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.

BTW, I'm still into believing there are only half a dozen real species of salmon, and all of them taste good ~ these guys are just playing with words (not biology at all) as they create these strange ways of identifying species.

Right now, all we have to do is figure out how the tens of millions, or even billions or trillions of virus genes in the sea get moved from that environment into our own genomes where some of them "do stuff".

"So the definition of the word kind in the number 2 has been removed as a meaning which is a scientific determination of a division by you and the rest of the world must follow suit."

Not by me, by taxonomists. Blame Linnaeus. *Kind* hasn't been used for centuries. It's a Biblical word with no scientific meaning.

"The word is legitimate to use as a scientific word it may not be the word of the day but it can define a situation as well as the word species."

No, it can't, as you proved earlier when you called the family canidae the *dog kind* and then said all the species within it were also the *dog kind*. The word can be stretched by creationists to mean a species to a taxonomic family, or order, or class, depending on how desperate the creationist gets. That's why you couldn't tell me how many *kinds* there were of insects, or of birds, as there are millions of insect species, and about 10,000 bird species. There is no consistent way to divide biological populations by *kind*, nor is there even a hint of one. Species distinctions can get blurry, mainly because of the fact that they are not fixed in time. But *kind*? Could mean anything. Which is the way creationists want it.

Again, it wasn't I who discarded *kind* from taxonomy; it was scientists hundreds of years ago, long before Darwin. Blame Linnaeus.

re: these guys are just playing with words (not biology at all) as they create these strange ways of identifying species)))

Implicit is the demand that you accept the terms that they decide to set. It's called "controlling behavior" and is a symptom of OC disorder.

I am often ingrigued by the similarities of trout and salmon. The flesh of both tastes somewhat alike to me (of course, the trout is more delicate.) But trout even look like salmon.

A quote from "Darwin's Ghost: The Origin of Species Updated" by Steve Jones, 2000, should clear up your argument. Species in scientific definition means separated by the inability to breed. Family has a precise scientific definition which has nothing to do with relatives.

From DG: "Linnaeus--a classifier of what he saw as a fixed biological universe--named the domestic dog as a separate species, Canis familiaris. For him, it had the same status as the elephant or the tiger: an animal so removed from its relatives as to demand a label of its own. The International Commission of Zoological Nomenclature defines its species more stringently. Dogs as an entity were doomed because, once out of its owner's sight, any dog is happy to have sex with any other--even, given the chance, with a wolf....

In 1993 the Smithsonian Institution's "Mammal Species of World", the Who's Who of mamals, admitted the domestic dog only as a subspecies of the wolf Canis lupus. As Canis lupus familiaris it joins the American wold Canis lupus occidentalis and the European wolf, Canis lupus lupus; each so much alike as to be ranked as mere varieties of the same animal." So there you have it:

Genus: Canis; Species: Lupus; Varieties: familiaris, occidentalis and lupus.

Variation within it's kind does not mean evolution. A Doberman and Pekingese are still dogs. A Shetland and a Clydesdale are still horses.A short person and tall person are still humans.

And a hummingbird and a condor are both birds - but can they mate with each other and produce offspring?

Websters, Second College Edition, New World Dictionary of the American Language ----kind---- 1. [Archaic] (a) origin (b) nature (c) manner; way 2. a natural group or division (the rodent kind) sometimes used in compounds (human-kind) 3.essential character 4. sort; variety: class

They are a variety of the bird kind. It does not mean that all bird are interfertile. I am not saying that. My argument is that kind is a definitive word of the variations of the diferent classes of animals. You do not have to use the word species to define a variety class or sort.

Daddy long legs can not breed with a Black widow does that mean by your reasoning that they are not of the arachnoid kind.

Millions of viruses in the sea.

The genetic material from lower life forms in the sea probably got into the human genome via our ancestors the amphibians who probably got when they were still in the sea and passed it on to us.

That's one hypothesis. How about the last mosquito bite you ran into? How do you know some virus inserted into your bloodstream didn't make it's way to your gonads?

Did you ever notice that all the mammals and birds who had ancestors living in the trees have full color vision?

Yup, it's a fact.

Did you ever wonder how such an amazing feat of parallel evolution took place?

You have written a considerable number of posts in your first few weeks here, all showing an interest and commitment to ideas and are, I think, thoughtful.

Why is Noah a particular problem?

The Bible is full of claims of miracles. Noah is not a particularly hard one to wrap your mind around. There are others--Joshua commanding the sun to stand still, for instance. This is one that bothers a lot of literally-minded--but I've heard more outlandish claims from PhDs in physics. Cats and boxes of bouncing light...?

How about that turning water into wine? Think of the problems, what vintage, what varietal? And the account in John insists that the wine was the best--

Either God is omnipotent or He's not. Either he commands Time or He doesn't. If He is the master of time and space and matter, he can cram a hundread thousand species in a rowboat if He pleases.

Noah is not the problem of faith. The problem of faith is a pediatric oncology ward--there's where you'll find your challenges.

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