Coevolutionary diversification between hosts and their nutritional symbionts in phytophagous hemipterans

Hemipteran insects feeding on plant sap rely on obligate microbial symbionts to compensate for the nutritional deficiencies of phloem and xylem sap. Because these bacteria are transmitted maternally over evolutionary timescales, Hemiptera have become a major model system for studying long-term host-microbe codiversification. Recent phylogenomic and comparative genomic studies have greatly expanded taxonomic coverage beyond classical model systems, revealing that ancient codiversification between hosts and nutritional symbionts is widespread across the order, but also that multipartite symbioses, recurrent symbiont acquisition, and symbiont replacement are far more common than previously appreciated. Here, we review current knowledge on the evolutionary dynamics of nutritional symbioses in plant-sucking Hemiptera. We discuss how genome reduction, metabolic complementation, and cellular compartmentalization contribute to the long-term stability of host-symbiont associations, and how horizontal gene transfer and repeated recruitment of novel microbial partners counteract the consequences of genome erosion. We also highlight major gaps in our understanding, including the limited taxonomic sampling across many hemipteran lineages, the ecological and evolutionary consequences of symbiont turnover, and the molecular mechanisms governing bacteriocyte development and symbiont integration. Integrating comparative phylogenomics with functional approaches will be essential for understanding how nutritional symbioses have shaped the diversification and ecological success of Hemiptera.