), could provide crucial clues to a fundamental question in biology: How did solitary cells band together long ago to form multicellular coalitions capable of moving, hunting and hiding?
Most choanoflagellates live simple, solitary lives. So when cell biologist Nicole King, a Howard Hughes Medical Institute investigator at the University of California, Berkeley and her colleagues discovered hundreds of these organisms locked together in a sample taken from a splash pool along the coast of the Caribbean island of Curaçao, they were surprised. The cells formed a concave sheet, with their tail-like flagella extending from the cupped side.
What happened next stunned the scientists. In unison, the organisms making up the sheet inverted into a ball-like shape, tiny flagella flailing outward like tiny oars, allowing the organisms to swim much more swiftly. Accordingly, the team dubbed the new species
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“It was this crazy behavior unlike anything we’d ever heard of in choanoflagellates, ” King says, “We just had to figure out how they pulled it off.”
This collective behavior emerges from the simple actions of cells responding to changes in light, King and her colleagues report in the Oct. 18
. The researchers suggest that the new species could offer clues to how a key step in animal evolution happened. “Plus, it’s just a really cool phenomenon, ” King says.
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William Ratcliff, an evolutionary biologist at Georgia Tech in Atlanta who wasn’t involved in the study, agrees. While it’s impossible to go back in time to observe how the common ancestor of animals and choanoflagellates evolved into more complicated multicellular creatures, he says, “this study breaks down this huge jump and shows how single cells can adapt and become more complex at the multicellular level.”
King’s team brought the organisms back to the lab. Each individual resembles a sort of smooshed sphere. From one end, many tiny, tentacle-like protrusions form a collar that’s accented with a single, longer flagellum that extends beyond the collar.
Individual choanoflagellates join together by touching these collars. In the concave form, the flagella all point inward, “which aids feeding on bacteria, ” King says. When the organisms flip into a more of a sphere, the flagella all point outward, becoming hundreds of tiny paddles that help with swimming.
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, a new species of single-celled microbes called choanoflagellates, crowds together to form groups of many individuals. The organisms form a ball shape with their tail-like flagella pointing out to help with swimming, but can quickly switch to a relaxed sheet shape with their flagella pointing in when feeding. The GIF shows (at roughly 2x speed)
’s transformation remained a mystery until the researchers noticed that the flipping stopped when the organisms were exposed to a microscope’s light for too long. On a whim, King tried turning off the lights then turning them back on. In the dark,
Inverted into a ball shape. “And then we did it again, and did it again, and did it again, and every time we changed the illumination, they flipped.”
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The researchers haven’t fleshed out the full mechanism, but they’ve confirmed that a light-sensitive protein known as rhodopsin plays a role. And the collective behavior doesn’t seem to be the result of complicated communication among the cells. Rather, it stems from a simple, musclelike tightening or loosening of each choanoflagellate’s collar appendages. In sheet mode, the collars of all cells are tighter, pulling the cells into a slightly cupped shape. When the light changes, each cell’s collar widens, collectively forcing the sheet to invert into a sphere.
This change in a single choanoflagellate wouldn’t amount to much, Ratcliff says. But together, this simple individual action adds up to produce a whole new behavior — swimming or staying put to feed. “It’s a beautiful example of how simple groups of cells gain these emergent multicellular traits, ” he says.
King isn’t sure why changes in light trigger this response. But she notes that a consequence of swimming faster in darkness and staying put in light is that
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The importance of this kind of shape-shifting extends far beyond choanoflagellates, King says. Key components of animal development involve the folding of tissues as an embryo develops. “Our study shows that the basic cellular machinery necessary for this kind of folding predates the origin of animals, ” she says.Ancient DNA reveals gene flow between Eurasian and North American horses New findings show connections between the ancient horse populations in North America, where horses evolved, and Eurasia, where they were domesticated
A new study of ancient DNA from horse fossils found in North America and Eurasia shows that horse populations on the two continents remained connected through the Bering Land Bridge, moving back and forth and interbreeding multiple times over hundreds of thousands of years.
The new findings demonstrate the genetic continuity between the horses that died out in North America at the end of the last ice age and the horses that were eventually domesticated in Eurasia and later reintroduced to North America by Europeans.
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“The results show that DNA flowed readily between Asia and North America during the ice ages, maintaining physical and evolutionary connectivity between horse populations across the Northern Hemisphere, ” said corresponding author Beth Shapiro, professor of ecology and evolutionary biology at UC Santa Cruz and a Howard Hughes Medical Institute investigator.
The study highlights the importance of the Bering Land Bridge as an ecological corridor for the movement of large animals between the continents during the Pleistocene, when massive ice sheets formed during glacial periods.
Dramatically lower sea levels uncovered a vast land area known as Beringia, extending from the Lena River in Russia to the MacKenzie River in Canada, with extensive grasslands supporting populations of horses, mammoths, bison, and other Pleistocene fauna.
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Paleontologist Aisling Farrell holds a mummified frozen horse limb recovered from a placer gold mine in the Klondike goldfields in Yukon Territory, Canada. Ancient DNA recovered from horse fossils reveals gene flow between horse populations in North America and Eurasia.
Paleontologists have long known that horses evolved and diversified in North America. One lineage of horses, known as the caballine horses (which includes domestic horses) dispersed into Eurasia over the Bering Land Bridge about one million years ago, and the Eurasian population then began to diverge genetically from the horses that remained in North America.
The new study shows that after the split, there were at least two periods when horses moved back and forth between the continents and interbred, so that the genomes of North American horses acquired segments of Eurasian DNA and vice versa.
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“This is the first comprehensive look at the genetics of ancient horse populations across both continents, ” said first author Alisa Vershinina, a postdoctoral scholar working in Shapiro's Paleogenomics Laboratory at UC Santa Cruz. “With data from mitochondrial and nuclear genomes, we were able to see that horses were not only dispersing between the continents, but they were also interbreeding and exchanging genes.”
Mitochondrial DNA, inherited only from the mother, is useful for studying evolutionary relationships because it accumulates mutations at a steady rate. It is also easier to recover from fossils because it is a small genome and there are many copies in every cell. The nuclear genome carried by the chromosomes, however, is a much richer source of evolutionary information.
The researchers sequenced 78 new mitochondrial genomes from ancient horses found across Eurasia and North America. Combining those with 112 previously published mitochondrial genomes, the researchers reconstructed a phylogenetic tree, a branching diagram showing how all the samples were related. With a location and an approximate date for each genome, they could track the movements of different lineages of ancient horses.
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“We found Eurasian horse lineages here in North America and vice versa, suggesting cross-continental population movements. With dated mitochondrial genomes we can see when that shift in location happened, ” Vershinina explained.
The analysis showed two periods of dispersal between the continents, both coinciding with periods when the Bering Land Bridge would have been open. In the Middle Pleistocene, shortly after the two lineages diverged, the movement was mostly east to west. A second period in the Late Pleistocene saw movement in both directions, but mostly west to east. Due to limited sampling in some periods, the data may fail to capture other dispersal events, the researchers said.
Alisa Vershinina works in the Paleogenomics Lab at UC Santa Cruz where ancient DNA is extracted from fossils for sequencing and analysis.
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The team also sequenced two new nuclear genomes from well-preserved horse fossils recovered in Yukon Territory, Canada. These were combined with 7 previously published nuclear genomes, enabling the researchers to quantify the amount of gene flow between the Eurasian and North American populations.
“The usual view in the past was that horses differentiated into separate species as soon as they were in Asia, but these results show there was continuity between the populations, ” said coauthor Ross MacPhee, a paleontologist at the American Museum of Natural History. “They were able to interbreed freely, and we see the results of that in the genomes of fossils from either side of the divide.”
The new findings are sure to fuel the ongoing controversy over the management of wild horses in the
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