- Early Life Hedged Its Bets to Survive
- Baby Neutron Star Found Inside Supernova Remnant
- Spider’s Color-Changing Camouflage Is a Mystery
- Darwin’s Wolf Mystery Solved
Posted: 04 Nov 2009 10:00 AM PST
By forcing bacteria to evolve in ever-changing conditions, scientists have induced a behavior in which colonies formed by microbes with identical genes take radically different forms, as if one sibling in a pair of identical quadruplets could sprout gills.
Technically known as "stochastic switching between phenotypic states" — or, more conversationally, hedging your bets — the ability may have been critical to the success of primitive forms of life.
Bet hedging "may have been among the earliest evolutionary solutions to life in variable environments," even preceding the ability to turn genes on and off, wrote researchers in a study published Wednesday in Nature.
Scientists have known for decades about bet hedging, which is widespread in the natural world. One well-known example comes from disease-causing bacteria, which randomly produce different surface proteins, a few of which are bound to escape immune system detection. For all its ubiquity, however, bet-hedging behavior was at first considered counter-intuitive, even baffling. After all, in any given instance, it's better to have the right surface protein.
But it's not always possible to know what's right in advance, especially in highly variable environments. In the 1960s, evolutionary biologists made mathematical models suggesting that bet hedging made sense over the long run. Some researchers even speculated that it was a basic component in the toolbox of early life, allowing primitive microbes to adapt rapidly, without being able to sense their environments or adjust gene activity — a sophisticated ability that probably took hundreds of millions of years to emerge.
But for all this theorizing, the evolution of bet-hedging had until now never been directly observed.
"Almost every biologist knows about this and is fascinated by it," said study co-author Hubertus Beaumont, a Leiden University biologist. "We go one step further, and see this evolving in real time."
Beaumont started the experiment with a population of genetically identical Pseudomonas fluorescens, a common bacterium that divides every 45 minutes and has a relatively small genome, making it easy to study.
From that strain, they seeded 12 different bacterial lines, each growing in a tube of undisturbed, nutrient-rich broth. After three days, a sample was taken and spread on agar plates to see what type of colonies formed. The bacteria divided and spread across each plate. The researchers then took a single sample of the healthiest colony and transferred it to a tube of shaken broth. After another three days of growth, the P. fluorescens in that tube were again sampled, spread on agar, and the healthiest put back into unshaken broth.
From a human perspective, it was as if tribes that thrived in a forest were suddenly tossed in a desert, then thrown back as soon as they'd started to adjust. The switch was performed a total of 16 times, with the researchers sequencing the survivors' genomes at each step.
Earlier research by Paul Rainey, a Massey University evolutionary geneticist and co-author of the study, showed that different types of broth drove the evolution of different colony types. Shaken broth favored colonies that, in their aggregates of millions of microbes, had a smooth, rounded appearance. Unshaken conditions favored the evolution of wrinkled, fast-spreading colonies. As the rounds of selection continued, some P. fluorescens lines evolved back and forth between wrinkly and smooth types.
But in two of the lines, something special happened: In the very same tube, sharing the very same genetic inheritance, were cells that formed completely different types of colonies. Some were wrinkled, and others were smooth. It was as if those P. fluorescens strains had planned for an unpredictable future.
When the researchers looked at the genomic histories, they found that bet hedging required nine genetic mutations. The first eight were linked to traits that helped microbes survive in shaken and static tubes. The ninth, involving a gene important in metabolism, triggered the ability to produce multiple colony forms. The researchers ran the experiment multiple times, with similar results. An average of one line in twelve would evolve bet hedging, always as a result of the same accumulation of mutations.
This ability "could reasonably—one might think—take tens of thousands of generations to evolve," wrote the researchers. Instead, it took a few months. That it emerged so rapidly hints at the role it may have played for microbes that hadn't yet evolved ability to to sense changes in temperature or nutrient availability, much less respond to them.
"For them, the world was completely unpredictable," said Beaumont. "I suspect that if you go back in time, you'd find organisms with one genotype that could express a wide range of strategies."
Richard Lenski, a Michigan State University evolutionary biologist known for his decades-long studies of evolutionary dynamics in E. coli colonies, said that it's difficult to know exactly what happened early in life's history. "But their results do show that such adaptations evolve pretty easily, so it's certainly possible," said Lenski, who was not involved in the study.
As for what caused colonies to take radically different forms from their genetically identical neighbors, or why that ninth mutation in particular was so critical, Beaumont doesn't yet know. Although we know the mutations, the details of the mechanisms underlying evolution, even in simple bacteria, are often "still hidden in a black box," he said.
"We want to know what's going on in that box," said Beaumont. "We're going beyond theory. We're doing experiments with evolution itself."
Image: Hubertus Beaumont
Citation: "Experimental evolution of bet hedging." Hubertus J. E. Beaumont, Jenna Gallie, Christian Kost, Gayle C. Ferguson & Paul B. Rainey. Nature, Vol. 461 No. 7269, November 4, 2009.
Posted: 04 Nov 2009 10:00 AM PST
Scientists have finally identified the mysterious source of X-ray emissions at the center of our galaxy's youngest supernova: Inside the remains of Cassiopeia A sits a baby neutron star surrounded by a thin layer of carbon dioxide.
Twenty times heavier than our sun and 11,000 light years away from Earth, Cassiopeia A was a dense star that exploded roughly 330 years ago. The supernova left behind a dense central core 12.5 miles wide that was first spotted in 1999 by NASA's Chandra X-ray Observatory. But until now, astronomers hadn't come up with a model to explain the object's confusing X-ray emission spectrum. Previous attempts had come up with a stellar radius too small to be a neutron star, or a non-uniform surface temperature, which didn't make sense.
Now, combining data from two prior studies, researchers have discovered that Cassiopeia's X-ray emission pattern can be explained by the presence of a very young neutron star with a low magnetic field and an unusually thin carbon dioxide atmosphere.
Published Wednesday in Nature, the findings make Cassiopeia's core the youngest neutron star scientists have ever encountered.
"This discovery helps us understand how neutron stars are born in violent supernova explosions," astrophysicist Craig Heinke of the University of Alberta said in a press release. "This neutron star was born so hot that nuclear fusion happened on its surface, producing a carbon atmosphere just 10 centimeters thick."
Images: 1) A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A, NASA/CXC/Southampton/W.Ho. 2) A close-up of the same image, with an artist's rendering of the neutron star at the center of the remnant, NASA/CXC/M.Weiss.
Posted: 04 Nov 2009 06:25 AM PST
Crab spiders can scuttle, but apparently they can't hide.
Long touted as an example of cryptic coloring, the female Misumena vatiaspider switches her body color over the course of days depending on the flower where she lurks. Contrary to the textbook scenario, though, a white spider on a white flower doesn't catch more prey than a white spider moved to a yellow flower, researchers report online November 3 in Proceedings of the Royal Society B
Nor does a yellow spider on a yellow flower get a color-coordination bonus, says study coauthor Rolf Brechbühl of the University of Fribourg in Switzerland. He and his colleagues reached this conclusion after videotaping some 2,000 occasions when an insect buzzed over to a flower that held a spider. Sitting on a bloom ready to pounce on pollinators, the spider supposedly shifts to match her background by switching between white and yellow. To human eyes, she looks as if she's becoming harder for her prey to see.
The study "finally shatters the myth of crypsis by color matching in crab spiders," comments behavioral ecologist Marie Herberstein of Macquarie University in Sydney, who was not part of the study. "I suspect that textbooks may now need to be rewritten."
Color changing probably has some adaptive benefit for the spiders, according to ecologist Thomas C. Ings of Queen Mary University of London. What those benefits might be still isn't clear, he says, "but this paper is exciting, as it shows that we may be focusing our attention in the wrong direction."
Another possible direction — protection from the spider's own predators — also doesn't look encouraging in the new study. Brechbühl says that his research focused on spider prey, but he points out that all this videotaping took place in a field with plenty of birds and other possible menaces around. Even though he frequently moved spiders to flowers of the wrong color, he recorded only one predator (a bird) nabbing a spider.
Ideas about crab spider coloration have been unraveling since 2001 when Lars Chittka, also of Queen Mary, pointed out that bees see ultraviolet wavelengths but that non–UV-reflecting spiders often sit on UV-reflecting flowers.
To test for an effect of color on M. vatia crab spiders' hunting, Brechbühl and his colleagues set up clusters of yellow, white and violet wildflowers in a field. The researchers filmed each spider for three days, tallying all potential prey. Spiders caught only 3.5 percent of insect visitors, and in terms of volume of insect meat, color-coordination didn't make a difference to the catch.
Musing about other possible benefits of color changing, Ings notes that only adult females change color. "So is there a specific advantage to crypsis in mature females about to lay eggs?" he says. Or perhaps the color change worked against other predators or prey in the past and has not been lost.
Posted: 03 Nov 2009 11:24 AM PST
Genetic analysis of the now-extinct Falkland Islands Wolf has answered a biological riddle that caught the attention of a young Charles Darwin, and helped shape his understanding of evolution.
During his voyage aboard the HMS Beagle, Darwin observed that the wolves — like his now-famous finches — varied widely in size between different islands, suggesting that the traits of species were not immutable, but changed over time in response to their environments.
Darwin also wondered at the origins of the wolves, which were unusually small, and had reddish fur and relatively short jaws. He dubbed them foxes, and was the first of many scientists to suspect that the strange canids weren't wolves at all. Others thought they were descended from dogs brought by the islands' first human settlers. Indeed, not a single mammal species other than the wolf was native to the Falkland Islands, located 300 miles off the southeastern tip of South America.
In a study published Tuesday in Current Biology, researchers address these questions with a genetic analysis of five museum specimens. Their findings are twofold. First, the specimens last shared a common ancestor 70,000 years ago, or a full 50,000 years before humans sailed to the Falklands; and the animals' closest relative is the maned wolf, still found on the savannas of South America.
Moreover, the split from the maned wolf appears to have occurred 6.7 million years ago — some four million years before wolves are known to have lived in South America. At that time, maned wolves lived in North America, and it seems that all of South America's canids originated in the north.
Unfortunately, by the time Darwin arrived in the Falklands, the wolves were being killed for their fur, and their numbers were in decline. "Within a very few years after these islands shall have become regularly settled, in all probability this fox will be classed with the dodo, as an animal which has perished from the face of the earth," he wrote.
Forty years later, the Falklands Islands wolf was gone.
Image: From Zoology of the Voyage of the H.M.S. Beagle
Citation: "Evolutionary history of the Falklands wolf." By Graham J. Slater, Olaf Thalmann, Jennifer A. Leonard, Rena M. Schweizer, Klaus-Peter Koepﬂ, John P. Pollinger, Nicolas J. Rawlence, Jeremy J. Austin, Alan Cooper, and Robert K. Wayne. Current Biology, Vol. 19 Issue 20, November 3, 2009.
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