What Makes Aphids Grow Wings When Crowded?

ByKen

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What makes aphids grow wings when crowded? In many aphid species the answer starts with the mother, not the young. A review of aphid wing morphs says that “Many aphids exhibit discretely different winged and unwinged morphs.” The switch to wings is linked to conditions in the mother’s environment, and crowding is one of the named triggers. This article follows the steps the sources describe, from touch to brain signals to hormones, using quoted text from two review articles.

Quick answer

  • Crowding is a trigger for winged daughters. “increased crowding” is listed among the environmental cues that lead mothers to produce winged daughters.
  • Touch starts the response. “Early work established that wing polyphenism is triggered by active tactile stimulation that happens in a crowded and/or resource-challenged social environment”
  • The brain relays the signal. “crowded pea aphids increase expression of the vesicular glutamate transporter 2 (ApvGluT2) specifically in the maternal brain.”
  • Hormones carry the decision. “ecdysone signaling controls morph determination in the pea aphid, with lower ecdysone levels causing more winged offspring.”

Two morphs, two strategies

A winged aphid and a wingless aphid are not simply the same animal with or without wings. The first review notes that “The morphs are distinguished not just by whether they have wings, but also by additional morphological differences, their behaviour and their life history.” The review explains that “Winged aphids of course have the ability of flight, which enables them to migrate to new host plants if aphid densities become high or if the quality of the host plant deteriorates.”

The wingless form pays a different cost. The review says that “winged aphids have lower reproductive output, producing less offspring overall and producing offspring that are generally smaller when compared with unwinged aphids.” It also notes that “Unwinged aphids have a faster development time and larger body size.” So the wings buy movement at the expense of reproduction, and the switch is a trade-off rather than an upgrade.

The same review places crowding in a list of cues. It says that “mothers producing winged daughters after they experience certain environmental cues such as declining food quality, increased crowding and increased interspecific interactions.” The list shows that crowding is one of several signals, along with poorer food and competition from other species.

Why the mother decides, not the nymph

A key question is which generation perceives the crowding. The first review gives a direct answer for the pea aphid: “Indeed, all experimental evidence in the pea aphid suggests that it is the mother that perceives cues such as crowding and transmits this information to her daughter embryos.” The review adds a test result that supports this view: “If nymphs are crowded, they give rise to no greater proportion of winged offspring than control groups.”

This means the signal passes from the adult female to the embryos she is carrying. The second review describes the same structure in asexual females, where “stress-inducing conditions result in winged daughters, whereas benign environments yield wingless offspring.”

Touch and the brain

The second review describes how crowding is sensed. It states that “Early work established that wing polyphenism is triggered by active tactile stimulation that happens in a crowded and/or resource-challenged social environment.” Touch from neighboring aphids is therefore part of the trigger, not only the number of aphids nearby.

The review then describes the nerve chemistry. It says that “Upstream neurotransmitter pathways, including glutamate and possibly dopamine, translate tactile crowding into endocrine responses.” In the pea aphid, one specific transporter is involved. The review reports that “crowded pea aphids increase expression of the vesicular glutamate transporter 2 (ApvGluT2) specifically in the maternal brain.” When this transporter is disabled, the response weakens: “Knockdown of ApvGluT2 or injection of the vGluT2 inhibitor significantly reduced winged offspring production.”

The review also reports a rescue experiment. It says that “The receptor agonist, which mimics increased glutamate signaling, rescued the ApvGluT2 phenotype, restoring winged offspring production.”

Hormones and the maternal signal

Hormones form the next link. The second review explains that “Research from the 1950s to 1970s on the aphid wing plasticity largely focused on JH as the key regulator of wing polyphenism.” JH stands for juvenile hormone. More recent work has shifted attention toward ecdysone. The review says that “Recent studies of ecdysone signaling have been fruitful, providing clearer evidence for its causal role in regulating the aphid wing plasticity.”

The decapitation experiments show that the mother’s head is part of the signal. The review describes aphids that were induced to make winged young and then decapitated: “they almost immediately switch from producing winged to wingless offspring.” From this result the review concludes that “there is a maternal, winged morph-promoting signal” that is “released from the mother’s head to promote wingedness”. The exact molecule is not named in the quoted text.

The review also covers insulin signaling. It describes “a peripheral connection between IIS and aphid wing polyphenism.” IIS stands for the insulin and insulin-like growth factor signaling pathway.

The whole chain

The second review sums up the process in one sentence: “Collectively, these studies outline a multi-step process, from environmental sensing to neuroendocrine integration, hormonal signaling, and epigenetic maintenance, that governs aphid wing plasticity.” Put simply, the chain runs from sensing the crowd, to nerve signals, to hormones, and then to the daughters’ wing development.

The review also states that the polyphenism is a stable pattern in asexual females: “live-bearing asexual females produce genetically identical fully winged or fully wingless daughters depending on a stressful or benign environment, respectively.” The same genetic mother can therefore produce either form, depending on the conditions she experiences.

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How we chose

The two sources are review articles on aphid wings. Their titles state the scope: “Aphid wing dimorphisms: linking environmental and genetic control of trait variation” covers how the environment and genes shape the two morphs, and “Aphid wing plasticity: hormonal and epigenetic control mechanisms” covers the hormonal and epigenetic control. Together they give the cues, the sensing route, and the hormone links that this article explains.

What we could not verify

The sources describe mechanisms in a few aphid species, mostly the pea aphid. They do not give a general rule that applies to every aphid on every plant, and this article does not claim one.

What the sources do not say

  • The sources do not give a crowding density or a number of aphids per leaf that triggers winged offspring.
  • The sources do not say how long a winged generation takes to appear after crowding begins on a given plant.
  • The sources describe the pea aphid and a few other species, so the findings are not a complete account of every aphid species.
  • The exact molecule behind the maternal winged-morph signal is not named in the quoted text.

Frequently asked questions

Do all aphids grow wings when crowded?

No. The first review says that “Among aphids, some species alternate between environmentally sensitive (polyphenic) and genetic (polymorphic) control of wing morph determination in their life cycle.” Crowding acts through the environmental route, so it does not explain every species.

Is touch or smell the trigger for winged aphids?

The sources point to touch. The second review says that “Early work established that wing polyphenism is triggered by active tactile stimulation that happens in a crowded and/or resource-challenged social environment.” The quoted text does not describe a smell-based trigger.

Why do winged aphids produce fewer young?

The first review explains the trade-off: “winged aphids have lower reproductive output, producing less offspring overall.” The first review says that “The trade-off between the morphs likely relates directly to where they invest their energy.”

Does the young aphid decide to grow wings?

The evidence in the first review points to the mother. It states that “If nymphs are crowded, they give rise to no greater proportion of winged offspring than control groups.”

Can a single aphid mother make both wingless and winged young?

Yes, depending on conditions. The second review says that “live-bearing asexual females produce genetically identical fully winged or fully wingless daughters depending on a stressful or benign environment, respectively.”

Sources

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