Division of labour within psyllids: Metagenomics reveals an ancient dual endosymbiosis with metabolic complementarity in the genusCacopsylla

Abstract

Hemipteran insects are well-known for their ancient associations with beneficial bacterial endosymbionts, particularly nutritional symbionts providing the host with essential nutrients such as amino acids or vitamins lacking from the host’s diet. Thereby, these primary endosymbionts enable the exploitation of nutrient-poor food sources such as plant sap or vertebrate blood. In turn, the strictly host-associated lifestyle strongly impacts the genome evolution of the endosymbionts, resulting in small and degraded genomes. Over time, even the essential nutritional functions can be compromised, leading to the complementation or replacement of an ancient endosymbiont by another, more functionally versatile, bacterium. Herein, we provide evidence for a dual primary endosymbiosis in several psyllid species. Using metagenome sequencing, we produced the complete genome sequences of both the primary endosymbiont ‘CandidatusCarsonella ruddii’ and an as yet uncharacterizedEnterobacteriaceaebacterium from four species of the genusCacopsylla. The latter represents a new psyllid-associated endosymbiont clade for which we propose the name ‘CandidatusPsyllophila symbiotica’. Fluorescentin situhybridisation confirmed the co-localization of both endosymbionts in the bacteriome. The metabolic repertoire ofPsyllophilais highly conserved across host species and complements the tryptophan biosynthesis pathway that is incomplete in the co-occurringCarsonella. Unlike co-primary endosymbionts in other insects, the genome ofPsyllophilais almost as small as the one ofCarsonella, indicating an ancient co-obligate endosymbiosis rather than a recent association to rescue a degrading primary endosymbiont.

Seasonal wild dance of dual endosymbionts in the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea): Endosymbiont titers tell two different stories

“…This resulted in the establishment of obligate co-diverging host-symbiont associations, accompanied by drastic reductions in the genome size of the symbiotic bacteria until only core housekeeping genes and biosynthetic pathways for the nutrients required by the insect hosts are retained [7][8][9] . Many sap-feeding hemipteran lineages, such as sternorrhynchans (aphids, adelgids, psyllids, scales, mealybugs) and auchenorrhynchans (planthoppers, spittlebugs, cicadas), are associated with more than one obligate endosymbiont [10][11][12][13][14][15][16][17][18][19][20][21][22][23] . In most dual endosymbiotic systems studied to date, the primary endosymbiont supplies the host with the majority of essential amino acids (EAAs), whereas the co-primary endosymbiont complements the genes or pathways that are no longer present in the primary endosymbiont [23][24][25][26][27][28][29] .…”

“…Many sap-feeding hemipteran lineages, such as sternorrhynchans (aphids, adelgids, psyllids, scales, mealybugs) and auchenorrhynchans (planthoppers, spittlebugs, cicadas), are associated with more than one obligate endosymbiont [10][11][12][13][14][15][16][17][18][19][20][21][22][23] . In most dual endosymbiotic systems studied to date, the primary endosymbiont supplies the host with the majority of essential amino acids (EAAs), whereas the co-primary endosymbiont complements the genes or pathways that are no longer present in the primary endosymbiont [23][24][25][26][27][28][29] .…”

“…The symbiosis between psyllids (Hemiptera: Psylloidea) and their maternally inherited primary endosymbiont "Candidatus Carsonella ruddii" (thereafter Carsonella) is well characterized 6,13,17,[48][49][50] . In many species, an additional endosymbiont co-occurs with Carsonella in the bacteriome 13,23,28 . Depending on the psyllid lineage, Carsonella is associated with the coprimary endosymbiont "Ca.Profftella armatura" (thereafter Profftella) in Diaphorina spp.…”

“…28,51,52 or "Ca. Psyllophila symbiotica" (thereafter Psyllophila) in several Cacopsylla species 23,53,54 . Carsonella synthesizes most (in the case of Cacopsylla spp.)…”

“…or all (Diaphorina spp.) EAAs for the host, while Psyllophila complements the production of the remaining EAAs and both Psyllophila and Profftella produce vitamins and carotenoids 23,28,52 . The most comprehensive study on the density of obligate endosymbionts in psyllids showed that the titers of both Carsonella and Profftella increased throughout the development and with the age of D. citri 55,56 .…”