insects and relatives, Cardinium infection showed a clear and consistent tendency to increase with temperature. For Wolbachia, a tendency to increase with temperature in temperate climates, is counteracted by reduced prevalence in the tropics, resulting in a weak negative trend overall. We discuss the implications of these results for natural and introduced symbionts, in regions affected by climate change.
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