White Horses and other Beautiful Mutants
Mutations fuel evolution, and we all carry a few, according to a new paper published in Science. The mutation that led to a snow-white racehorse in New Jersey is striking, but not all of ours are so quite so obvious.
White Horses and other Beautiful Mutants
Here's my weekly column, which will also run Monday on the cover of the Philadelphia Inquirer's Health and Science section:
There was no hanky-panky involved when a fairy-tale white foal was born to two brown Standardbreds at the Four Winds Farm in New Jersey. DNA tests confirm that the snowy foal, born May 6, is a mutant, but that’s nothing to be ashamed of. So are most humans, according to a new analysis.
Geneticists and veterinarians say this unusual foal’s lack of color comes from a spontaneous or “de novo” mutation — a spelling error in the DNA carried in either the sperm or egg from which he was conceived.
“De novo mutations are the fuel of evolutionary change,” said Joshua Akey, a geneticist from the University of Washington. He’s been studying de novo mutations in humans, and found hundreds of them. Most of these new mutations haven’t caused anything as dramatic as this foal’s white coat, but Akey says they are probably influencing our susceptibility to diseases.
There are no exact numbers for the frequency of white foals, but experts say it’s extremely rare to see one crop up spontaneously. Most horses that appear white are really pale gray, said Hannah Galantino-Homer, a veterinarian and researcher with the Penn Veterinary School’s New Bolton Center. Gray horses tend to fade as they age. (That may explain why there are both a pale horse and a white horse ridden by the four horsemen of the apocalypse — they truly are different colors.)
White horses can be the product of several possible mutations, said Galantino-Homer. Albino horses result from mutations that prevent production of the compound melanin — a pigment that gives color to skin, hair, and eyes. But such animals usually have pink eyes. This foal’s eyes are blue.
Other mutations that can lead to white animals prevent pigment-producing cells from migrating around an embryo during development, Galantine-Homer said. Such mutations also crop up in mice, pigs, and dogs.
Melanin-producing cells, called melanocytes, form along the proto-spinal cord of an embryo — what’s called the neural crest, she said. During development, those cells normally receive signals that prompt them to migrate away and become distributed over the embryo.
If a mutation interferes with this process, an animal can come out white with a smattering of color on the head or along the mane. A few animals with these mutations are also born deaf, Galantino-Homer said. That’s because some melanin-producing cells are important for inner-ear development.
There’s no indication the white foal is deaf. The mutation he carries is probably located in a gene called KIT, said Cornell University veterinarian Samantha Brooks. There are 18 known spelling errors in this gene that lead to white horses, she said. Like other horses with KIT mutations, the foal has some spots of brown along the spine – the product of cells that were waiting for a signal to migrate.
Some horses inherit a KIT mutation from a white parent, but in this case, the mutation is new and unique to this horse.
Until now, scientists weren’t sure how common such de novo mutations are in our species. In a study published earlier this month in Science, Washington’s Akey showed that we humans are riddled with them.
He and his colleagues took DNA from 2,440 people of both European and African descent and examined just the parts that hold the code for specific proteins. The result: “We all carry our own personal mutations and an even larger number of mutations that are extremely rare,” he said.
Some of these may have no biological effects, but he suspects many have a subtle influence on risks for such diseases as Alzheimer’s, multiple sclerosis, heart disease, diabetes, cancers, and mental illness.
It’s not that we’ve stopped evolving, Akey said. But the winnowing hand of natural selection has gone easy on our species in recent centuries. Charles Darwin observed that natural selection works because each generation of plants and animals produces many more offspring than can possibly survive. The “struggle for existence,” as Darwin described it, leaves just those best equipped to survive and reproduce.
Humans are still subject to evolution, said Akey, but we’re experiencing a population explosion. New mutations are appearing much faster than natural selection can filter out the deleterious ones.
If the white foal’s mutation is indeed in KIT, then it’s dominant, meaning that if he became a sire, it would affect half of his offspring. How far he spreads his new mutation depends on whether his owners think he has a talent for racing. So far, the signs are good, said his owner, Peter Congilose. “He can turn on a dime. He’s agile, he’s quick. … I’ve never seen a horse that can move like this.”
References: http://www.sciencemag.org/content/early/2012/05/21/science.1219240
Contact Faye Flam at 215-854-4977, [URL]fflam@phillynews.com;/[URL], or [URL]@fayeflam;/[URL] on Twitter. Read her blog at [URL]philly.com/evolution;.
Think of the beautiful diversity integral to nature, vs. the sad, divisive role it frequently plays in human society.— jxxphilly- Another great article. Thanks, Faye (or Ms. Flam, however you prefer to be addressed by anonymous internet trolls). What's great about this is that it shows how easy it is for white hair to evolve in mammals, so in the Arctic where white is adaptive, we have white wolves, bears, foxes, hares, and sheep. But in tree canopy, no green mammals because melanins cannot produce green, so natural selection cannot preserve green-haired phenotypes. Plenty of green snakes and lizards in trees, because these sauropsids (birds and "reptiles") get color from porphyrins, unlike mammalian hair.— anaxyrus
- During the rainy season, sloths' fur (hair) holds blue-green algae which gives them a mossy green appearance. Symbiosis.— jxxphilly
- Akey, the geneticist, states that de novo mutations increase human susceptibility to multiple sclerosis, heart disease, diabetes, cancer, mental illness, and Alzheimers's. Furthermore, Akey admits that new mutations are apprearing faster than natural selection can eliminate deleterious mutations. The point is that the given the current harmful or slightly harmful mutation rate and mutations accumulating, the human race is only thousands of years in existence. John Sanford in "Genetic Entroy" wrote a whole book on this issue. Fay has again presented scientific evidence that humans and other mammals came into existence less than 20,000 years ago.— Ted Siek
- @Ted Siek. You can cherry pick all you like but the science doesn't state what you want it to. Intelligent design is for the willfully ignorant.— oldstuff
- Oldstuff-Thank you for correcting the poor science student, Teddy. Furthermore, if we are created in "gods image", which is the highest state of perfection (trying not to laugh), why are random mutations allowed in the equation, and why are we riddled with DNA from virus and bacteria? We could easily evolve/mutate into something very far from "god's image" e.g. Republicans. Sorry, I didn't mean to say that out loud.— dhedrick
- Humans are still evolving, and may be at quite a fast pace (see http://www.biosciencehypotheses.co.uk/bihy/editorials/human%20evolution.pdf). The nature of the selective forces has changed, though, especially in the West. But, yes, we are also undergoing a population explosion, so more mutants are allowed to persist in the gene pool than (say) 500 years ago. The result may be several human sub-species, or even genuine new, reproductively isolated species, in a geological eyeblink (100k years? less).— William_B
- Some years back, I did some science at Seneca Army Depot in NY State. A biologist for the Army Corps of Engineers told me that a sizable deer population that was fenced in when the depot was built. Because there is no hunting allowed there, they use it to study what happens to deer populations in the absence of predation. Turns out that a more or less constant percentage of the deer population are pure white. It's an element in the natural variability of the population that is ordinarily deleterious to the individual, but would allow the population to evolve towards white coloration fairly rapidly, if selective pressures required it.— DigthePast
- Faye, another great post. The point is that evolution does not stop and indeed cannot stop. Not your fault that some people still manage, with a little bit of ingenuity, to miss the point. And kudos for giving the link to the primary literature.
Ted Siek, you're wrong, since you don't take into account the rate at which mutations are bred out. Same fallacy as the argument for the age of the oceans by dividing their volume by the annual river flow.— PaulBraterman
In pursuit of her stories, writer Faye Flam has weathered storms in Greenland, gotten frost nip at the South Pole, and floated weightless aboard NASA’s zero-g plane. She has a degree in geophysics from the California Institute of Technology and started her writing career with the Economist. She later took on the particle physics and cosmology beat at Science Magazine before coming to the Inquirer in 1995. Her previous science column, “Carnal Knowledge,” ran from 2005 to 2008. Her new column and blog, Planet of the Apes, explores the topic of evolution and runs here and in the Inquirer’s health section each Monday. Email Faye at fflam@phillynews.com.
Reach Planet of the at fflam@phillynews.com.
