Researchers Evolve A Complex Genetic Trait In The Laboratory

[Guest guest]

Researchers Evolve A Complex Genetic Trait In The Laboratory

http://www.medicalnewstoday.com/medicalnews.php?newsid=37289

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.

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.