Conserved microbiota among young Heliconius butterfly species

BACKGROUND: Insects are the most diverse group of animals which have established intricate evolutionary interactions with bacteria. However, the importance of these interactions is still poorly understood. Few studies have focused on a closely related group of insect species, to test the similarities and differences between their microbiota. Heliconius butterflies are a charismatic recent insect radiation that evolved the unique ability to use pollen as a protein source, which affected life history traits and resulted in an elevated speciation rates. We hypothesize that different Heliconius butterflies sharing a similar trophic pollen niche, harbor a similar gut flora within species, population and sexes. METHODS: To test our hypothesis, we characterized the microbiota of 38 adult male and female butterflies representing six species of Heliconius butterflies and 2 populations of the same species. We sequenced the V4 region of the 16S rRNA gene with the Roche 454 system and analyzed the data with standard tools for microbiome analysis. RESULTS: Overall, we found a low microbial diversity with only 10 OTUs dominating across all individuals, mostly Proteobacteria and Firmicutes, which accounted for 99.5% of the bacterial reads. When rare reads were considered, we identified a total of 406 OTUs across our samples. We identified reads within Phyla Chlamydiae, found in 5 butterflies of four species. Interestingly, only three OTUs were shared among all 38 individuals (Bacillus, Enterococcus and Enterobacteriaceae). Altogether, the high individual variation overshadowed species and sex differences. Thus, bacterial communities were not structured randomly with 13% of beta-diversity explained by species, and 40 rare OTUs being significantly different across species. Finally, 13 OTUs, including the intercellular symbiont Spiroplasma, varied significantly in relative abundance between males and females. DISCUSSION: The Heliconius microbial communities in these 38 individuals show a low diversity with few differences in the rare microbes between females, males, species or populations. Indeed, Heliconius butterflies, similarly to other insects, are dominated by few OTUs, mainly from Proteobacteria and Firmicutes. The overall low microbial diversity observed contrasts with the high intra-species variation in microbiome composition. This could indicate that much of the microbiome maybe acquired from their surroundings. The significant differences between species and sexes were restricted to rare taxa, which could be important for microbial community stability under changing conditions as seen in other host-microbiome systems. The presence of symbionts like Spiroplasma or Chlamydiae, identified in this study for the first time in Heliconius, could play a vital role in their behavior and evolution by vertical transmission. Altogether, our study represents a step forward into the description of the microbial diversity in a charismatic group of closely related butterflies.