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Large, wax-covered colonies of the North American aphid Grylloprociphilus imbricator (Fitch) are known to last over several months on exposed twigs of American Beech (Fagus grandifolia Ehrhart). We hypothesized that the colonies could not persist for such a long period without defense against predators, and found that nymphs of the second generation attacked moth larvae that had been artificially introduced into the aphid colony. Nymphs of all four instars participated in the attack and stung the larvae with their stylets. Of 69 nymphs that attacked the larvae, 36 (52.2%) were 4th instar. Unlike older nymphs of other eriosomatines, wingpadded 4th-instar nymphs of G. imbricator were slender in shape with long legs, and actively walked around on the twig. This is the first report that wingpadded nymphs are the main defenders of an aphid colony. /// Se sabe que colonias grandes y lanudas del afido norteamericano Grylloprociphilus imbricator (Fitch) duran varios meses en ramitas expuestas de Fagus grandifolia Ehrhart. Formamos la hipótesis que las colonias no podrían durar por tanto tiempo sin defensa contra predadores, y encontramos que ninfas de segunda generación atacaron larvas de polilla que habían sido introducidas artificialmente a la colonia áfida. Ninfas de todos los cuatro instares participaron en el ataque y picaron las larvas con sus estiletes. De 96 ninfas que atacaron las larvas, 36 (52.2%) fueron 4toinstar. A diferencia de esos otros eriosomatinos, ninfas de 4toinstar \"wingpadded\" de G. imbricator fueron delgadas en forma con patas largas, y caminaron activamente en la ramita. Este es el primer reporte que ninfas \"wingpadded\" son los principales defendedores entre los áfidos.

The relative proportions of free amino acids as well as the amino acid compositions of hydrolysed unprecipitated peptides and hydrolysed whole carcasses were quantified for two aphid species: the gall-dwelling social aphid Pemphigus spyrothecae and the pea aphid Acyrthosiphon pisum. The whole-tissue amino acid profiles of the two taxonomically distant species had a surprisingly high level of correspondence. In contrast, when comparing the A. pisum profiles obtained in the current study to those obtained in an earlier study, major differences were identified. It is concluded that there are good prospects for developing an artificial diet for P. spyrothecae. There may also exist considerable scope for tailoring the existing diets of A. pisum to suit specialised populations which develop poorly on the standard diet. The amino acid profile of P. spyrothecae is the first such profile that has been reported for a gall-forming aphid.

Social aphid species provide ideal systems to study the ecological influences upon the evolution of sociality because they consist of discrete colonies which are entirely clonal and therefore devoid of any genetic conflict over altruistic behaviour. Although selfishness can be discounted as an obstacle preventing the evolution of altruistic defenders, the vast majority of aphid species are not social. To examine the key life-history and ecological characteristics that interact to facilitate social evolution, we designed a matrix population model based on the natural history of one of the unique aphid species with soldiers, Pemphigus spyrothecae. In addressing the life-history factors, our special interest was to examine the optimal trade-off faced by colonies that can increase their defence investment by producing defenders at birth and/or increasing the duration of the defensive stage. The level and period of exposure to predation and a declining colony birth rate were key factors that selected for social defence. The model demonstrated that, in species which have soldiers that can facultatively develop to make a direct contribution to colony fitness, temporal extension of the soldier stage is a key mechanism of increasing defence investment. This extension is particularly favoured when predation is high and the lifetime of a colony is long. An increase in production of defenders at birth was favoured when mortality due to predation was strongly biased towards defenders. The model suggests that, in species which have the defensive flexibility of choosing whether soldiers remain as such, there is little requirement for flexibility in the morph allocations made at birth. All these predictions were found to be fully compatible with the available empirical data.

The horned soldier aphids of the Cerataphidini, unlike most social insects that reside in nests, live on the open surface of plants. The lack of a nest and other obvious ecological correlates makes it unclear why secondary-host soldiers might have evolved. Here I present a molecular phylogenetic analysis of 32 species of the Cerataphidini, including 10 species from the genera Ceratovacuna and Pseudoregma that produce horned soldiers. The phylogeny suggests that horned soldiers evolved once and were lost once or twice. Most horned soldiers are a morphologically specialized caste and two species that have unspecialized soldiers are independently derived from species with specialized castes. The genus Ceratovacuna appears to have undergone a relatively rapid radiation. Mapping secondary-host plants and geographic ranges onto the phylogeny suggests that bamboos were the ancestral secondary-host plants and that the Asian tropics and subtropics were the ancestral geographic regions for the genera Astegopteryx, Ceratoglyphina, Ceratovacuna Chaitoregma, and Pseudoregma and possibly for the entire tribe. There is evidence for vicariant events that separate the tropical and subtropical lineages in all of the major lineages of the tribe and for dispersal of some lineages. Based on these results, I present hypotheses for the causes and consequences of horned-soldier evolution.

The genus Pemphigus comprises several species that produce soldiers (defensive morphs) in galls on the primary host. At the moment, it is unclear if host-alternating species also produce defenders on their secondary host. We therefore examined how P. bursarius morphs of the secondary host generations respond to predators to test whether they show defensive behavior. We further examined how this response compares with the antipredator behavior of soldiers in P. bursarius and P. spyrothecae occurring on the primary host. We performed two manipulative experiments using two predatory species to quantify the behavior of the different morphs in response to predators. In both experiments, secondary host morphs of P. bursarius showed no attacking behavior and antipredator behavior in these morphs was limited to escaping natural enemies by walking away. In contrast, the first instars of the primary host generations in both Pemphigus species showed attacking behavior and were capable of killing the predators.[PUBLICATION ABSTRACT]

Background Nuclear receptors (NRs) constitute a superfamily of transcription factors that regulate diverse biological processes. In insects, NRs not only govern essential physiological functions including metabolism, development, and reproduction, but also play pivotal roles in regulating caste differentiation and division of labor within social insect colonies. Pseudoregma bambucicola is a species of social aphid in which adults exhibit a specialized reproductive division of labor. This unique system produces first-instar nymphs and soldiers, which share an identical genetic background yet exhibit distinct morphological and behavioral traits. Although NRs exhibit pleiotropic regulatory capacities, their roles in the unique developmental patterns of P. bambucicola remain unclear. Results This study identified 21 NR genes based on the genomic data of P. bambucicola and analyzed the duplication and loss events of these genes through phylogenetic analysis. Additionally, differential expression of NR genes was analyzed using transcriptomic data. The TLL exhibited significant differential expression in adults with distinct reproductive behaviors, suggesting its involvement in the regulation of reproductive division of labor. E75 and HNF4 were found to be important for the post-embryonic development of soldiers. Furthermore, quantitative real-time PCR confirmed caste-specific expression patterns of HR4 and HR39 , indicating their potential involvement in morphological differentiation and developmental regulation among castes. Conclusions This study conducted bioinformatic identification of NR genes in the social aphid P. bambucicola , and investigated their potential roles in morphological differentiation and behavioral division through analysis of differential gene expression. The findings provide preliminary evidence for the functional significance of NR genes in social aphids, while offering novel insights for subsequent research exploration.

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.

Living in nests is an almost universal feature of eusocial animals. In some aphids, however, sterile soldier castes have evolved in open colonies without a nest. To clarify the factors promoting the evolution of eusociality in these colonies, we used newly developed microsatellite markers to compare the kin structure of the open colonies of two aphid species on bamboo: the non-eusocial colonies of Astegopteryx bambucifoliae and the eusocial colonies of Pseudoregma alexanderi on Dendrocalamus latiflorus.Our samples, from over 1000 hectares, contained 99 clones of A. bambucifoliae and 19 of P. alexanderi. Clonal mixing occurred in both species: average pairwise relatedness within a colony was 0.54 in A. bambucifoliae and 0.71 in P. alexanderi. Each clone of A. bambucifoliae occurred in a unique location, whereas those of P. alexanderi occurred in multiple locations and more than 90% of individuals came from just four clones. There was significant genetic variation among different colonies in the same clump (stem-cluster) in A. bambucifoliae but not in P. alexanderi, indicating that P. alexanderi colonies in a single clump are genetically homogenised, functioning as a large colony. In P. alexanderi, the proportion of sterile soldiers to normal first-instar nymphs was significantly different across the four clones.Our results indicate that the lack of input of migrants from the primary host and feeding on a large, stable host plant are important ecological factors that might favour the evolution of eusociality, enabling the production of genetically homogenised, large, and long-lived colonies. After eusociality evolves on the secondary host, the optimal strategy of soldier production might vary between different clones.Significance statementNest living has often been considered to be a necessary condition for the evolution of eusociality. In a small number of aphid species, however, sterile soldier castes have evolved in open colonies without a nest. To understand why these aphids are unique, we examined the kin structure and genetic relatedness of individuals within eusocial and non-eusocial open colonies of two aphid species on bamboo. We found that clonal mixing occurred in both species, but the eusocial colonies are more genetically homogenised, functioning as a large colony. Our results suggest that ecological conditions that promote genetically homogenised, large and long-lived colonies are important for the evolution of eusociality in these aphids. We propose that the open colonies of social aphids provide an ideal model system in which to study the evolution of altruism.

In the social aphid Nipponaphis monzeni, a unique gall-repairing behaviour has been known: when a hole is made on the gall, many soldier nymphs discharge body fluid on the breach, which promptly solidifies and plugs the hole. Here, we experimentally investigated the subsequent fate of repaired galls and their inhabitants. Irrespective of natural repair by soldier nymphs or artificial repair with adhesive, repaired galls survived significantly better than non-repaired galls. Within a month after repair, the plant tissue around the hole proliferated and sealed up the hole. Many soldier nymphs were localized at the hole area and extermination of inhabiting aphids by insecticides aborted the gall regeneration, indicating that the gall regeneration requires inhabiting aphids, wherein soldier nymphs are likely to play a major role. This study provides an unprecedented case of scab formation and wound healing, which occurs at an animal-plant interface: scab derived from insect body fluid promptly plugs damaged plant tissue and subsequently the insects actively stimulate regeneration of the plant tissue, whereby the compromised plant tissue recovers. We suggest that the novel system may have evolved in the aphid lineage through enhancement and recruitment of the pre-existing capabilities of haemolymph coagulation and gall formation.

Housekeeping and colony defense behaviors are crucial for social aphids, as they help maintain a habitable living environment and enhance their ecological adaptability. However, over the past decades, numerous studies have focused on housekeeping and colony defense behaviors in species living in primary hosts, but little attention has been given to the secondary host stage. This constrains a deeper understanding of the altruistic behavior of social aphids, as well as the ecological and evolutionary significance of such behavior. We employed indoor video recordings to document and analyze the behaviors displayed by the soldiers of the sugarcane wooly aphid, C. lanigera, on secondary hosts. C. lanigera soldiers continuously patrol around the colony to detect potential threats. When encountering potential threats or obstacles, soldiers actively initiate cleaning behavior. The soldiers use their frontal horns to disengage the hardened honeydew, corpses, or honeydew simulants (rock sugar) that are attached to the surface of host plant leaves. Subsequently, they transport these materials away from the colony using their frontal horns or forelegs, either discarding or flicking them directly. When soldiers identify obstacles—such as predator eggshells—as natural enemies, they attack them with their frontal horns. Our findings contribute to a broader understanding of altruistic behavior in social aphids and the evolutionary success of their sociality.