MULTILEVEL INTERACTIONS BETWEEN PLANTS, MICROBES AND INSECTS: ECOLOGICAL AND EVOLUTIONARY CONSTRAINTS UNDERLYING INTERACTIONS

The microbiota, including commensal, symbiotic and pathogenic microbes (bacteria, archaea, protists, fungi, viruses), has a great influence on the physiology and ecology of many multicellular organisms. So much so that many organisms should be treated as a unique biological system together with their microbiota (holobionts). Insects are one of the major focuses of microbiota studies since they constitute the most diverse animal class on earth, play a central role in many terrestrial ecosystems, include several economically important species and are easily manipulable in laboratory conditions. Phytophagous insects are particularly interesting in this context because of their ecological and economic importance, but also because many microbial taxa are fundamental in allowing the use of plant tissues as a food source (macromolecules digestion, detoxification, essential nutrients supply). The main aim of this thesis is to investigate the principal factors shaping the composition and diversity of the bacterial component of the microbiota in a group of phytophagous insects, the leaf beetles (Coleoptera: Chrysomelidae). This thesis is composed of three main studies. The first one, with a metabarcoding approach targeting two regions (V1-V2 and V4) of the bacterial 16S rRNA, characterize the bacterial microbiota of a selection of Euro Mediterranean species of Chrysomelidae and investigate the effects of the breadth of the diet spectrum in shaping its composition and diversity. It has been demonstrated that generalist species harbour a more diversified microbiota than specialists, and also evidences that this phenomenon can be interpreted both as an adaptive trait and as a result of environmental stochastic acquisition conveyed by diet, are provided. The second study, with a similar approach, explores the bacterial microbiota of seven species of Chrysomelidae from the same environment, with a focus on the differences between the microbiota of male and female insects. Males leaf beetles showed a richer microbiota than females, especially in the low-abundance transient component, and differences in the distribution of bacterial primary symbionts and reproductive manipulators have been also identified. This phenomenon may be related to constrains on the vertical transmission mechanisms acting differently in the two sexes. The third study report the preliminary results of a genomic approach applied to characterize two putative bacterial symbionts identified in three Eumolpinae species of leaf beetles. These results support the presence of widespread symbioses in Eumolpinae, possibly similar to the well-known symbioses of two other Chrysomelidae subfamilies (Cassidinae and Donacinae) where the symbionts are involved in providing essential nutrients and digestive enzymes.