It is dawn in a European forest, and gypsy moth caterpillars are looking for somewhere to hide. With early birds starting to rise, the caterpillars will spend the day in bark crevices or buried in soil. But one of them is behaving very strangely. While its peers head downwards, this one climbs upwards, to the very top of the highest leaves. It has come to die.
At the top of its plant, the caterpillar liquefies. Its body almost seems to melt. As it does, it releases millions of viruses, dripping them onto plants below and releasing them into the air. These viruses are the agents that compelled the caterpillar to climb, and eventually killed it. They are baculoviruses, and they cause a condition known aptly as Wipfelkrankheit – the German for “tree top disease”.
If “cyanide two-ways” sounds like an unappetising dish, you’d do well to stay clear of the bird’s-foot trefoil. This common plant flowers throughout Europe, Asia and Africa, and its leaves are loaded with cyanide. The plants are also often crawling with the caterpillars of the burnet moth, which also contain a toxic dose of cyanide
The poisons in the insect are chemically identical to those of the plant, and they are produced in exactly the same way. But both species evolved their cyanide-making abilities separately, by tweaking a very similar trinity of genes. This discovery, from Niels Bjerg Jensen at the University of Copenhagen, is one of the finest examples of convergent evolution – the process where two species turn up for life’s party accidentally wearing the same clothes.
Recently, several studies have shown that the convergence runs very deep. Many animals have hit upon the same adaptations by altering the same genes. Rattlesnakes and boas evolved the ability to sense body heat by tweaking the same gene. Three desert lizards evolve white skins through different mutations to the same gene. The literally shocking abilities of two groups of electric fish have the same genetic basis.
These cases are perhaps understandable, since the species in question aren’t too distantly related from one another. It’s perhaps more surprising to learn that bats and whales evolved sonar via changes to the same gene, or that venomous shrews and lizards evolved toxic proteins in the same way. But the cyanide-making genes of the trefoil and the moth take these disparities to a whole new level. Here is a case of convergent evolution between entirely different kingdoms of life!
This is an old article, reposted from the original WordPress incarnation of Not Exactly Rocket Science. I’m travelling around at the moment so the next few weeks will have some classic pieces and a few new ones I prepared earlier.
In the meadows of Europe, colonies of industrious team-workers are being manipulated by a master slacker. The layabout in question is the Alcon blue butterfly (Maculinea alcon) a large and beautiful summer visitor. Its victims are two species of red ants, Myrmica rubra and Myrmica ruginodis.
The Alcon blue is a ‘brood parasite’ – the insect world’s equivalent of the cuckoo. David Nash and European colleagues found that its caterpillars are coated in chemicals that smell very similar to those used by the two species it uses as hosts. To ants, these chemicals are badges of identity and the caterpillars smell so familiar that the ants adopt them and raise them as their own. The more exacting the caterpillar’s chemicals, the higher its chances of being adopted.
The alien larvae are bad news for the colony, for the ants fawn over them at the expense of their own young, which risk starvation. If a small nest takes in even a few caterpillars, it has more than a 50% chance of having no brood of its own. That puts pressure on the ants to fight back and Nash realised that the two species provide a marvellous case study for studying evolutionary arms races.
When hornworm caterpillars eat tobacco plants, they doom themselves with their own spit. As they chew away, a chemical in their saliva reacts with airborne substances that are released by the beleaguered plants. This chemical reaction sends out a distress signal that is heard and answered by the predatory big-eyed bug. The bug eats hornworm caterpillars. Drawn by the chemical SOS of plants under distress, it finds plenty to devour.
The night sky is the setting for an arms race that has been going on for millions of years: a conflict between bats and moths. Many bats can find their prey by giving off high-pitched squeaks and listening out for the echoes that return. This ability – echolocation – allows them to hunt night-flying insects like moths, which they skilfully pluck out of the air. But moths have developed countermeasures; some have evolved ears that allow them to hear the calls of a hunting bat and take evasive action. And bats, in turn, have adapted to overcome this defence.
Holger Goerlitz from the University of Bristol has found that the barbastelle bat is a stealth killer that specialises in eating moths with ears. Its echolocation calls are 10 to 100 times quieter than those of other moth-hunting bats and these whispers allow it to sneak up on its prey. It’s the latest move in an ongoing evolutionary dogfight and for now, the barbastelle has the upper wing.
The masked birch caterpillar creates its own home by weaving leaves together with silk. Once built, it vigorously defends its territory but, like many animals, it prefers to intimidate its rivals before resorting to blows. To display its strength and claim its territory, it drums and scrapes its jaws against the leaf. It also drags its anus across the surface to create a complex scratching noise. This “anal scraping” message seems utterly bizarre, but its origins lie in a far more familiar activity – walking.
Warding a rival off with your anus might seem unseemly to us, but caterpillars that do this turn out to be rather civilised species. The scraping is based on the same walking movements that their ancestors used to chase after rivals. The other parts of their signalling repertoire – drumming and scraping jaws – are ritualised versions of fighting moves like biting, butting and hitting. While their earlier cousins might resort to such fisticuffs, the anal-scrapers conduct their rivalries with all the restraint of Victorian gentlemen.
These signals and their evolution have been decoded by Jaclyn Scott from Carleton University. They a great examples of how ritualised animal communiqués evolve from much simpler actions that have little if anything to do with communication – walking, breathing, hunting and the like. Crickets, for example, sing by rubbing their wings together, which may originally have been done to release pheromones or to prep the wings for flight. The whistling of wind through the feathers of crested pigeons has turned into an alarm. The competitive knee-clicks of eland antelopes are made by tendons that slide as a natural part of their gait.
Often, these origins are hard to test and scientists need to be careful if they aren’t to rely on fanciful just-so stories. To avoid that, Scott analysed 36 species of caterpillars from two different families. Some of them had simple struts called “pro-legs” on their end segment, which they use to inch their way along. Other species lacked these structures and in their place, they had a pair of “anal oars” – thicker, harder, spatula-shaped versions of the caterpillar’s normal hairs. These are the instruments that the larvae use to scrape their leaves.
These two groups of caterpillars put their bums to different uses – walking and talking – but the movements they make are the same. They lift the anal segment forward, place it on the leaf and their push backwards against it. The big difference is that in the walkers, the end stays put and the front half launches forward, while in the talkers, the front stays attached and the bum moves backwards. When the masked birch caterpillar makes its anal scrapes, it is essentially talking by walking on the spot.
In autumn, as green hues give way to yellows and oranges, some leaves develop mysterious green islands, where life apparently holds fast against the usual seasonal decay. These defiant patches still continue the business of photosynthesis long after the rest of the leaf has withered. They aren’t the tree’s doing. They are the work of tiny larval insects that live inside it – leaf-miners.
The larvae were laid within the leaf’s delicate layers by their mother. They depend on it for shelter and sustenance, and they can’t move away. If their home dies, they die, so they have a vested interest in keeping at least part of the leaf alive. These are the miniature landscape architects that create the green islands, and they don’t do it alone – to manipulate the plant, they wield bacteria.
Wilfried Kaiser and scientists from Rabelais University discovered this partnership after realising that some bacteria and fungi can also cause green islands. He reasoned that microbes might be helping insects to achieve the same ends. So he searched for them in one particular species, a tiny moth called the spotted tentiform leaf-miner, Phyllonorycter blancardella. Its larva makes its home in the leaves of apple trees.
Kaiser found that the leaf-miners are host to just one detectable type of bacteria – Wolbachia. That’s hardly surprising. Wolbachia infects around 60% of the world’s insect species, making it a strong candidate for the title of world’s most successful parasite. Without exception, every leaf-miner that Kaiser tested, from all over the Loire Valley, carried Wolbachia in their tissues.
Not Exactly Pocket Science is a set of shorter write-ups on new stories with links to more detailed takes by the world’s best journalists and bloggers. It is meant to complement the usual fare of detailed pieces that are typical for this blog.
Geneticist sequences own genome, finds genetic cause of his disease
If you’ve got an inherited disease and you want to find the genetic faults responsible, it certainly helps if you’re a prominent geneticist. James Lupski (right) from the Baylor College of Medicine suffers from an incurable condition called Charcot-Marie-Tooth (CMT) disease, which affects nerve cells and leads to muscle loss and weakness.
Lupski scoured his entire genome for the foundations of his disease. He found 3.4 million placed where his genome differed from the reference sequence by a single DNA letter (SNPs) and around 9,000 of these could actually affect the structure of a protein. Lupski narrowed down this list of candidates to two SNPs that both affect the SH3TC2 gene, which has been previously linked to CMT. One of the mutations came from his father and the other from his mother. Their unison in a single genome was the cause of not just Lipson’s disease but that of four of his siblings too.
It’s a great example of how powerful new sequencing technologies can pinpoint genetic variations that underlie diseases, which might otherwise have gone unnoticed. The entire project cost $50,000 – not exactly cheap, but far more so than the sequencing efforts of old. The time when such approaches will be affordable and commonplace is coming soon. But in this case, Lupski’s job was easier because SH3TC2 had already been linked to CMT. A second paper tells a more difficult story.
Jared Roach and David Gallas sequenced the genomes of two children who have two inherited disorders – Miller syndrome and primary ciliary dyskinesia – and their two unaffected parents. We don’t know the genetic causes of Miller syndrome and while the four family genomes narrow down the search to four possible culprits, they don’t close the case.
For great takes on these stories and their wider significance, I strongly recommend you to read Daniel Macarthur’s post on Genetic Future, Mark Henderson’s piece in the Times and Nick Wade’s take in the NYT (even if he does flub a well-known concept). Meanwhile, Ivan Oranksy has an interesting insight into the political manoeuvres that go into publicising two papers from separate journals. And check out this previous story I wrote about how genome sequencing was used to reverse the wrong diagnosis of a genetic disorder.
Male moths freeze females by mimicking bats
Flying through the night sky, a moth hears the sound of danger – the ultrasonic squeak of a hunting bat. She freezes to make herself harder to spot, as she always does when she hears these telltale calls. But the source of the squeak is not a bat at all – it’s a male moth. He is a trickster. By mimicking the sound of a bat, he fooled the female into keeping still, making her easier to mate with.
The evolutionary arms race between bats and moths has raged for millennia. Many moths have evolved to listen out for the sounds of hunting bats and some jam those calls with their own ultrasonic clicks, produced by organs called tymbals. In the armyworm moth, only the males have these organs and they never click when bats are near. Their tymbals are used for deceptive seductions, rather than defence.
Ryo Nakano found that the male’s clicks are identical to those of bats. When the males sung to females, Nakano found that virtually all of them mated successfully. If he muffled them by removing the tymbals, they only got lucky 50% of the time. And if he helped out the muted males by playing either tymbal sounds or bat calls through speakers, their success shot back up to 100%. Nakano says that this is a great example of an animal evolving a signal to exploit the sensory biases of a receiver.
More on bats vs. moths from me
Reference: Biology Letters http://dx.doi.org/10.1098/rsbl.2010.0058
It’s not every day that you hear about spy missions that involve a lack of sex, but clearly parasitic wasps don’t pay much attention to Hollywood clichés.
These insects merge the thriller, science-fiction and horror genres, They lay their eggs inside other animals, turning them into slaves and living larders that are destined to be eaten inside-out by the developing grubs. To find their victims, they perform feats of espionage worthy of any secret agent, tapping into their mark’s communication lines, tailing them back to their homes and infiltrating their families.
Two species of parasitoid wasp – Trichogramma brassicae and Trichogramma evanescens – are particularly skilled at chemical espionage. They’ve learned to home in on sexual chemicals used by male cabbage white butterflies. After sex, a male coats the female with anti-aphrodisiac that turns off other suitors and protects the male’s sexual investment. These chemicals are signals from one male to another that say, “Buzz off, she’s taken.”
But the wasps can sense these chemicals. They feed on the nectar of the same plants that the cabbage white visit and when they do, the wasps jump her. They are tiny, smaller even than the butterfly’s eye (see the image below), and they hitch a ride to the site where she’ll lay her eggs. There, they lay their own eggs inside those of the butterfly. Amazingly, the wasps use the same trick for different species of cabbage white butterflies, which secrete very different anti-aphrodisiacs. They can even sense when the anti-aphrodisiacs are wafting among the general scent of a freshly mated female. It’s all part of a sophisticated “espionage-and-ride” strategy.
Walk through the rainforests of Ecuador and you might encounter a beautiful butterfly called Heliconius cydno. It’s extremely varied in its colours. Even among one subspecies, H.cydno alithea, you can find individuals with white wingbands and those with yellow. Despite their different hues, they are still the same species… but probably not for much longer.
Even though the two forms are genetically similar and live in the same area, Nicola Chamberlain from Harvard University has found that one of them – the yellow version – has developed a preference for mating with butterflies of its own colour. This fussiness has set up an invisible barrier within the butterfly population, where traits that would typically separate sister species – colour and mate preferences – have started to segregate. In time, this is the sort of change that could split the single species into two.
Heliconius butterflies defend themselves with foul chemicals and advertise their distasteful arsenal with bright warning colours on their wings. The group has a penchant for diversity, and even closely related species sport different patterns. But the butterflies are also rampant mimics. Distantly related species have evolved uncanny resemblances so that their warnings complement one another – a predator that learns to avoid one species will avoid all the ones that share the same patterns. It’s a mutual protection racket, sealed with colour.
The result of this widespread mimicry is that populations of the same species can look very different because they are imitating different models. This is the case with H.cydno – the yellow form mimics the related H.eleuchia, while the white form mimics yet another species, H.sapho.
How can we be sure that the pairs of butterflies that look alike aren’t in fact more closely related? For a start, scientists have shown that the frequencies of the yellow and white versions of alithea in the wild match those of the species they mimic. Genetic testing provides the clincher. It confirms that the two mimics are indeed more closely related to each other than they are to their models.
Genetics also tells us how alithea achieves its dual coats. Colour is determined by a single gene; if a butterfly inherits the dominant version, it’s white and if it gets two copies of the recessive one, it’s yellow. Pattern is controlled in a similar way by a second gene. These variations aside, there are no distinct genetic differences between the two alithea forms. They are still very much a single population of interbreeding butterflies.
But that may change, and fussy males could be the catalyst. Chamberlain watched over 1,600 courtship rituals performed by 115 captured males. Her voyeuristic experiments showed that yellow males strongly preferred to mate with yellow females, although white males weren’t so fussy.
This isn’t just a whimsical preference – Chamberlain thinks that the colour gene sits very closely to a gene for mate preference. The two genes may even be one and the same. Either way, their proximity on the butterfly’s genome means that their fates are intertwined and they tend to be inherited as a unit. That’s certainly plausible, for the same pigments that colour the butterflies’ wings also serve to filter light arriving into their eyes. A change in the way those pigments are produced could alter both the butterfly’s appearance and how it sees others of its kind.
To see what happens when this process goes further, you don’t have to travel far. Costa Rica is home to another H.cydno subspecies called galanthus, and a closely related species called H.pachinus. They represent a further step down the road that alithea is headed down. Galanthus and H.pachinus look very different because they mimic different models – the former has white wingbands reminiscent of H.sapho, while the latter has green bands inspired by H.hewitsoni.
Nonetheless, the two species could interbreed if they ever got the chance. Two things stand in the way. The first is geography – H.cydno galanthus stays on the eastern side of the country, while H.pachinus remains on the west. The second is, as with alithea, sex appeal. Males prefer females bearing the same wing colours as they do so even if the two sexes of the two species were to cross paths, they’d probably fly right past each other.
Genetically, these species have also diverged far further than the two forms of alithea have. They differ at no less than five genes involved in colour and pattern, two of which are practically identical to the ones that causing alithea to segregate. They also provide more evidence that the genes for colour and mate preference are closely linked, for crossbreeding the two species yields offspring with half-way colours and half-way preferences.
These butterflies are by no means the only examples of speciation in the wild. In this blog alone, I’ve discussed a beautiful case study of diversity creating itself among fruit flies and parasitic wasps, explosive bursts of diversity in cichlid fish fuelled by violent males, and a giant predatory bug that’s splitting cavefish into isolated populations.
But Heliconius butterflies may be the most illuminating of all these case studies. They’re easy to capture, breed and work with. And as Chamberlain’s study shows, they can marshal together the contribution of experts in genetics, ecology, evolution and animal behaviour in an effort to understand that most magnificent of topics – the origin of species.
Reference: Science 10.1126/science.1179141
More on speciation: