Posted on 02/13/2006 5:01:39 PM PST by FlameThrower
Researchers Evolve A Complex Genetic Trait In The Laboratory
Duke University biologists have evolved a complex trait in the laboratory -- using the pressure of selection to induce tobacco hornworms to evolve the dual trait of turning black or green depending on the temperature during their development. The biologists have also demonstrated the basic hormonal mechanism underlying the evolution of such dual traits.
Frederik Nijhout shows the "polyphenic" hornworm he and Yuichiro Suzuki evolved. (Photo Credit: Les Todd)Their experiments, they said, offer important insight into how complex traits involving many genes can abruptly "blossom" in an organism's evolution.
The researchers -- Professor of Biology Frederik Nijhout and graduate student Yuichiro Suzuki -- published their findings in the Feb. 3, 2006, Science. Their work was funded by the National Science Foundation.
The complex traits, or "polyphenisms," they studied are instances in which animals with the same genetic makeup can produce quite different traits, or phenotypes, in different environments. For example, genetically identical ants can develop into queens, soldiers, or workers, according to their early hormonal environment. Or, the same butterfly can assume very different coloration in winter or summer. A kind of polyphenism is also likely at work in mammals -- for example in the seasonal development of antlers or changes in plumage or coat colors, said Nijhout and Suzuki.
While biologists have understood the basic machinery underlying polyphenisms, the mystery remained how such complex traits, which involve mutations in multiple genes, could evolve and persist.
"It's long been known that polyphenisms are controlled by hormones, with the brain sensing environmental signals and altering the pattern of hormonal secretions," said Nijhout. "In turn, these hormonal patterns turn sets of genes on or off to produce different traits. However, we understood only the developmental mechanism, and how it is possible with a single genome in an animal to produce two very different phenotypes," he said.
"There had been theoretical models to explain the evolutionary mechanism -- how selective pressures can maintain polyphenisms in a population, and why they don't converge gradually into one form or another," said Nijhout. "But nobody had ever started with a species that didn't have a polyphenism and generated a brand-new polyphenism. Such a demonstration could offer important insights into the evolutionary mechanism underlying such traits."
In their experiments, Suzuki and Nijhout chose a species of finger-sized tobacco hornworm, Manduca sexta, which normally produces only green larvae. Because a related species, Manduca quinquemaculata, develops black or green larvae when exposed to lower or higher temperatures, the researchers theorized that they could use temperature shocks to evolve a similar polyphenism in M. sexta.
Suzuki and Nijhout conducted their experiments on a black mutant form of M. sexta, which is black because of lower production of a key hormone called juvenile hormone. They subjected the black mutant caterpillars to heat during a critical period, and over multiple generations selected for two different lines of mutant caterpillars. One polyphenic line was selected to show increased greenness on heat treatment, and one monophenic line selected to show decreased color change upon heat treatment.
After rearing and selecting ten generations of caterpillars, with about 300 caterpillars per generation, the researchers found that they had, indeed, created the two distinct strains. The polyphenic strain would develop a green color at higher temperatures, altering abruptly at a temperature of about 28 degrees C. (83 degrees F.) In contrast, the monophenic strain remained black at all temperatures.
The researchers could compare these strains to understand the origin of the polyphenism. Their experiments revealed that it was the level of juvenile hormone in the caterpillars that regulated whether they would turn black or green.
For example, by applying a spot of juvenile hormone extracted from a green caterpillar to a black caterpillar during a critical period, Suzuki could produce a green spot on that caterpillar.
Also, by tightening a tiny noose around a developing caterpillar's head to prevent the juvenile hormone -- produced in the head -- from flowing to the rest of the body of the heated polyphenic worm, Suzuki could prevent the caterpillar from turning green.
According to Nijhout, the generation of polyphenism in the caterpillar demonstrates an evolutionary phenomenon called "genetic accommodation." In this process, a mutation in a regulatory pathway such as a hormonal pathway changes the hormonal level to bring it closer to a threshold level that could be affected by environmental variation.
Thus, the black mutant hornworm had "dialed-down" levels of juvenile hormone, so that the caterpillar's color-producing machinery would be more likely to be affected by temperature. By selecting for a temperature-sensitive strain, the researchers established polyphenism in the caterpillar.
"Our work is really the first demonstration that genetic accommodation actually can happen," said Nijhout. "In this case, it happens in the laboratory by artificial selection; but as with all such experiments, we assume that this is a microcosm of what is actually going on in nature."
Nijhout theorized that such "homeostatic" mechanisms that maintain, for example, the color of a caterpillar, can act to mask a great deal of mutations present within the genetic machinery.
"Homeostatic mechanisms tend to stabilize a phenotype such as color and, therefore, allow the accumulation of underlying, covert mutations just as an electrical capacitor acts to accumulate charge. And eventually, these mutations could 'break out' of that constraint to produce a sudden phenotypic change; and one way for them to break out is for a mutation to happen -- for example, one that alters a hormonal level -- releasing all this variation.
"The reason this 'capacitor' concept is important in understanding evolution and the origin of complex traits is that the common model is that a new trait gets started by a fortuitous single mutation," said Nijhout. "And while that likely happens, we believe that another important mechanism involves the accumulation of many mutations in many genes without any apparent effect because they are buffered by a homeostatic mechanism; then all of a sudden one of them alters the homeostatic mechanism and lots of genetic variation suddenly explodes and is revealed as a tremendous increase in the phenotypic variability of the species. This variation then serves as raw material for selection to mold a new adaptive trait. And so that's why we think these kinds of experiments demonstrate an important novel mechanism for the evolution of novel traits."
In further studies, Nijhout and his colleagues will seek to determine whether the type of evolutionary mechanism they demonstrated in the laboratory also occurs in nature. Also, they will seek to demonstrate the phenomenon of the genetic 'capacitor,' in which mutations can accumulate 'invisibly' without obviously affecting a trait, and whether natural selection tends to filter out deleterious mutations in such cases.
"No, I mean, define *kind*. Specifically. And scientifically."
*Kind* has no scientific meaning at all.
" Is that specific and scientific enough for you."
No, it's incredibly vague and has nothing to do with biological classification.
" Because species is used now as the word of the day does not cancel out the word "kind" as a scientific word."
Yes it does.
There are 35 kind of dog in the dog kind.
Dogs are not interbreeding. New one on me.
Oh, almost forgot. There are over 5 to 6 million insect species. Can you tell me how many *kinds* of insects there are? How would one make such a determination? And how did Noah take care of these *kinds* on the Ark? You DO know that most would not have survived a global flood that last for months, right? Most are very highly adapted to a particular environment, especially the plants they eat or the prey they catch.
they studied are instances in which animals with the same genetic makeup can produce quite different traits, or phenotypes, in different environments.
You're misreading the passage, although admittedly it's not as clear as it could be. It's not saying that the evolved versions had the "same genetic makeup" as the originals, it's saying that the evolved version was situationally able to produce two different traits out of the same (new) genetic code.
It most certainly does. Why don't you toodle off and learn some biology before you attempt to lecture us again?
But while you're at it, feel free to define "kind" in a way that can be objectively determined. Is the vertebrate taxon a "kind"? Why or why not, and how have you determined this? How about "mammals"? "Eukaryotes"? "Eutherians"?
"Carnivores"? "Ungulates"? Share your wisdom with the people who do this kind of thing for a living, they're just dying to acquire the hard-won insights of someone who has read a few creationist pamphlets.
A Doberman and Pekingese are still dogs.
But they are no longer the same, genetically or phenotypically, as their ancestors. Nor are they even the same species as their more distant ancestors, the wolves. that's evolution.
A Shetland and a Clydesdale are still horses.
But they are no longer the same, genetically or phenotypically, as the ancestral horse from which they descended. That *is* evolution. It's not *speciation*, but then no one but clueless anti-evolutionists think that evolution consists only of speciation.
A short person and tall person are still humans.
No one said they weren't. Enjoy beating up that straw man.
Wrong, but thanks for playing. Your analogy fails on every level.
No, it isn't, since it provides absolutely no method by which other groups can be similarly classified -- i.e., determined whether they too are a "kind" or not.
Nor did he even state how he determined (if he actually did, which is unlikely) whether the items in his laundry list really are "kinds" or not. What criteria is he using, other than "whatever I feel like saying at randomly and slapping the label 'kind' on"?
This is what separates actual science from creationist ramblings.
The definition of "species" is quite variale,
True, for good reason -- contexts vary. For example, living things can be classified by their ability to interbreed, whereas obviously that is an impractical way to classify extinct animals, since you can't test dead things by attempting to breed them.
and about as specific as that simple list, btw. Same order of magnitude of specificity.
Utterly false, but thanks for revealing your utter lack of knowledge on this topic. Biological definitions of species involve testable criteria, not just laundry lists of "we call these few things a species, now guess what else might qualify" like "Creationist's" cop-out.
I'm quite curious as to what this "new genetic code" might be?
Perhaps aug is now a stop codon?
There's a "hint" that changes in critters are much more akin to putting fuel injectors in '57 Chevys than anyone ever imagined.
You can find both terms used in scientific peer reviewed journals. I prefer to use the one that suggests folks are into this for the purpose of turning obscure knowledge into real money, probably via foundation or government research grants.
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