Bacterial Gut Symbionts Contribute to Seed Digestion in an Omnivorous Beetle

Authors: Lundgren, Jonathan; Lehman, R - Michael

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2010
Publication Date: 5/26/2010
Citation: Lundgren, J.G., Lehman, R.M. 2010. Bacterial Gut Symbionts Contribute to Seed Digestion in an Omnivorous Beetle. PLoS One. 5(5):e10831.doi:10.1371/journal.pone.0010831.

Interpretive Summary: Gut bacteria are important to the digestion of foods in many insects, and may allow them to diversify their diet. Carabid beetles are beneficial insects best appreciated for their ability to consume insect pests in cropland, but many species also eat weed seeds. We hypothesized that gut bacteria facilitate seed consumption in these abundant natural enemies. Harpalus pensylvanicus beetles were randomly assigned to one of five dietary treatments: untreated field populations, those fed weed seeds (lambsquarters; Chenopodium album), those fed seeds plus antibiotics (to remove or reduce their gut bacteria), those fed insect prey (eggs of house crickets; Acheta domesticus), and those fed insect prey plus antibiotics. Using sequenced clone libraries and tRF profiles, we described the bacterial communities present in the stomachs of beetles from these different treatments. We also examined the effects of antibiotics on seed and egg consumption levels. Finally, we correlated members of the bacterial communities with seed consumption rates, to identify bacteria that may be most influential to seed consumption. Nearly all bacteria found are commonly symbiotic with higher organisms, and there were none that were clearly obligate symbionts ubiquitously associated with H. pensylvanicus. The bacteria communities were fairly simple, and each beetle averaged 3-4 bacteria species. Antibiotics and laboratory diets were found to affect bacterial abundance and diversity in these beetle stomachs. Antibiotics was found to reduce seed consumption by the beetles, but prey consumption was similar in both dietary treatments. A stepwise general linear model revealed that the bacterium Enterococcus faecalis was significantly correlated with seed consumption rates. This symbiotic relationship may be one reason that H. pensylvanicus has been able to diversify its diet to include seeds.

Technical Abstract: Obligate bacterial symbionts alter the diets of host animals in numerous ways, but the ecological roles of facultative bacterial residents that consistently colonize insect guts remain unclear. Carabid beetles are a common group of beneficial insects appreciated for their ability to consume insect prey and seeds, but the contributions of microbes to diet diversification in this and similar groups of facultative granivores are largely unknown. Using 16S rRNA gene clone libraries and terminal restriction fragment (tRF) length polymorphism analyses of these genes, we examined the bacterial communities within the guts of the facultative granivores, adult Harpalus pensylvanicus (Carabidae), fed one of five dietary treatments: 1) an untreated Field population, 2) Seeds (aposymbiotic, i.e. fed antibiotics for 10 d to reduce gut bacterial populations), 3) Seeds (symbiotic), 4) Prey (aposymbiotic), and 5) Prey (symbiotic). Seeds of Chenopodium album (Chenopodiaceae) and prey of eggs of Acheta domesticus (Gryllidae) were used as food. The number of seeds and eggs consumed by each beetle were recorded after antibiotic treatment. Harpalus pensylvanicus possess a fairly simple gut community of approximately 3-4 bacterial operational taxonomic units (OTU) per beetle. These bacteria were affiliated with the Gammaproteobacteria, Bacilli, Alphaproteobacteria, and Mollicutes classes in decreasing order of abundance. Diet and antibiotic treatment had dramatic effects on the bacterial communities of the host. The field population and treatment fed seeds (symbiotic) had the closest matching bacterial communities according to the tRF analysis, and the communities in the two treatments administered antibiotics were more closely related to each other than they were to those of the symbiotic treatments. Antibiotics reduced and altered the bacterial communities found in the beetle guts. Beetles fed antibiotics ate fewer seeds, but this relationship was driven by only a portion of the beetles responded to the treatment. Those beetles that retained the bacterium Enterococcus faecalis consumed greater numbers of seeds than those lacking this symbiont. We conclude that the bacterium E. faecalis contributes to this factultative granivore’s ability to consume seeds as part of its diet.