Some animal groups associate with the same vertically transmitted microbial symbionts over extended periods of evolutionary time, punctuated by occasional symbiont switches to different microbial taxa. Here we test the oft-repeated suggestion that symbiont switches are linked with host diet changes, focusing on hemipteran insects of the suborder Auchenorrhyncha. These insects include the only animals that feed on plant xylem sap through the life cycle, as well as taxa that feed on phloem sap and plant parenchyma cells. Ancestral state reconstruction provides strong statistical support for a xylem feeding auchenorrhynchan ancestor bearing the dual symbiosis with the primary symbiont Sulcia (Bacteroidetes) and companion symbiont 'β-Sym' (β-proteobacteria). We identified seven dietary transitions from xylem feeding (six to phloem feeding, one to parenchyma feeding), but no reversions to xylem feeding; five evolutionary losses of Sulcia, including replacements by yeast symbionts, exclusively in phloem/parenchyma-feeding lineages; and 14-15 losses of β-Sym, including nine transitions to a different bacterial companion symbiont. Our analysis indicates that, although companion symbiont switching is not associated with shifts in host diet, Sulcia is probably required for xylem-feeding. Furthermore, the ancestral auchenorrhynchan bearing Sulcia and β-Sym probably represents the sole evolutionary origin of xylem feeding in the animal kingdom.
Keywords: mutualism; mutualism loss; nutritional symbiosis; sap; symbiont switch; xylem.