Symbiotic microbes can enable their host to access untapped nutritional resources but may also constrain niche space by promoting specialization. Here, we reconstruct functional changes in the evolutionary history of the symbiosis between a group of (semi-)aquatic herbivorous insects and mutualistic bacteria. Sequencing the symbiont genomes across 26 species of reed beetles (Chrysomelidae, Donaciinae) spanning four genera indicates that the genome-eroded mutualists provide life stage-specific benefits to larvae and adults, respectively. In the plant sap-feeding larvae, the symbionts are inferred to synthesize most of the essential amino acids as well as the B vitamin riboflavin. The adult reed beetles' folivory is likely supported by symbiont-encoded pectinases that complement the host-encoded set of cellulases, as revealed by transcriptome sequencing. However, mapping the occurrence of the symbionts' pectinase genes and the hosts' food plant preferences onto the beetles' phylogeny reveals multiple independent losses of pectinase genes in lineages that switched to feeding on pectin-poor plants, presumably constraining their hosts' subsequent adaptive potential.
Plant microbiomes have become one of the hottest topics in plant biology. Driven by the increased availability of metagenome sequencing methods, analyses of plant-associated microbiomes have been skyrocketing during the last decade. They have generally taken one of two main perspectives: (1) a focus on the microbiome itself, where researchers describe microbiome diversity and attempt to understand its drivers, or (2) a focus on the consequences of microbiomes, where researchers analyse effects of microbiomes on plants. We briefly discuss these two perspectives, and we argue that for both a genotype-by-environment (G x E) framework will be key for achieving a deeper and more general understanding of plant microbiomes.
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