When infection is unavoidable, fruit flies ramp up recombination

So, you wanna head back to my place after this and make some recombinant offspring?

Imagine you find yourself in the midst of a large-scale epidemic, similar to the scenarios portrayed in movies like Contagion or Outbreak (or both!). The disease is extremely contagious, and the probability of becoming infected is high. Now imagine that scientists fail to discover a cure. There is no Dustin Hoffman-led team of military virologists available to develop a vaccine and save humanity, and the disease persists, with the potential to infect subsequent generations. In this harsh, disease-ridden environment, how could you ensure that your future offspring would survive?

It turns out, if you were a fruit fly, you might rely on recombination.

Disease is thought to have played a major role in shaping the reproductive strategies of animals. The Red Queen hypothesis predicts that species experiencing parasite-related selection pressures are more likely to evolve sexual reproduction, along with increased rates of outcrossing and recombination. This is because, in the ongoing evolutionary arms race between hosts and parasites, a little bit of genetic variation can make it a lot harder for the parasite to “win.”

But while strategies for increasing genetic variation may improve disease resistance, they often come at a cost. Increased recombination, in particular, can reduce fitness by breaking up locally adaptive combinations of alleles. One potential way to get around this issue is to increase recombination rates only when the risk of infection is high. However, we have yet to observe direct evidence of parasite-induced recombination in animals.

In a study recently published in Science, Singh et al. sought to investigate the capacity of fruit flies to plastically increase recombination in response to infection. To do this, the researchers infected Drosophila melanogaster females with a variety of parasites, and observed the proportion of recombinant offspring the females produced.

In order to track recombination events, researchers took advantage of the known genetic basis of two visible phenotypic traits. The ebony locus and the rough locus occupy nearby positions on the same chromosome in D. melanogaster, and recessive mutations at each of these loci have easily identifiable effects on the phenotype. For this study, the researchers generated females heterozygous at both ebony and rough.

Next, the researchers infected females with one of several different types of parasites. Two distinct (but similarly disturbing-sounding) methods were used to infect flies, depending on the type of parasite involved. In some trials, the researchers stabbed adult flies in the thorax with a needle covered in disease-causing bacteria. In other trials, the researchers housed larval flies with female parasitic wasps, allowing the wasps to inject their eggs directly into the larvae. Seriously, these flies must have been terrified.

A parasitic wasp (Leptopilina heterotoma) probes for fruit fly larvae with her ovipositor.

A parasitic wasp (Leptopilina heterotoma) probes for fruit fly larvae with her ovipositor. (Photo courtesy of Dr. Michael Martin)

Finally, the researchers backcrossed infected females to double-mutant males, and examined the resulting offspring. Sorting through thousands of individual flies, researchers identified recombinant offspring as those that exhibited one mutant trait but not the other.

As predicted by the Red Queen hypothesis, infected females produced significantly more recombinant offspring than non-infected females. The researchers saw this pattern across all types of infection studied, including infection by species that parasitize D. melanogaster in the wild. Furthermore, the effect persisted across host life stages, with females producing more recombinant offspring even when infection occurred during the larval stage of development.

The study also provided some insight on the underlying mechanism for making more recombinant offspring, which – surprisingly – appears not to involve an actual increase in recombination rate. Instead, the culprit looks to be some form of transmission distortion, whereby recombinant gametes are promoted at the expense of non-recombinants.

This study highlights the remarkable ability of individual organisms to rapidly respond to changes in the environment, as well as the central role disease has played in shaping the evolutionary trajectory of animals.

But the reason I’m REALLY excited about these findings is because of their potential to reinvigorate the post-apocalyptic science fiction genre.

Picture this: 50 years after the emergence of an unprecedentedly deadly cross-species pathogen, the majority of the planet’s human population has been wiped out. The only people remaining are the highly recombinant offspring of those infected with (and ultimately killed by) the disease. In a world where survival of the fittest reigns supreme, these exceptionally disease-resistant individuals must attempt to rebuild society as they contend with resource shortages, lawless bands of savages, and the unknown genetic ramifications of the extreme levels of heterozygosity within their population.

It sounds like the beginnings of a pretty solid screenplay to me.

While you’re waiting for my movie to hit theaters, you can read the full text of the Science article here. And check out the video below (courtesy of Dr. Michael Martin), which shows a parasitic wasp female attempting to deposit her eggs in some (probably pretty freaked out) fruit fly larvae.

Junk science

Mating Ladybirds

Birds do it, beetles do it … (Flickr: Henry Burrows)

Last spring, the journal Current Biology published a report describing something new under the entomological sun: A genus of tiny cave-dwelling insects, dubbed Neotrogla, in which females, not males, have penises.

Or, rather, the females have a thing that they stick inside the males. Once it’s in there, that thing inflates and latches into the male with tiny barbs, binding the couple together in a copulation lasting two to three days, while the thing collects a packet containing sperm and a whole lot of (potentially) nutritious protein. What to call the females’ thing seems to have puzzled even the scientists who described it. In the text of their paper, they call it a gynosome (literally, a “female body”); but in the title, it’s a “female penis.”

This synonymy went from confusing to controversial the moment it hit the popular science press, which almost uniformly chose to go penis-first. “Female insect uses spiky penis to take charge” read the headline in the prestigious journal Nature. “Meet the female insect with giant PENIS whose steamy sex sessions last 70 HOURS,” said the Daily Mirror, caps-locked emphasis sic. Most of the stories, even the Mirror’s, got around to using the word “gynosome” eventually, and many went into more detail about how the organ in question wasn’t really a penis as we know it. LiveScience noted it was “a complex organ composed of muscles, ducts, membranes and spikes,” before adding that its size, relative to the body of a Neotrogla female, was “the equivalent of a man who is 5 feet 9 inches (1.75 meters) tall having a penis about 9.8 inches (24.9 centimeters) long.”

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Did Marine Mammals Merge Molecularly? Maybe.

Morphological convergence is one of the most striking patterns in evolution. Just among mammals there are spectacular and bizarre examples of distantly related species that share surprisingly similar adaptations. I bet you’ve heard of saber-toothed cats. But what about marsupial saber-toothed cats? Raccoons are surely familiar, but have you heard of raccoon dogs? Or the earless, eyeless oddity that is the golden mole, which somehow looks almost exactly like the equally earless and eyeless notoryctid marsupial mole? My favorite, though, might be the lesser hedgehog tenrec from Madagascar, which bears the same tiny coat of spiked armor as the common hedgehog but is more closely related to an elephant.

Skull from the marsupial saber-toothed “cat” Thylacosmilus.

Skull from a placental saber-toothed cat Smilodon.

Until recently, most scientists studying evolutionary convergence have focused on the converged phenotype (external appearance), but with the arrival of ever-cheaper DNA sequencing technologies, scientists can efficiently study patterns of convergent genotypes across thousands of genes in species that appear to have converged at the phenotypic level.

Now, I know dancing sharks are the preferred marine species of the moment, but allow me to reignite your interest in some other denizens of the sea. Last month, a team of researchers published a study in Nature examining how genes in three marine mammal lineages might have converged independently on the same solution to the very hard physiological problem of living in the ocean after millions of years evolving on land (Foote et al. 2015). Their results are hardly conclusive but do illustrate a compelling new way to think about and study convergence now that genomes are getting so cheap to produce.

Katy Perry and her dancing sharks at the 2015 Super Bowl

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Under selection, an endangered species runs low on evolutionary “fuel”

Atlapetes pallidiceps

A pale-headed brushfinch, Atlapetes pallidiceps. (Wikimedia Commons:Aves y Conservación/NBII Image Gallery)

The pale-headed brushfinch, Atlapetes pallidiceps, is a conservation success story, or at least the first chapter of one. The birds were thought to be extinct, until a 1998 survey [PDF] of Ecuador’s Yunguilla Valley found four nesting pairs, and observed them foraging for insects and fruit. Following that rediscovery, the Fundacion Jocotoco secured a reserve encompassing the brush finches’ known territory, and took steps to control brood-parasitic cowbirds that were threatening them. Now, the population is five times bigger, with as many as 200 of the birds living in the reserve.

Have the brush finches’ rebounded enough to secure their population for the future? Populations that decline so precipitously can lose genetic variation, and may not regain it even if their numbers increase again. With reduced genetic variation, species that have undergone such a “population bottleneck” event may be unable to respond to natural selection imposed by disease or changing environments.

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Bringing back the “king” of American forests

The American chestnut used to be one of the most common trees in North American hardwood forests, providing enormous crops of nuts that supported birds and other wildlife, and a source of robust, rot-resistant lumber for human use. But American chestnuts were nearly wiped out by the introduction of a virulent chestnut blight from Asia.

But now, after years of selective breeding and some careful genetic engineering, biologists at the State University of New York and the American Chestnut Foundation have produced blight-resistant chestnuts and they’re getting ready to start restoring the population with a crowd-funding campaign. If American chestnuts couldn’t evolve to cope with blight on their own, they may be one of the first species to get an evolutionary helping hand from humans.

Natural selection at the movies: Only the bad guys evolve

You can thank evolution for making xenomorphs so gosh darn scary.

You can thank evolution for making Xenomorphs so gosh darn scary. (Flickr: Maggie Osterberg)

It’s almost Halloween, and if you’re anything like me, you celebrate the season by watching scary movies. Although the horror movie marathon is a typical annual tradition of mine, this year I set out with a specific task: to identify as many movies as possible where the villain is somehow associated with evolution by natural selection. As it turns out, there are a lot of them.

Think classic horror films like Alien and Jaws, and also more recent movies like Chronicle, Resident Evil, and Slither. The trend also isn’t restricted to horror movies, with references to natural selection cropping up everywhere from science fiction/adventure films like Edge of Tomorrow to sports dramas like Rocky IV. Nor is it limited to movies alone- television shows like The Walking Dead can give you your fix of “survival of the fittest” references on a weekly basis. Even the urbandictionary.com definition of the word “villains” involves natural selection.

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Life, um, finds a way—except when it doesn’t

This week the LA Review of Books has my review of Unnatural Selection, a nifty new book in which ecological toxicologist Emily Monosson describes how living things evolve their way around the things we humans do to try and contain them.

… the introduction of the insecticide DDT rapidly led to the evolution of resistant mosquitoes, houseflies, and, yes, bedbugs. Decades of farming with the herbicide glyphosate, better known under the brand name Roundup, have led to the evolution of resistance in dozens of weed species. One after another, Monosson ticks off cases, dividing them into chapters corresponding roughly to biological classification. She goes beyond these headline examples to describe lesser-known triumphs of “resistance evolution,” such as viruses evading human immune responses and inadequate vaccination, cancer cells overcoming chemotherapy, and fish that survive water polluted by biochemical toxins.

This hits some of the same themes as that recent review about using evolutionary biology to solve major problems in the coming century, though I might have liked it if Unnatural Selection spent a bit more time discussing the cases when life doesn’t find a way—the myriad reasons we’re in the middle of the sixth mass extinction in the history of the planet. But I highly recommend the book for the folks in your life who may not realize how personal evolutionary biology can be.