Genomic characterization of symbiont contributions in tortoise leaf beetles (Coleoptera: Cassidinae). Bacterial symbionts in Cassidinae leaf beetles

Numerous adaptations are gained in light of a symbiotic lifestyle. Here, we examined the streamlined partnership between Cassidinae leaf beetles and their pectinolytic Stammera symbionts to detail how small changes to the bacterium’s metabolic range can expand the digestive physiology and upgrade the adaptive potential of its host. Genome sequencing of 14 representative Stammera strains revealed high functional conservation, highlighted by the universal presence of polygalacturonase, a primary pectinase targeting nature’s most abundant pectic class, homogalacturonan. Given that conservation, we unexpectedly note the disparate distribution of rhamnogalacturonan lyase, a secondary pectinase hydrolyzing the pectic class rhamnogalacturonan I (RG-I). Consistent with the annotation of rhamnogalacturonan lyase in Stammera, corresponding beetles demonstrate an ability to monomerize RG-I to its most basic subunits, in contrast to cassidines whose symbionts lack the gene. Accounting for the omnipresence of RG-I in foliage and the increased digestive efficacy mediated by rhamnogalacturonan lyase, we propose the secondary pectinase as a key innovation for Cassidinae beetles, contributing to their specialization across a wider range of host plants. While polygalacturonase-supplementing Stammera associate with beetles restricted to feeding on angiosperms within the lamiid eudicot clade, cassidines endowed with symbionts additionally contributing rhamnogalacturonan lyase evolved specializations on lamiids, campanulids, and fabids.