Coexistence of Three Dominant Bacterial Symbionts in a Social Aphid and Implications for Ecological Adaptation

Simple Summary

Most insects are associated with a variety of symbionts that play a crucial role in insect life history. Symbiosis of aphids and their symbionts is a good model system to study insect–symbiont interactions. Pseudoregma bambucicola is a typical social aphid that lives parthenogenetically throughout the year on bamboos in subtropical areas, and it is the only aphid that exclusively feeds on the hard stalks of bamboo. In this study, we surveyed the symbiotic bacterial community associated with P. bambucicola. Our results showed that the diversity of P. bambucicola microbiome was low, but three symbionts, namely the primary endosymbiont Buchnera and two secondary symbionts (Pectobacterium and Wolbachia), were stable coexisting with a high infection rate. Combined with the biology of P. bambucicola, we speculate that Pectobacterium may help P. bambucicola feed on the stalks of bamboo, and Wolbachia may regulate the loss of sexual reproduction or has a nutritional role in P. bambucicola. These findings will advance our knowledge of the microbiomes of social aphids and set the foundation for further studies on the functional roles of P. bambucicola symbionts.

Abstract

Aphids are associated with an array of symbionts that have diverse ecological and evolutionary effects on their hosts. To date, symbiont communities of most aphid species are still poorly characterized, especially for the social aphids. In this study, high-throughput 16S rDNA amplicon sequencing was used to assess the bacterial communities of the social aphid Pseudoregma bambucicola, and the differences in bacterial diversity with respect to ant attendance and time series were also assessed. We found that the diversity of symbionts in P. bambucicola was low and three dominant symbionts (Buchnera, Pectobacterium and Wolbachia) were stably coexisting. Pectobacterium may help P. bambucicola feed on the hard bamboo stems, and genetic distance analysis suggests that the Pectobacterium in P. bambucicola may be a new symbiont species. Wolbachia may be associated with the transition of reproduction mode or has a nutritional role in P. bambucicola. Statistical tests on the diversity of bacterial communities in P. bambucicola suggest that aphid populations attended by ants usually have a significantly higher evenness than populations without ant attendance but there was no significant difference among aphid populations from different seasons.

Plant Manipulation by Gall-Forming Social Aphids for Waste Management

Many social aphids form spectacular galls on their host plants, in which hundreds to thousands of aphids thrive for several months or even for over a year. Here, in addition to colony defense against natural enemies, waste disposal is an important task for the gall dwellers to sustain their social life. In open galls, soldier nymphs actively clean colony wastes such as honeydew droplets, cast-off skins, and cadavers by pushing them with their head out of the gall opening. In the gall, the excreted honeydew is coated with aphid-derived powdery wax to form "honeydew balls," which prevents the aphids from wetting and drowning with their own excretion. How the aphids deal with the accumulated honeydew in closed galls has been a mystery. Here, we report a novel gall-cleaning mechanism: the gall inner surface absorbs and removes the liquid waste through the plant vascular system. Such a plant-mediated water-absorbing property is commonly found in aphids forming closed galls, which must have evolved at least three times independently. By contrast, the inner surface of open galls is wax-coated and water-repelling, and in some cases, the inner surface is covered with dense trichomes, which further enhance the water repellency. In conclusion, gall-forming aphids induce novel plant phenotypes to manage the waste problems by manipulating plant morphogenesis and physiology for their own sake. This review describes our recent studies on waste management strategies by gall-forming social aphids and discusses future directions of this research topic.

Exaggeration and cooption of innate immunity for social defense

Social insects often exhibit striking altruistic behaviors, of which the most spectacular ones may be self-destructive defensive behaviors called autothysis, "self-explosion," or "suicidal bombing." In the social aphid Nipponaphis monzeni, when enemies damage their plant-made nest called the gall, soldier nymphs erupt to discharge a large amount of body fluid, mix the secretion with their legs, and skillfully plaster it over the plant injury. Dozens of soldiers come out, erupt, mix, and plaster, and the gall breach is promptly sealed with the coagulated body fluid. What molecular and cellular mechanisms underlie the self-sacrificing nest repair with body fluid for the insect society? Here we demonstrate that the body cavity of soldier nymphs is full of highly differentiated large hemocytes that contain huge amounts of lipid droplets and phenoloxidase (PO), whereas their hemolymph accumulates huge amounts of tyrosine and a unique repeat-containing protein (RCP). Upon breakage of the gall, soldiers gather around the breach and massively discharge the body fluid. The large hemocytes rupture and release lipid droplets, which promptly form a lipidic clot, and, concurrently, activated PO converts tyrosine to reactive quinones, which cross-link RCP and other macromolecules to physically reinforce the clot to seal the gall breach. Here, soldiers' humoral and cellular immune mechanisms for wound sealing are extremely up-regulated and utilized for colony defense, which provides a striking case of direct evolutionary connection between individual immunity and social immunity and highlights the importance of exaggeration and cooption of preexisting traits to create evolutionary novelties.

Mechanisms regulating caste differentiation in an aphid social system

For evolution and maintenance of the social systems of insect colonies, caste production should be controlled in response to external cues so that caste ratio in the colony is kept at an optimal range. Recent developments using artificial diet rearing techniques have revealed an underlying mechanism for adaptive control of caste production in a social aphid, Tuberaphis styraci, which has a sterile soldier caste in the 2(nd) instar. Aphid density was the proximate cue that acts on 1(st) instar nymphs and embryos to induce soldier differentiation. The final determination of soldier differentiation occurred postnatally, probably at a late 1(st) instar stage. Direct contact stimuli from live non-soldier aphids mediated the density effect. While coexisting non-soldiers facilitated soldier differentiation in 1(st) instar nymphs, coexisting soldiers acted to suppress such differentiation. These results suggest that caste production in aphid colonies is controlled by positive and negative feedback mechanisms consisting of density-dependent induction and suppression of soldier differentiation. Here, we demonstrate the mechanisms that coordinate aphid society, and provide a striking case of clonal superorganism system where simple responses of colony members to local extrinsic stimuli are integrated into a highly organized regulation of the whole colony.