Meeting Abstract
Like many herbivores, leaf beetles (Coleoptera: Chrysomelidae) depend on a range of plant cell wall-degrading enzymes to break down ingested material for nourishment and development. Here, we highlight the role of an obligate endosymbiont in mediating this process in the leaf beetle Cassida rubiginosa. Our survey of the bacterial community associated with C. rubiginosa revealed a universal partnership with a γ-proteobacterial species, one that phylogenetically clusters alongside other nutritional mutualists of insects, including Buchnera (aphids) and Blochmannia (ants). Fluorescence in situ hybridization (FISH) revealed symbiont localization to be restricted to a pair of sac-like organs connected to the foregut, as well as, in females, a pair of accessory glands associated with the ovipositor. Symbiont vertical transmission is accomplished through the singular topping of a “caplet” on the exterior surface of each egg. Piercing the egg caplet disrupts the transmission cycle, resulting in symbiont-free beetles that suffer retarded growth and higher mortality. Genome sequencing revealed that the symbiont possesses a drastically reduced genome (257,000 bp). However, unlike its close γ-proteobacterial relatives, C. rubiginosa’s mutualist lacks all the necessary genes to synthesize essential amino acids or vitamins. Instead, we discovered genes involved in the synthesis and export of pectinase, an enzyme catalyzing the breakdown of pectin. Comparative transcriptomic analysis across different regions of the insect’s body revealed pectinase expression to be restricted to the symbiotic organs. Consistent with our expectation that the symbionts contribute towards host fitness by mediating the breakdown of pectin, agarose diffusion assays using protein extracts from the symbiotic organs revealed strong pectinolytic activity.
