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Abstract Background Aphis gossypii is a worldwide sap-sucking pest with a variety of hosts and a vector of more than 50 plant viruses. The strategy of wing polyphenism, mostly resulting from population density increasing, contributes to the evolutionary success of this pest. However, the related molecular basis remains unclear. Here, we identified the effects of postnatal crowding on wing morph determination in cotton aphid, and examined the transcriptomic differences between wingless and wing morphs. Results Effect of postnatal crowding on wing determination in A. gossypii was evaluated firstly. Under the density of 5 nymphs·cm− 2, no wing aphids appeared. Proportion of wing morphs rised with the increase of density in a certain extent, and peaked to 56.1% at the density of 20 nymphs·cm− 2, and reduced afterwards. Then, transcriptomes of wingless and wing morphs were assembled and annotated separately to identify potentially exclusively or differentially expressed transcripts between these two morphs, in which 3 126 and 3 392 unigenes annotated in Nr (Non-redundant protein sequence) database were found in wingless or wing morphs exclusively. Moreover, 3 187 up- and 1 880 down-regulated genes were identified in wing versus wingless aphid. Pathways analysis suggested the involvement of differentially expressed genes in multiple cellular signaling pathways involved in wing morphs determination, including lipid catabolic and metabolism, insulin, ecdysone and juvenile hormone biosynthesis. The expression levels of related genes were validated by the reverse transcription quantitative real time polymerase chain reaction (RT-qPCR) soon afterwards. Conclusions The present study identified the effects of postnatal crowding on wing morphs induction and demonstrated that the critical population density for wing morphs formation in A. gossypii was 20 nymphs·cm− 2. Comparative transcriptome analysis provides transcripts potentially expressed exclusively in wingless or wing morph, respectively. Differentially expressed genes between wingless and wing morphs were identified and several signaling pathways potentially involved in cotton aphid wing differentiation were obtained.

Abstract: Polyphenism constitutes a key example of the adaptability of a genotype to a shifting environment. In aphids, the relative production of apterous and alate individuals is influenced by cues resulting from increases in population density, changes in plant quality, and natural enemies. The ability to respond to multiple environmental cues offers aphids an effective way to optimize their fitness. Understanding the mechanisms behind alate morph production is particularly important in aphid species that are agricultural pests. We tested the effects of crowding and host plant quality cues on morph determination in Aphis glycines Matsumura (Hemiptera: Aphididae). We exposed A. glycines to pre‐natal and post‐natal crowding under laboratory conditions. In the field, we reared nymphs of A. glycines at high‐density vs. low‐density conditions in clip cages set on the top vs. bottom nodes of infested and uninfested soybean plants, Glycine max (L.) Merrill (Fabaceae). Apterous adult body size was used as an estimate of aphid performance, and the proportion of morphs developing under each condition was determined. Both pre‐ and post‐natal crowding induced the production of asexual alate individuals by A. glycines. In the field, we observed additive negative effects of lower overall plant quality (infested vs. uninfested plants), within‐plant differences in nutritional quality (bottom vs. top nodes), and high vs. low levels of crowding on aphid size. These three factors also interacted to increase the proportion A. glycines nymphs that developed wings. Our results suggest two levels in the production of asexual alate individuals by A. glycines: (1) a moderate level in response to aggregation, triggered by crowding cues, and (2) a high level in response to plant quality deterioration, triggered by both plant quality and crowding cues. These two processes ensure a flexible production of asexual alate individuals, allowing aphids to adapt to changing environmental conditions. [ABSTRACT FROM AUTHOR]

The aphid alarm pheromone is known to trigger wing induction in pea aphids (Acyrthosiphon pisum). In reaction to alarm pheromone, aphids drop off the plant or walk away. While searching for a new feeding site they repeatedly encounter other members of the aphid colony and this increased contact rate is assumed to be important for wing induction ("pseudo-crowding" hypothesis). Because the encounter rate is a function of aphid colony size, wing induction in aphids in the presence of a predator should be a function of the number of aphids on the plant. We placed two, seven or 13 adult pea aphids on bean plants, and exposed the different-sized colonies to synthetic alarm pheromone to test the density-dependence of predator-induced wing induction. The mean percentage of winged morphs among the offspring produced on the plants ranged from 10 to 80 percent and increased both with aphid number and exposure to alarm pheromone. There was no synergy between aphid number and alarm pheromone exposure indicating that both factors are additive. The implications for aphid metapopulation dynamics are discussed.

Pea aphids ( Acyrthosiphon pisum Harris; Hemiptera: Aphididae) exhibit transgenerational wing polyphenism, in which unwinged females produce genetically identical winged offspring in response to environmental cues such as overcrowding and predation risk that indicate poor habitat quality., Laboratory experiments were carried out to explore the intensity of the wing polyphenic response of pea aphids exposed to cues from ladybird predators and crowding, and their response was compared with pea aphids that were not exposed to any cues (control)., The study used cues from two different ladybird species - Coccinella septempunctata L. ( Coleoptera: Coccinellidae) and Hippodamia convergens Guérin- Méneville ( Coleoptera: Coccinellidae) - to investigate whether the wing polyphenic response of pea aphids to predator cues can be generalised., The intensity of the wing polyphenic response of pea aphids to crowding was found to be much stronger than their response to predator cues. There was no response to H. convergens cues and the response to C. septempunctata cues was mixed. [ABSTRACT FROM AUTHOR]

Genetic variation in anti-predator traits has been shown for a variety of species. Aphid alarm pheromone, ( E)-β-farnesene, is released by attacked aphids and causes a variety of behavioral defense reactions in the signal receivers. In pea aphids, Acyrthosiphon pisum Harris (Homoptera: Aphididae), ( E)-β-farnesene mediates the production of winged offspring in the presence of natural enemies. While variation in the propensity for pea aphids to produce winged offspring is well-documented, little quantitative information is available about clonal differences in ( E)-β-farnesene emission or the amount of alarm pheromone released in aphid colonies. We tested the wing induction response of four clones when attacked by a predatory lacewing larva, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae), and found that three of the four clones increased the proportion of winged offspring under predator attack. We then investigated the emission of aphid alarm pheromone of these clones of pea aphid under attack. Alarm pheromone emission in aphid colonies of initially 25 adults varied from 81.2 to 10 851.0 ng per aphid colony over 24 h. There were no differences between clones in total emission or in emission per consumption event. These results show that there is substantial variability in alarm pheromone emission within clones and that the propensity to produce winged offspring in some clones is not a simple function of the propensity of alarm pheromone production in these clones. [ABSTRACT FROM AUTHOR]

1. The production of winged morphs is a well known mechanism of induced defence in aphids to escape from natural enemies, and is also a reaction to poor resource quality. 2. Host plants of aphids often associate with endophytic fungi that have been shown to reduce the fitness of some species of aphids. 3. It was hypothesised that endophyte infection of host plants that represent a low quality plant resource should increase the aphid’s induced response to a predator because both low plant quality and predator presence represent a stronger cue for wing production than predator presence alone. 4. In a laboratory experiment, bird cherry-oat aphids Rhopalosiphum padi L. were exposed to the factors predator threat and endophyte infection and the effects of these factors on the proportion of winged morphs produced by the aphid colonies was analysed. 5. The presence of endophytic fungi strongly decreased aphid colony sizes. When a predator threat was present, all colonies on endophyte-free grasses produced winged morphs whereas only a few colonies were able to produce winged morphs on endophyte-infected grasses. However, these few colonies produced larger proportions of winged morphs than colonies on endophyte-free grasses. Without a predator threat, no colonies on endophyte-infected grasses produced any winged morphs. 6. These results show that aphids in stressed conditions and with reduced fitness will only invest in inducible defences when predators are present but are unable to produce winged morphs in response to endophyte presence. [ABSTRACT FROM AUTHOR]

The aphid alarm pheromone (E)-β-farnesene (EBF) is the major example of defence communication in the insect world. Released when aphids are attacked by predators such as ladybirds or lacewing larvae, aphid alarm pheromone causes behavioural reactions such as walking or dropping off the host plant. In this paper, we show that the exposure to alarm pheromone also induces aphids to give birth to winged dispersal morphs that leave their host plants. We first demonstrate that the alarm pheromone is the only volatile compound emitted from aphid colonies under predator attack and that emission is proportional to predator activity. We then show that artificial alarm pheromone induces groups of aphids but not single individuals to produce a higher proportion of winged morphs among their offspring. Furthermore, aphids react more strongly to the frequency of pheromone release than the amount of pheromone delivered. We suggest that EBF leads to a‘pseudo crowding’ effect whereby alarm pheromone perception causes increased walking behaviour in aphids resulting in an increase in the number of physical contacts between individuals, similar to what happens when aphids are crowded. As many plants also produce EBF, our finding suggests that aphids could be manipulated by plants into leaving their hosts, but they also show that the context-dependence of EBF-induced wing formation may hinder such an exploitation of intraspecific signalling by plants. [ABSTRACT FROM AUTHOR]

1. The presence of an across-species trade-off between dispersal ability and competitive ability has been proposed as a mechanism that facilitates coexistence. It is not clear if a similar trade-off exists within species. Such a trade-off would constrain the evolution of either trait and, given appropriate selection pressures, promote local adaptation in these traits.2. This study found substantial levels of heritable variation in competitive ability of the pea aphid,Acyrthosiphon pisumHarris (Homoptera: Aphididae), measured in terms of relative survival when reared with a single clone of the vetch aphid,Megoura viciaeBuckton (Homoptera: Aphididae).3. Pea aphids can move to new patches by either flying (longer distance dispersal) or walking (local dispersal) from plant to plant. There was considerable clonal variation in dispersal ability, measured in terms of the proportion of winged offspring produced, and ability to survive away from their host plant.4. Winged individuals showed longer off-plant survival times than wingless forms of the same pea aphid clone.5. There was no evidence of a relationship between clonal competitive ability and either measure of dispersal ability, although the power of the test is limited by the number of pea aphid clones used in the trial.6. However, there was a positive correlation between clonal fecundity and the proportion of winged offspring produced. Although speculative, it is suggested that clones that are more likely to either overwhelm their host plant or attract higher numbers of natural enemies as a result of having higher fecundity are more likely to produce winged morphs. [ABSTRACT FROM AUTHOR]

Transgenerational phenotypic changes, whereby offspring have an altered trait or a distinct alternate phenotype, frequently occur in response to increased maternal predation risk. The cotton aphid,Aphis gossypii(Glover), is unique, however, as offspring consist of four distinct phenotypes (“normal” light green apterae, “normal” dark green apterae, “dwarf” yellow apterae, and alatae), all with divergent life history patterns and resulting population dynamics. Here, we show that increased predation risk induces transgenerational phenotypic changes in cotton aphids. When exposed to search tracks from larval or adult convergent ladybird beetles,Hippodamia convergensGuérin-Méneville, cotton aphids produced greater numbers of winged offspring. In a subsequent experiment, apterous and alate individuals on clean plants were found to have primarily normal and dwarf offspring, respectively. We suggest that elevated predation risk may cause phenotypic changes in aphids over multiple generations, resulting in a more precipitous decline in herbivore populations than could be explained solely by increased predation rates. [ABSTRACT FROM AUTHOR]

Natural enemies not only influence prey density but they can also cause the modification of traits in their victims. While such non-lethal effects can be very important for the dynamic and structure of prey populations, little is known about their interaction with the density-mediated effects of natural enemies. We investigated the relationship between predation rate, prey density and trait modification in two aphid-aphid predator interactions. Pea aphids (Acyrthosiphon pisum, Harris) have been shown to produce winged dispersal morphs in response to the presence of ladybirds or parasitoid natural enemies. This trait modification influences the ability of aphids to disperse and to colonise new habitats, and hence has a bearing on the population dynamics of the prey. In two experiments we examined wing induction in pea aphids as a function of the rate of predation when hoverfly larvae (Episyrphus balteatus) and lacewing larvae (Chrysoperla carnea) were allowed to forage in pea aphid colonies. Both hoverfly and lacewing larvae caused a significant increase in the percentage of winged morphs among offspring compared to control treatments, emphasising that wing induction in the presence of natural enemies is a general response in pea aphids. The percentage of winged offspring was, however, dependent on the rate of predation, with a small effect of predation on aphid wing induction at very high and very low predation rates, and a strong response of aphids at medium predation rates. Aphid wing induction was influenced by the interplay between predation rate and the resultant prey density. Our results suggests that density-mediated and trait-mediated effects of natural enemies are closely connected to each other and jointly determine the effect of natural enemies on prey population dynamics. [ABSTRACT FROM AUTHOR]

Summary 1. Winged morph production in aphids is a phenotypic trait that has traditionally been seen as a response to unfavourable environmental conditions. The evidence to support this theory is reviewed and the ecological and evolutionary significance of the findings is discussed. 2. The common assertion of poor host-plant nutritional quality leading to increased production of winged morphs does not always apply, particularly when the host-plant quality is exceptionally poor. The available data are skewed heavily towards Myzus persicae, and for this species the dynamical change in plant quality appears to be important with respect to wing induction. 3. Crowding may be a less influential stimulus for wing induction as study methods approach natural conditions experienced by aphids on their host plant. 4. The growing evidence that interactions with other organisms can induce the production of winged morphs by aphid colonies is reviewed. In the case of natural enemies, such a response by an aphid colony may be regarded as induced defence. Wing induction may also act as a means of transmission for a virus or fungal pathogen. [ABSTRACT FROM AUTHOR]

1. Winged dispersal is vital for aphids as predation pressure and host plant conditions fluctuate. 2. Ant-tended aphids also need to disperse, but this may represent a cost for the ants, resulting in an evolutionary conflict of interest over aphid dispersal. 3. The combined effects of aphid alarm pheromone, indicating predation risk, and ant attendance on the production of winged aphids were examined in an experiment with Aphis fabae ( Homoptera: Aphididae) ( Scopoli 1763) aphids and Lasius niger ( Formicidae: Formicinae) ( Linné, 1758) ants. 4. This study is the first to investigate the joint effects of alarm pheromone and ant attendance, and also the first to detect an influence of alarm pheromone on the production of winged morphs in A. fabae. 5. After a period of 2 weeks, it was found that aphid colonies exposed to intermittent doses of alarm pheromone produced more winged individuals, whereas ant tending had the opposite effect. The effects were additive on a log scale, and ant attendance had a greater proportional influence than exposure to alarm pheromone. A tentative conclusion is that ants have gained the upper hand in an evolutionary conflict about aphid dispersal. [ABSTRACT FROM AUTHOR]