Donaciinae symbiont genomes. Donaciinae

Symbiotic microbes in insects can provide novel metabolic capabilities that enable their host to access untapped nutritional resources, but may also constrain niche space by limiting the host’s adaptive potential. However, tracing evolutionary changes in the symbiont-provided benefits and linking them to ecological transitions in the host remains a major challenge. Here, we reconstructed functional changes in the evolutionary history of the symbiotic association between a group of aquatic herbivores and mutualistic bacteria, providing evidence that an initial symbiont-mediated adaptation to a novel niche was followed by constraints on host plant utilization upon losses of the symbionts’ enzymatic capabilities. By sequencing the symbiont genomes across 26 species of (semi-)aquatic reed beetles (Chrysomelidae, Donaciinae) spanning the four major genera, we show that the genome-eroded mutualists provide life stage-specific benefits to larval and adult beetles, respectively.