Are aphids the flamingos of the insect world?
A few weeks ago I discovered the hard way that green apple aphid (Aphis pomi), when squashed in large amounts (for science not cruelty), dye skin a yellowy green. My current study organism of choice, woolly apple aphid (Eriosoma lanigerum), however, is bright red when squashed. How is it that two organisms, feeding on the exact same plant, are two such different colours? Insects are a very diverse animal group in terms of pigmentation, but how do they get them?
We’ve all heard the urban myth that flamingos are pink because they eat shrimp, presumably because the cooked prawns we eat are pink. In fact, that isn’t a million miles from the truth, as the pink pigment, beta carotene, is found in algae in the water and within the shrimp that they eat. When flamingos hatch, they are grey but they break beta carotene into pigments which are deposited in their feathers and skin, giving their famous pink colour, which is unique to each flamingo.
On to aphids. The typical yellow-green aphid colour is created by the carotenoid lutein which they acquire from their diet. Different aphid colours occur with different dietary carotenoids. So far, so flamingo. The pea aphid (Acyrthosiphon pisum) shows differential red/green colouration, also determined by carotenoids, with aphids of different colours thought to have different proportions of carotenoids consumed. This colour differentiation is driven by the pressure to avoid predation from either a ladybird species or a parasitoid wasp. More red clones were present when there is high pressure from parasitoids, and more green clones when predation risk from ladybirds was high.
Despite the fact that most animals can’t make their own caroteinoids, relying solely on what they can get in their diets, pea aphids are different: they have been found to synthesise their own carotinoids. The carotenoid synthesis enzymes can be inherited from parent to offspring, which fits in with the pressure to avoid predation.
But why would this happen? The rest of the animal kingdom seems to be getting along fine relying on dietary carotenoids. The section of the aphid genome which encodes caroteinoid biosynthesis enzyme has been found to have fungal origin, meaning that at one point in the far distant past the genome of a fungus with the ability to biosynthesise carotenoids integrated into the aphid genome, giving it the genetic code for the enzyme.
This integration happened a long time ago in evolutionary history because the fungal gene has been found not only in multiple aphid species, but also in adelgids which are closely related to aphids, suggesting that the integration might have happened in a common ancestor of the two groups. The fungal genes continued to evolve alongside aphids; aphids which show different colour forms have more copies of this fungal genes than single-colour aphids.
So, what about the apple aphids? They feed on different parts of the plant; green apple aphid feeds on leaves but woolly apple aphid feeds on woody parts which might bring them into contact with different carotenoids. It may also be defensive: green apple aphids are ant attended which offers them some protection against predators and the green colouring might help protect them against parasitoids. Woolly apple aphids are red but have a protective wax coat which protects them from natural enemies, so it may have another purpose for them.
Even with an insect-centric bias it has to be said that there is a lot more going on with aphids than flamingoes!
