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Why Do Luna Moths Have Such Absurdly Long Tails?

You don’t need a field guide to recognise a luna moth. This large insect, found throughout the eastern half of North America, is unmistakeable. It has a fuzzy white body, red legs, feathery yellow antennae, and huge lime-green wings that can stretch up to 4.5 inches across. And at the end of its hindwings are a pair of long, streaming tails that can double the moth’s length.

In 1903, an entomologist named Archibald Weeks suggested that the tails direct predators away from the moth’s body. “Again and again may predator bat or bird, in an effort to capture a moth or butterfly, successively tear away sections of the tails, of which a sacrifice can be readily afforded, without disabling it or retarding its flight,” he wrote.

He was roughly right. More than a century on, Jesse Barber from Boise State University has shown that the luna moth’s tails are the equivalent of eyespots on fish and butterflies. These distinctive markings are typically found on dispensable body parts like tails and outer wings. They serve to draw a predator’s attention away from more vulnerable regions; better to lose a tail than a head.

Eyespots are visual defences, and bats—the main nemeses of moths—are not visual hunters. They find their prey with sonar—they make high-pitched squeaks and visualise the world using the rebounding echoes. To divert a bat, you need something that makes distracting echoes.

That, according to Barber, is what the luna moth’s tails do. They are “auditory deflectors”. Bat distractors.

Luna moth close-up. By Oliver Dodd. CC-BY-2.0
Luna moth close-up. By Oliver Dodd. CC-BY-2.0

Barber pitted luna moths against bats in a dark room, and filmed their encounters with infrared cameras. Under normal circumstances, the bats only managed to snag 35 percent of the moths. But if Barber cut off the insects’ tails beforehand, the bats caught 81 percent of them. That’s not because they become worse fliers—in fact, the tails don’t seem to affect their aerial abilities at all.

When bats aim their sonar at insects, they analyse the rebounding echoes for the distinctive signatures of beating wings. But the luna moths tails, which spin behind them as they fly, also produce echoes that resemble wingbeats. To the bat, they either sound like a very conspicuous part of their target, or like a different target entirely. As a result, they fumble their attacks.

When bats attack, they usually use their wings and tail to scoop an insect towards their faces, so they can deliver a killing bite to their victim’s body. But when bats attack luna moths, they aim about half their attacks at the tails. That’s a mistake—only 4 percent of those attacks succeed. Sometimes, the bat misses the moth entirely (see above). Other times, it bites off a tail while the moth escapes—down one inessential body part, and still alive (see below).

The tails also make the luna moths bigger, which might make them harder for the bats to handle and dispatch. But when Barber pitted bats against the polyphemus moth—an even bigger species that lacks tails—he saw that the predators killed 66 percent of their targets. The luna moths, despite being smaller, were harder to catch. “Clearly, tails provide an anti-bat advantage beyond increased size alone,” Barber wrote.

It’s possible that female moths also judge the health and quality of a male by looking at the size of his tails. But this doesn’t fit with the moths’ behaviour. Female moths spend most of their time hiding in protected nests and drawing males to them by releasing pheromones. They also mate with the first males they find, so there’s no evidence that they’re choosy—much less that they choose on the basis of tail length.

Luna moths belong to a group of large moths called the saturniids—a group that contains members like Copiopteryx and Eudaimonia, with even more extreme tails. By comparing the tail lengths of 113 saturniid species, Barber showed that these moths have evolved long tails on at least four separate occasions. He now wants to know if these other species are also good at foiling bats.

Reference: Barber, Leavell, Keener, Breinhoff, Chadwell, McClure, Hill & Kawahara. 2015. Moth tails divert bat attack: Evolution of acoustic deflection. PNAS http://dx.doi.org/10.1073/pnas.1421926112

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The Supergene That Paints a Liar

The females of the common mormon butterfly are masters of disguise. Some of them look like the black-and-white males, but others embellish their wings with white brushstrokes and red curlicues to mimic distantly related swallowtail butterflies that are toxic. By copying them, the delectable female mormons fool predators into thinking that they are similarly distasteful.

The picture below shows how varied these disguises can be. The right halves are all different butterfly species, and the left halves are all common mormon females.

The left halves are all females of the common mormon butterfly (Papilio polytes). The right halves are either common mormon males (top) or different species of swallowtails (bottom). Adapted from Kunte et al, 2014.
The left halves are all females of the common mormon butterfly (Papilio polytes). The right halves are either common mormon males (top) or different species of swallowtails (bottom). Adapted from Kunte et al, 2014.

British scientists Sir Cyril Clarke and Philip Sheppard studied these butterflies in the 1960s and, through cross-breeding experiments, showed that the insects never mix and match their patterns. You don’t get intermediate butterflies with red streaks from one pattern and white blotches from another. Instead, each pattern is inherited as one.

The duo reasoned that the patterns were controlled by a “supergene”—a cluster of genes that each control different parts of the wings, but are inherited as a single block. Imagine a row of switches that control the appliances in your house, but are taped together so that flipping one flips them all.

The supergene concept has been very influential and scientists have identified many such clusters in plants, snails, and other butterflies. But until now, no one had found the common mormon supergene. Krushnamegh Kunte from the Tata Institute of Fundamental Research in India finally did it and his results are a complete surprise—one that both confirms and refutes Clarke and Sheppard’s hypothesis.

Kunte, together with Marcus Kronforst at the University of Chicago, compared non-mimetic females that resemble males, or mimetic ones that look like the common rose swallowtail. They searched for parts of the butterfly’s genome that were linked to the mimetic patterns, and eventually homed in on a small region containing five genes. Four were similar in all the females, regardless of their patterns.

The fifth gene, known as doublesex, was another story.

Kunte and Kronforst found over 1,000 mutations separating the versions of the gene (alleles) in the mimetic and non-mimetic females. This astonishing variety was all the more surprising since doublesex has a reputation for consistency. It’s much the same in flies, beetles, ants, and every other insect group. And yet, in the common mormon—and only between the mimetic and non-mimetic females—this typically conservative gene was a hotbed of evolution.

The butterflies don’t switch their wing patterns by inheriting different versions of a cluster of genes, as Clarke and Sheppard suggested, but by inheriting different versions of doublesex. The supergene is not a collective, but a single gene. There isn’t a row of switches all taped together; there’s just one switch that controls everything.

The varying patterns of the common mormon butterfly (Papilio polytes). Credit: Wei Zhang
The varying patterns of the common mormon butterfly (Papilio polytes). Credit: Wei Zhang

The doublesex discovery is doubly surprising because this gene already has a well-defined role: it sends developing butterflies down either a male or female path. It’s like finding that the light in your bedroom also starts your car.

Kunte now wonders if scientists have been typecasting doublesex as a sexual differentiation gene, when it could potentially do much more.  “Throughout the animal kingdom, you see tremendously different males and females,” he said. Maybe this particular gene family, involved in sex determination throughout the animal kingdom, is also involved in making deer antlers or peacock tails.”

It’s still not clear how doublesex alleles produce the patterns on the butterfly’s wings. With a thousand mutations to look at, the team understandably had some trouble matching these to specific wing elements. Still, they found some hints about how the gene does its thing.

When genes are activated, the instructions encoded in their DNA are first converted into a related molecule called RNA. These RNA transcripts are then used to build proteins. Kunte and Kronforst found that doublesex RNA is sliced and rejoined into four distinct ‘isoforms’, like different paths through the same choose-your-own-adventure book. One of these isoforms is found in males, and the other three are found in females.

You might think that each of the three female isoforms corresponds to a different pattern, and that’s what Kunte and Kronforst first suspected too. They were wrong. Every female has all three forms, regardless of their pattern. Instead, it’s the way these isoforms are used that matters. The mimetic females make more of them than the non-mimetic ones, especially in their wings and especially in parts that produce white markings.

So, the same gene gets processed in different ways and switched on at different strengths in different parts of the butterfly’s wing to produce a variety of patterns.

But that doesn’t explain why the patterns are so stable. Whenever a new generation is born, different versions of the same gene line up and shuffle their DNA, creating new combinations. In the common mormon, the doublesex mutations that produce one mimetic pattern should eventually shuffle with those that produce another, producing new blends. Clearly, that doesn’t happen.

Kunte and Kronforst found out why: the mimetic version of doublesex is inverted relative to the non-mimetic one, so it sits in a different orientation in the genome. This inversion stops the two versions from lining up properly and prevents them from shuffling, ensuring that the gene’s thousand-plus mutations are all inherited together.

And that is, more or less, what Clarke and Sheppard thought!

They envisioned many elements acting in concert to produce the right copycat wings, and passing to the next generation as a single block. They were right except for one detail: those elements don’t have to be individual genes. They can be different parts of the same gene. As Mark Scriber from Michigan State University told me, it’s amazing that Cyril Clarke basically got the right answer 60 years ago, without any powerful genetic tools that today’s scientists can use.

Matthieu Joron from the CNRS was also impressed. He has found supergenes in a different group of butterflies—the long-winged Heliconius species. These are more in line with Clarke and Sheppard’s ideas: clusters of individual genes, locked together by a genomic inversion. Both lineages of butterflies—the longwings and the mormons—have evolved mimicry in a similar way.

That’s perhaps surprising, since the two lineages use mimicry for very different reasons. The mormons are Batesian mimics—they deceive predators into thinking that they are toxic by resembling species that genuinely are. But the longwings are Mullerian mimics—they are all actually toxic and they reinforce their warning colours by resembling each other, so that a predators which learns to avoid one species also knows to avoid them all.

The Batesian mormons are liars that mooch off the warning signals of their better-defended peers. The Mullerian longwings are honest communicators that find safety in their shared warnings. And yet, both groups have evolved their lookalike patterns in much the same way.

Note: This is an expanded version of a news story in The Scientist.

Reference: Kunte, Zhang, Tenger-Trolander, Palmer, Martin, Reed, Mullen & Kronforst. 2014. doublesex is a mimicry supergene. Nature http://dx.doi.org/10.1038/nature13112

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3-D Scans Reveal Caterpillars Turning Into Butterflies

The transformation from caterpillar to butterfly is one of the most exquisite in the natural world. Within the chrysalis, an inching, cylindrical eating machine remakes itself into a beautiful flying creature that drinks through a straw.

This strategy—known as holometaboly, or complete metamorphosis—partitions youngsters and adults into completely different worlds, so that neither competes with the other. It’s such a successful way of life that it’s used by the majority of insects (and therefore, the majority of all animals). Butterflies, ants, beetles and flies all radically remodel their bodies within a pupa as they develop from larvae to adults.

But what goes on inside a pupa? We know that a larva releases enzymes that break down many of its tissues into their constituent proteins. Textbooks will commonly talk about the insect dissolving into a kind of “soup”, but that’s not entirely accurate. Some organs stay intact. Others, like muscles, break down into clumps of cells that can be re-used, like a Lego sculpture decomposing into bricks. And some cells create imaginal discs—structures that produce adult body parts. There’s a pair for the antennae, a pair for the eyes, one for each leg and wing, and so on. So if the pupa contains a soup, it’s an organised broth full of chunky bits.

We know this because scientists have dissected lots of pupae, although they’ve mostly trained their scalpels on fruit flies and blowflies.  By its nature, such work always destroys the insect that’s being observed. It also only provides a snapshot in time. If you want to work out what happens as metamorphosis progresses, you need to cut open many pupae that you think are at different stages of development.

But now, two teams of scientists have started to captured intimate series of images showing the same caterpillars metamorphosing inside their pupae. Both teams used a technique called micro-CT, in which X-rays capture cross-sections of an object that can be combined into a three-dimensional virtual model.

By dissecting these models rather than the actual insects, the teams could see the structures of specific organs, like the guts or breathing tubes. They could also watch the organs change over time by repeatedly scanning the same chrysalis over many days. And since insects tolerate high doses of radiation, this procedure doesn’t seem to harm them, much less kill them.

One team analysed the caterpillar of the stunning blue morpho just before it started metamorphosis and a week into the process. They analysed the structure of the tracheae—the network of breathing tubes that carry oxygen throughout the insect’s body. Their work was done with the BBC as part of a documentary on metamorphosis—it was publicised in March but hasn’t been published yet.

The second project had its origins in crime-fighting. Thomas Simonsen from London’s Natural History Museum started using micro-CT to look at the pupae for blowflies. These insects lay their eggs on fresh corpses, whether it’s “someone who has been murdered or a deer in a forest”. They appear so predictably that you can estimate a body’s time of death based on where its blowflies are in their life cycle. This gets trickier once the flies turn into pupae, since those all look the same from the outside. But by scanning their insides using micro-CT, Simonsen hoped to get better estimates for how old they are.

From flies, he turned his attention to his favourite subjects—butterflies and moths. He worked with Tristan Rowe and Russell Garwood from the University of Manchester, who regularly scanned the cocoons of painted lady butterflies, some every day.

Painted lady butterfly chrysalis during metamorphosis. Breathing tubes in blue; guts in red.
Painted lady butterfly chrysalis during metamorphosis. Breathing tubes in blue; guts in red.

The scans showed that the caterpillar’s guts quickly change shape, becoming narrower, shorter and more convoluted. Meanwhile, the tracheal tubes become bigger, although their arrangement barely changes. The common wisdom is that “almost everything is massively reorganised in the pupa,” says Simonsen. That’s largely true, but not for the tracheal system. From its first day as a chrysalis, the painted lady already has the breathing tubes of an adult butterfly. “If there is remodelling, it happens very quickly in the first hours of pupation,” says Garwood. Alternatively, it happens when the butterfly is still a caterpillar.

This doesn’t drastically change what we knew about metamorphosis. There are some small insights—it seems that midway through the transformation, the big breathing tube that delivers oxygen to the flight muscles reattaches itself to a different set of openings on the insect’s torso. But the big picture stays the same. “I think it will provide instructive images for textbooks, but I don’t think it provided surprising new insights,” says David Champlin at the University of Southern Maine, who studies metamorphosis.

There are other limitations. The technique’s resolution is rather low. You cannot stain individual tissues or proteins with coloured molecules, while still keeping the animal alive. And the scanners can only pick up a limited number of organs. Brains and nerves, for example, are invisible to them, although Garwood hopes that new technological advances will overcome that hurdle.

Micro-CT scans may not revolutionise what we know about metamorphosis but Garwood hopes that their advantages will give scientists new options for their experiments. For example, the scans use up fewer individuals, since you can scan the same ones over time. This could free up insect specialists to move beyond the usual suspects like fruit flies, and study the development of rare or valuable species without harming them. They could look at how pesticides affect the development of bees, or how mutations in different genes change the process of metamorphosis. Champlin agrees. “It would be great to compare the normal animal with a variety of mutant strains defective for specific genes,” he says.

Reference: Lowe, Garwood, Simonsen, Bradley & Withers. 2013. Metamorphosis revealed: three-dimensional imaging inside a living chrysalis. Interface. http://dx.doi.org/10.1098/rsif.2013.0304

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Liquefying virus uses one gene to make caterpillars climb

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”.


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Moth and plant hit on the same ways of making cyanide

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!


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Evolutionary arms race turns ants into babysitters for Alcon blue butterflies


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.


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Tobacco leaves emit warning chemicals that summon predators when mixed with caterpillar spit


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.


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Ninja bat whispers to sneak up on moths

Batman's logo became much more detailed over the yearsBatman’s logo became a lot more detailed over the years…

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.


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Caterpillars must walk before they can anally scrape

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.



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Caterpillars use bacteria to produce green islands in yellowing leaves

Green_islandIn 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.


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Pocket Science – geneticist hunts down the cause of his own genetic disorder, and male moths freeze females by mimicking bats

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.

Reference: http://dx.doi.org/10.1056/NEJMoa0908094  and http://dx.doi/org/10.1126/science.1186802

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


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Parasitic wasps hitchhike on butterflies by smelling for chemical chastity belts

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.


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Discriminating butterflies show how one species could split into two

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.

[This post was written as an entry for the NESCENT evolution blogging contest. For more details about this competition, visit their website.]

Reference: Science 10.1126/science.1179141

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Museum butterfly collections chronicle evolutionary war against male-killers

The drawers of the world’s museums are full of pinned, preserved and catalogued insects. These collections are more than just graveyards – they are a record of evolutionary battles waged between animals and their parasites. Today, these long-dead specimens act as “silent witnesses of evolutionary change”, willing to tell their story to any biologist who knows the right question to ask.

This time round, the biologist was Emily Hornett, currently at UCL, and her question was “How have the ratios of male butterflies to female ones changed over time?” You would think that the sex ratios of insects to mirror the one-to-one proportions expected of humans but not if parasites get involved.

The bacterium Wolbachia is arguably the world’s most successful parasite, infecting around 20% of all insects, themselves an extraordinarily successful group. It can infect eggs but not sperm, which means that females can pass the bacteria on to their offspring, but males cannot. As a result, Wolbachia has it in for males – they are evolutionary dead-ends, and the bacterium has many strategies for getting rid of them. It can kill them outright, it can turn them into females and it can prevent them from mating with uninfected females.  As a result, populations infected with Wolbachia can be virtually male-free.

To study the effect of Wolbachia on butterfly populations, Hornett (great name for an entomologist)turned to collections of the blue moon butterfly (Hypolimnas bolina). This beautiful species was heavily collected by entomologists between 1870 and 1930 and their efforts have stocked the museums of the world with specimens. While these were long dead, Hornett found that many of them contained viable DNA and she used them to develop a genetic test for Wolbachia infections.

She validated her Wolbachia test by using butterflies collected by the entomologist H.W.Simmons in Fiji over 70 years ago. Simmons carefully recorded the numbers of males and females in his butterflies and noted some very unusual all-female brood. Sure enough, Hornett confirmed that only mothers who tested positive for Wolbachia produced these skewed clutches, while those that were infection-free gave birth to the standard bisexual broods.

Satisfied that her test was accurate, Hornett cast her net further. She looked at specimens collected from five populations of blue moon butterflies collected from the Phillippines, Borneo, Tahiti, Fiji and Samoa between 73 and 123 years ago. The butterflies are well studied to this day, so Hornett could compare the proportion of Wolbachia infections then and now. In butterfly time, this represents a gap of 500 to 1000 generations separating the specimens from their modern descendants.

The results show that the butterfly and the bacterium have been engaging in a heated evolutionary battle throughout the Pacific. The male-killer’s dominance has fluctuated greatly, rising in some areas and falling in others, while the butterfly has repeatedly evolved to resist its sex-skewing antics. 


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I’m baaaack (with photos)…

Right, back from holiday and back to blogging. Something new about malaria coming up in a few hours but for now, I thought you might enjoy a couple of snaps taken at Oxford’s Botanical Gardens.