Division of labor within psyllids: metagenomics reveals an ancient dual endosymbiosis with metabolic complementarity in the genus .

Heritable beneficial bacterial endosymbionts have been crucial for the evolutionary success of numerous insects by enabling the exploitation of nutritionally limited food sources. Herein, we describe a previously unknown dual endosymbiosis in the psyllid genus , consisting of the primary endosymbiont " Carsonella ruddii" and a co-occurring Enterobacteriaceae bacterium for which we propose the name " Psyllophila symbiotica." Its localization within the bacteriome and its small genome size confirm that is a co-primary endosymbiont widespread within the genus . Despite its highly eroded genome, perfectly complements the tryptophan biosynthesis pathway that is incomplete in the co-occurring . Moreover, the genome of is almost as small as 's, suggesting an ancient dual endosymbiosis that has now reached a precarious stage where any additional gene loss would make the system collapse. Hence, our results shed light on the dynamic interactions of psyllids and their endosymbionts over evolutionary time.