Frederik Nijhout studying polyphenic hornworms Credit: Duke University
Most adaptations occur between generations, but occasionally traits evolve so an animal can adapt after its genes have been fixed. They’re essentially adaptations for adaptation. Ooh, how meta.
This sort of trait is a polyphenism—the ability of animals with identical genetic makeup to develop one of two or more distinct, alternative features, depending on environment pressures. Polyphenisms are pretty darn complicated, but two Duke University researchers managed to evolve a fun, dual-trait in caterpillars.
In a study published in the February 3rd issue of Science, biology professor Frederik Nijhout and graduate student Yuichiro Suzuki created hornworms that turn either green or black depending on the temperature they experience during development. They said their results show a single species can contain genetic variations that do not manifest until the population experiences revealing conditions.
“[We] learned that changes in the hormonal regulation can lead to exposure of hidden genetic variation that selection can act upon,” the authors said jointly via e-mail. “We think this may be a common process especially when populations’ environments change and the population needs to adapt.”
The researchers created the polyphenic caterpillars by taking a trait that naturally occurs to a small degree under only exceptional conditions—the caterpillars turn green when in hot temperature during development—and selected for it over 10 generations. When the scientists heat-shocked black caterpillars, it brought out their underlying genetic variation, showing which ones had the potential to become green under the stress of heat.
A naturally-occuring line of caterpillars turned green when exposed to 30°C heat, and the color intensified as the temperature increased to 33°C, when the hornworms hit their maximum verdant tint. The polyphenic line created by the researchers turned abruptly green when heat-shocked at only 28°C, while a completely monophenic line remained black no matter what temperature persisted through development.
The researchers noted that the controlled lab conditions allowed this polyphenism to evolve much faster than it would in nature. However, they said the trait does create an evolutionary advantage and could evolve under natural conditions.
“Polyphenisms are selected for when a species normally encounters two different environments that pose different selective pressures,” the authors said. “In the case of caterpillar polyphenism, dark coloration will be favored in cooler seasons because it absorbs heat much better and allows the organism to increase its metabolism. The green coloration would be favored in warmer conditions because it allows the caterpillar to be camouflaged and avoid predators.”
Nijhout and Suzuki said their experiment provides evidence for a mechanism they call a “genetic capacitor” in which mutations build up unseen, much like charge on a capacitor, and then appear, or discharge, when a specific stimulus occurs. This way, all of the hidden genetic variation is revealed.
“In the past it has been common to seek a single gene and a single mutation as an explanation for the evolution of certain traits,” the authors said. “But in reality, such simplistic explanations may not reflect the actual process by which traits evolve and become expressed.”
