Larval growth rate is associated with the composition of the gut microbiota in the Glanville fritillary butterfly

The rapidly increasing body of literature on commensal microbiota has revealed a large phylotypic and functional diversity of microbes associated with vertebrates and invertebrates. In insects, the gut microbiota plays a role in digestion and metabolism of the host as well as protects the host against pathogens. In the study reported here, we sampled gut microbiota of the larvae of the Glanville fritillary butterfly (Melitaea cinxia). The larvae were collected from the field or reared in the laboratory. This butterfly has two host plant species, Plantago lanceolata and Veronica spicata, and the host plant species is known from previous studies to influence larval growth rate. However, our results demonstrate that about 50 % of the variation in larval growth rate can be attributed to the effect of the gut microbial composition plus the joint effect of microbiota and the host plant species, while host plant species alone makes no significant contribution. Our results support previous studies showing that diet influences the gut microbiota but, more unexpectedly, that the composition of the gut microbiota significantly influences larval growth rate. We suggest that host plant effects on larval growth and development observed in many previous studies may be mediated via the gut microbiota. While we measured the growth rate only in laboratory-reared larvae, the similarity of the gut microbial composition between samples from field-collected and laboratory-reared larvae suggests that the results can be generalized to natural conditions.