A.ervi attacks a pea aphid, by Alexander Wild
In a British lab, a wasp has become (locally) extinct. And then, another wasp follows it into oblivion. That’s odd because these two insects are not competitors. They don’t attack one another, and they don’t even eat the same food. They do, however, remind us that it’s very hard to predict how the decline of one species will affect those around it.
Some consequences are obvious. If an animal goes extinct, its loss will cascade up and down the food web, so that its predators will suffer but its prey will probably thrive. But food webs are webs for a reason, rather than a set of isolated linear “food chains”. Consequences can ripple across, as well as up and down.
A black bean aphid is about to have a rough day. It has been targeted by a parasitic wasp, which lays several eggs inside its body. When the eggs hatch, the wasp grubs will try to eat the aphid from the inside out. If they succeed, the aphid will die, and the young wasps will burst from its corpse to find aphids of their own.
But the aphid isn’t necessarily doomed. There’s a chance that it will resist the attempt to usurp its body. If it does, the wasps will have done it a favour. When the mother wasp implanted its eggs, it also infected the aphid with bacteria that protect against parasitic wasps. It inadvertently vaccinated the aphid against its own kind.
Not Exactly Pocket Science is a set of shorter write-ups on new stories. It is meant to complement the usual fare of detailed pieces that are typical for this blog.
Chimp see, chimp do – back-scratching technique passes among disabled chimps
Tinka’s hands are paralysed. His fingers are permanently flexed, he can’t bend his wrists and to top it all off, he has a chronic skin condition. His body itches frequently and without dextrous hands, he can’t scratch himself properly. Fortunately, Tinka is an ingenious fellow. He uses his motionless hands to grab a liana (a thick, woody vine) and, stretching it taut, he rubs his itching body against it. Tinka’s a chimpanzee and he has found a way of getting to those hard-to-reach places, like a human towelling their back.
Tinka’s a member of the Sonso chimp community in Uganda, which has high rates of disability, inflicted by man-made snares. The snares were intended for duiker and bush pig, but with hundreds in the area, some chimps inevitably got caught. The snares have been removed but not before they inflicted permanent handicaps on a third of the Sonso chimps. But from this tragedy came an opportunity to study the spread of cultural traditions in wild chimps – an opportunity that’s been seized by Catherine Hobaiter and Richard Byrne form the University of St Andrews.
It’s clear that chimps can pick up new traditions from one another in captivity but their ability to do so in the wild is unclear. In natural conditions, it’s very hard to spot the birth of a new behaviour and to identify the individual who started it. It’s also difficult to work out if others are copying the innovator or just performing acts that were already within their repertoire. But Tinka bucks the trend. His liana-scratch technique is his invention. No other chimp in Sonso, or anywhere else in Africa, does the same thing. And Hobaiter and Byrne have found that at least 6 other chimps have taken up the technique, all of whom lived in the same area as Tinka.
There’s no element of active teaching here. After merely watching Tinka perform his special move, the apeing apes could do it themselves sometime later. Chimp see, chimp do. What’s more, many of these imitators aren’t handicapped and the technique doesn’t seem to offer them any benefits – they could just scratch or groom themselves if they wanted. Hobaiter and Byrne think that this is just a “behavioural fad”, reflective of the chimpanzee’s natural predilection for copying its peers.
Reference: PLoS ONE: http://dx.doi.org/10.1371/journal.pone.0011959
Aphids drop and roll when they detect mammal breath
For a colony of aphids, there can be few fates as humiliating as being suddenly eaten by a passing goat. Plant-eating mammals pose a great threat to plant-eating insects. One chomp can unwittingly take out an entire colony. But as hungry mammals get closer, they give off a warning –hot, wet breath. Pea aphids take this as a cue to stop, drop and roll – they let go of their plant en masse and fall away from the jaws of death.
Moshe Gish discovered this unusual strategy by allowing a goat to feed on alfalfa plants infested with pea aphids. He found that, at the last minute, two-thirds of the colonies dropped to the ground. When he flicked the leaves, only a quarter of the colonies dropped off and when he cast a shadow overhead, they did nothing. It was the animal’s breath that did it.
The aphids also drop when they sense predators like ladybirds, but they do so more quickly and consistently when they sense mammal breath. Gish could even make individual aphids abandon leaf by directing a puff of goat or human breath at them using a tube. By playing around with these artificial puffs, he found that it wasn’t the presence of any particular chemical that did the trick, but the combination of heat and humidity.
This is the first time that anyone has found an animal that defends itself against being accidentally eaten by plant-eaters. The ability to detect mammal breath isn’t unique to aphids though – mosquitoes use the same skill to find a blood meal, while other insects and arthropods respond to the hot and humid vapours by releasing toxic chemicals. But for the generally defenceless pea aphid, mammal breath means only one thing: drop or die.
Reference: Current Biology, citation to be confirmed
A ladybird larva is on the prowl on a witch hazel plant. The youngster is a voracious predator and it’s hunting for aphids. It seems to have found a bountiful feast – a swollen structure called a gall that houses an entire aphid colony. With so many meals in one place, the colony seems easy prey, but it has staunch defenders.
As the ladybird approaches, aphids pour out of the gall and grab the predator by their jaws and legs. It’s a suicide defence. The aphids secrete massive amounts of waxy liquid from their bodies, which quickly solidifies and glues the ladybird to the plant. Unable to walk or bite, the ladybird dies and the aphids go with it. In the video below, you can see what happens when one of these aphids is prodded with a needle.
There is more to these suicidal protectors that meets the eye. Keigo Uematsu and University of Tokyo found that all of them are ‘menopausal’. They are the parents of the other aphids in the gall but their reproductive days are long behind them. With no further opportunities to raise the next generation, their final role is to defend their offspring, with their lives if necessary.
Aphids, those sap-sucking foes of gardeners, come in a variety of colours. We usually think of them as green, but pea aphids sometimes wear a fetching red ensemble. That may not strike you as anything special; after all, lots of animals are red. But the aphid’s colour is unique in a couple of extraordinary ways.
The colour comes from pigments called carotenoids. Animals use them for all sorts of purposes; they act as antioxidants, and they contribute to red, orange and yellow colours. But the pea aphid is one of only a few known species (all aphids) that manufacture their own carotenoids; everyone else gets theirs from their food. But it’s the source of the pea aphid’s ability that’s truly remarkable – it stole the skill from fungi. By integrating fungal genes into its own genomes, it gained a superpower that almost all other animals lack.
These sorts of “horizontal gene transfers” go on all the time in bacteria, but they’re supposedly a rarity among more complex creatures like animals and plants. And yet, scientists have recently documented several examples of such transfers. Rotifers smuggle genes from fungi, bacteria and plants. “Space Invader” genes have jumped across animals as diverse as lizards and bushbabies. One bacterium, Wolbachia, has even inserted its entire genome into that of a fruit fly. And parasites can transfer their genes to humans.
In most of these cases, it’s unclear whether the imported genes are actually doing anything useful. But the story of the pea aphid, told by Nancy Moran and Tyler Jarvik, is very different. The colour of a pea aphid determines the predators that target it. Ladybirds (one of their major enemies) prefer to attack red aphids on green plants but parasitic wasps are more likely to lay their eggs in green aphids, to fatal effect. Colour clearly matters to an aphid, so here is a clear example of a transferred gene shaping an obvious trait in its new host and in doing so, shaping its evolution.
Viruses and bacteria often act as parasites, infecting a host, reproducing at its expense and causing disease and death. But not always – sometimes, their infections are positively beneficial and on rare occasions, they can actually defend their hosts from parasitism rather than playing the role themselves.
In the body of one species of aphid, a bacterium and a virus have formed a unlikely partnership to defend their host from a lethal wasp called Aphidius ervi. The wasp turns aphids into living larders for its larvae, laying eggs inside unfortunate animals that are eventually eaten from the inside out. But the pea aphid (Acyrthosiphon pisum) has a defence – some individuals are infected by guardian bacteria (Hamiltonella defensa) that save their host by somehow killing the developing wasp larvae.
H.defensa can be passed down from mother to daughter or even sexually transmitted. Infection rates go up dramatically when aphids are threatened by parasitic wasps. But not all strains are the same; some provide substantially more protection than others and Kerry Oliver from the University of Georgia has found out why.
H.defensa‘s is only defensive when it itself is infected by a virus – a bacteriophage called APSE (or “A.pisum secondary endosymbiont” in full). APSE produces toxins that are suspected to target the tissues of animals, such as those of invading wasp grubs. The phage infects the bacteria, which in turn infect the aphids – it’s this initial step that protects against the wasps.
Where there are plants, there are almost certainly aphids feeding on them. These ubiquitous insects are banquets for many predators, and some have evolved incredible defences against them. The cabbage aphid, for example, is a walking bomb.
Its body carries two reactive chemicals that only mix when a predator attacks it. The injured aphid dies. But in the process, the chemicals in its body react and trigger an explosion that delivers lethal amounts of poison to the predator, saving the rest of the colony.
The aphids’ chemical weapons are stolen from the plants they eat. Far from being sitting ducks, plants defend themselves from herbivores with a wide range of poisons. Cruciferous vegetables, for example, such as mustard, watercress and wasabi, use a group of chemicals called glucosinolates.
These are harmless on their own, but when they come into contact with an enzyme called myrosinase, the result is a violent chemical reaction that produces several toxic products. Among these is allyl isothiocyanate, the substance that gives mustard and horseradish their strong, pungent flavour.
Autumn is a time of incredible beauty, when the world becomes painted in the red, orange and yelllow palette of falling leaves. But there may be a deeper purpose to these colours, and the red ones in particular. In the eyes of some scientists, they aren’t just decay made pretty – they are a tree’s way of communicating with aphids and other insects that would make a meal of it. The message is simple: “I am strong. Don’t try it.”
During winter, trees withdraw the green chlorophyll from their leaves, and textbooks typically say that autumn colours are produced by the pigments that are left behind. That’s certainly true of yellows and oranges, but reds and purples are a different story.
They are the result of pigments called anthocyanins, which trees have to actively make. That uses up energy, which is lost to the tree when the leaf falls. An investment like that implies a purpose, and that’s what scientists have been trying to uncover.
Shortly before he died in 2000, the great William Hamilton (he of kin selection fame) suggested that autumn colours are a warning to insects. Many species, such as aphids, lay eggs in trees during autumn and their larvae feed off their host when spring arrives. That’s bad news for the tree, which defends itself with insecticidal poisons. Those that are particularly well-defended would benefit from advertising themselves as inhospitable hosts, and Hamilton suggested that they do this through red leaves.
Hamilton found some support for the idea – for example, he showed that trees that have the strongest autumn colours are also those that are plagued by the widest array of aphid pests. But his former student, Mario Archetti from the University of Oxford, has truly championed the theory and his latest findings provide the strongest support for it yet. They show that aphids avoid red-leaved apple trees, that they fare better on trees without them and that wild trees have far redder leaves than domesticated ones, which are less troubled by the challenges of insects.
Imagine that a massive hole appeared in a wall of your house, and you’d decided to fix it yourself. You head over to a DIY store and load up on plaster, tools and paint and look forward to many hard and tedious hours of work. If that seems like a chore, you might get some perspective by considering the plight of the gall aphid Nipponaphis monzeni. When holes appear in their homes, some unlucky individuals are tasked with repairing the damage using their own bodily fluids. They sacrifice themselves for the sake of some DIY.
Some species of aphids are heading towards the incredibly cooperative lifestyles of social insects like ants, bees and termites. They live in large hollow growths called galls, that sprout from the very plants whose sap they suck. The galls provide them with protection from predators, shelter from the elements and constant food. They are a precious resource indeed, and every colony of social aphids have a special caste of sterile individuals whose job it is to defend the gall and attack intruders. These are the soldiers.
But in a few species, such as N.monzeni, soldiers have a truly bizarre part-time job- they are suicide-plasterers. When their homes are breached, the soldiers leach their own bodily fluids onto the wounded area, mix it with their legs and plaster it over the hole. The liquids soon harden and within an hour, the gap has been plugged at the cost of the soldiers’ lives.
The aphids’ gall-repairing antics are remarkably similar to what happens when animals develop cuts and wounds. The fluid around the area clots and hardens to form a scab. This provides a temporary seal, that gives the surrounding cells enough time to grow, divide and restore the broken tissues. The exact same thing happens to the gall – the only difference is that its clots and scabs are provided not by the plant, but by the aphids it houses.
It’s a scene straight out of a horror film – you look around and see dead bodies everywhere. They haven’t just been killed either, they’ve been hollowed out from the inside-out leaving behind grotesque mummified shells. What would you do if you were confronted with such a macabre scene? Flee? Well, if you were an aphid, you’d probably just feel relieved and go about your business. Aphids, it seems, find security among the corpses of their peers.
Aphids, like almost all insects, are the targets of parasitic wasps that implant eggs inside their bodies. On hatching, the wasp grubs use the aphid as a living larder and eat their way out, leaving behind a mummified aphid-shaped husk.
These husks ought to be (quite literally) a dead give-away that parasites are afoot, valuable intel for any animal. But far from treating these bodies as a sign of danger, aphids actually see them as a reason to stick around. As Fievet says, “In human history, mummies had long been known to protect the dead; our study shows that in nature, mummies can also protect the living.”