A dual insect symbiont and plant pathogen improves insect host fitness under arginine limitation.

Some facultative bacterial symbionts are known to benefit insects, but nutritional advantages are rare among these non-obligate symbionts. Here, we demonstrate that the facultative symbiont enhances the fitness of its psyllid insect host, , by providing nutritional benefits. , an unculturable pathogen of solanaceous crops, also establishes a close relationship with its insect vector, , increasing in titer during insect development, vertically transmitting through eggs, and colonizing various tissues, including the bacteriome, which houses the obligate nutritional symbiont, supplies essential amino acids to its insect host but has gaps in some of its essential amino acid pathways that the psyllid complements with its own genes, many of which have been acquired through horizontal gene transfer (HGT) from bacteria.