If you’re pregnant with your sister-clone and haven’t been born yet…

Posted: June 22, 2015 in Uncategorized

I’m going to take a gamble here, and suggest that most of you reading this have never spared aphids that much thought. At a guess, you might be aware of them in one of two ways.

1) They are the things that gardeners do not like

2) They get eaten by ladybirds, which is why we like ladybirds.

In fairness, they don’t think of us much either, so don’t feel too bad. (Image from Wikimedia Commons)

Now, obviously, some people know a lot more about them but this, but such people are too busy using microscopes, wearing labcoats, and holding test tubes of bubbling green fluid up to the light and going “hmmm” to read this blog. So, in order to educate the general public more about aphids, and as yet another part of my overall mission to convince everyone that everything in nature is secretly absolutely awful, I have collected some facts about aphids that will make you appreciate them as something more than just bug food.

And by ‘appreciate’, I of course mean ‘recoil from in mild disgust’.

The first thing you should know about aphids is that there are a lot of them – some 4,400 species are known to exist. All are present in the family of insects known, surprisingly enough, as Aphididae. 4400 is also the name of a science fiction show I dimly remember from my childhood. Donning my tinfoil hat, I can state with certainty there is some diabolical and mysterious connection between these two facts. Of these, about 250 species are annoying enough to humans, in terms of destroying our plants, to be classed as ‘economically important’. Aphids are ‘true bugs’, members of the Hemiptera family, which we last saw wearing the desiccated corpses of ants and giving Charles Darwin chronic diseases. Slightly more scientifically, their mouthparts are modified so that they pierce their food and then suck up whatever fluid they feed on. In the case of aphids, this is mostly phloem fluid – plant sap, basically. This causes damage in two ways. Firstly, the plant loses important sugars and nutrients. Secondly, aphids are excellent viral vectors – its thought that some 275 plant viruses, many with names like Yellow Dwarf Virus and Tobacco Mosaic Virus – are transmitted by aphids; in a similar way to how blood born diseases are transmitted by sharing needles.

And for God’s sake don’t inject yourself with aphids. (Image credit: University of Minnesota),

Aphids can reproduce phenomenally fast, thanks in part to their reproductive cycles, which typically include elements of both asexual and sexual reproduction. Most species can reproduce asexually, so for a time the entire population will consist of females, all busily cloning themselves. Despite the fact that aphids are attacked by everything from parasitic fungi to ladybirds (one of which can eat 5000 aphids during their lifetime, according to this admittedly slightly biased source) to wasps which paralyse the aphid and then lay their eggs inside them, such a reproductive strategy means that, when times are good, the aphid population soars. One estimate stated that, if aphid reproduction was utterly unchecked, the aphid population within one year would be enough to cover the entire surface of the world to a depth of 150 km, suffocating all of man’s works beneath a mass of writhing, insect flesh. Only when conditions become less than optimal do males appear, presumably to allow sexual reproduction and thus increase genetic diversity.

“If you want an image of the future, Winston, imagine aphids, absolutely everywhere”. Image credit: Kent state university

What makes aphid reproduction truly weird, though, are ‘telescoping generations’. Basically, when a female aphid is pregnant, her offspring is developing inside her. That’s fair enough, its what happens in most humans, assuming, of course, you are actually human and not a vat grown android placed with unsuspecting human parents on the orders of the Shadow Government; ready to activate and commit unspeakable atrocities when a code word is broadcast over the television. But aphids are different; as another aphid is developing inside the daughter. So, basically, an aphid is born pregnant; and a mother will give birth to her granddaughter who is developing inside her daughter, who is also her identical twin sister, which, if nothing else, has the potential to cause really tense family arguments.

“So are you my mother, my daughter, my sister, my grandmother, or me?” Image credit: University of Minnesota

So, aphids have admittedly strange sex lives. But don’t we – I mean, err, a friend – all have weird sex lives? And amongst the invertebrates, where strategies like laying your eggs inside the living flesh of another animal are sadly all too common, a simple strategy like cloning yourself to produce an already pregnant granddaughter frankly comes as something of a relief. But where aphids may stand out is that they – maybe – trick plants.

Well, that’s what the evidence sort-of suggests, anyway. Briefly put, there are two main defence pathways in plants – one initiated by jasmonic acid, the other by salicylic acid. This makes sense – after all, if a plant is attacked by a fungus, the defence response needed will be different to if it got attacked by a virus, which in turn would be different if the Very Hungry Caterpillar turned up. Generally, bacteria and other pathogens tend to trigger a salicylic acid response, whilst animal attacks generally trigger a jasmonic acid response. Now, as you will hopefully be aware, aphids are insects, which are animals, which is why you are reading this on Horrific Animals of the World and not Horrific Bacteria of the World, Horrific Fungi of the world or, for that matter, A short history of Armenian Military Support of the Breakaway Azerbaijan Region calling itself the Nagorno-Karabakh Republic.

But it seems that when plants are attacked by aphids, both the salicylic acid and jasmonic acid pathways appear to be triggered – which is not what you would expect. Furthermore, it seems that generally, the salicylic acid pathway is not that effective against aphids, whilst the jasmonic acid pathway is. Another curious fact is that the two pathways are mutually antagonistic, generally speaking – which does make sense, as it means that plant does not divert resources towards a pathway that does. But the question is why are both pathways activated, when one appears, on the whole, not to be that effective against aphids – and in fact, may stop the more effective pathway from working at full efficiency.

Well, the theory goes this is actually what the aphids want. By promoting the less effective salicylic acid pathway, the jasmonic one is downregulated, and so the plant defences against aphids are downgraded.

(Of course, plant defence is an incredibly complex subject, and what’s true in one case may not be so in the other. There are literally thousands of chemicals, genes and proteins interacting with each other, added to which it can be quite hard to identify whether an effect is caused by X, or simply correlated with it. Ed.)

The fact that aphid excrement (called ‘honeydew’, because apparently aphids have one hell of a PR team) contains salicylic acid, and appears to dampen down the jasmonic acid pathway also adds support to this theory. And mechanically wounded plants infested with aphids have lower levels of jasmonic acid production than those not infested, which again suggests that aphids might ‘damp down’ that pathway – possibly also with the assistance of enzymes in their saliva which interfere with jasmonic acid production.

So, there you have it. Aphids can reproduce like crazy, have a mating system that wouldn’t look out of place on Jeremy Kyle, and ruthlessly use plant defences to gain benefits for themselves. They also might be able to harvest energy from light, and have weird and disturbing relationships with ants – which I’ll go into next time.

Still can’t stand up to a ladybird, though.

Unless that is what they WANT us to think.


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