Evolutionary dynamics of obligate endosymbiosis in the psyllid genus Cacopsylla.

Phloem-sucking hemipterans maintain obligate, heritable symbiotic bacteria to overcome nutritional deficiencies caused by their unbalanced diet. While some groups retain ancient primary symbionts, others supplement or replace them with additional symbionts conferring adaptive benefits. Here, we explore the diversity and evolutionary dynamics of multiple endosymbionts in psyllids of the genus Cacopsylla. While the primary symbiont Carsonella is found in all analysed species, the ancient co-primary symbiont Psyllophila is absent in Cacopsylla pyrisuga, which instead harbours a Sodalis symbiont with a larger genome, located in the syncytium of the bacteriome. Phylogenomic analyses demonstrated co-divergence of Carsonella and Psyllophila with their hosts, whereas the occurrence of closely related Sodalis across different psyllid lineages indicates several independent acquisitions. The presence of additional facultative Sodalis and Serratia symbionts further highlights the dynamic host-symbiont relationships in psyllids and their essential roles in insect niche adaptation and evolution.