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October 29, 2012--------News Archive Return to: News Alerts

Princeton researchers sequenced a poison-resistant protein in insect species that feed
on plants such as milkweed and dogbane – and which produce a class of cardiotoxins
called cardenolides. The surveyed insects included three orders:
Lepidoptera = butterflies and moths
Coleoptera = beetles and weevils
Hemiptera = aphids, bed bugs, milkweed bugs and other sucking insects

Above: Dogbane beetle (Photo courtesy of Peter Andolfatto)

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Far from Random, Evolution Follows a Predictable Genetic Pattern

Evolution, often perceived as a series of random changes, might in fact be driven by a simple and repeated genetic solution to an environmental pressure that a broad range of species happen to share, according to new research

Princeton University research published in the journal Science suggests that knowledge of a species' genes — and how certain external conditions affect the proteins encoded by those genes — could be used to determine a predictable evolutionary pattern driven by outside factors. Scientists could then pinpoint how the diversity of adaptations seen in the natural world developed even in distantly related animals.

"Is evolution predictable? To a surprising extent the answer is yes," said senior researcher Peter Andolfatto, an assistant professor in Princeton's Department of Ecology and Evolutionary Biology and the Lewis-Sigler Institute for Integrative Genomics.

The researchers carried out a survey of DNA sequences from 29 distantly related insect species, the largest sample of organisms yet examined for a single evolutionary trait. Fourteen of these species have evolved a nearly identical characteristic due to one external influence — they feed on plants that produce cardenolides, a class of steroid-like cardiotoxins that are a natural defense for plants such as milkweed and dogbane.

Though separated by 300 million years of evolution,
these diverse insects — beetles, butterflies and aphids —
experienced changes to a key protein called
sodium-potassium adenosine triphosphatase,
or the sodium-potassium pump,
which regulates a cell's crucial sodium-to-potassium ratio.

The protein in these insects eventually evolved
a resistance to cardenolides, which usually cripple
the protein's ability to "pump" potassium into cells
and excess sodium out.

Lead author Ying Zhen, Andolfatto, fourth author and graduate student Molly Schumer, and their co-authors sequenced and assembled all the expressed genes in 29 distantly related insect species, the largest sample of organisms yet examined for a single evolutionary trait.

They used these sequences to predict how a certain protein would be encoded in the genes of 14 distantly related species that evolved a similar resistance to toxic plants – and how the cell wall, sodium-potassium pump would be encoded in each of the species' genes based on cardenolide exposure.

Scientists using similar techniques could trace protein changes in a species' DNA to understand how many diverse organisms evolved as a result of environmental factors.

Andolfatto: "To apply this approach more generally a scientist would have to know something about the genetic underpinnings of a trait and investigate how that trait evolves in large groups of species facing a common evolutionary problem.

For instance, the sodium-potassium pump also is a candidate gene location related to salinity tolerance. Looking at changes to this protein in the right organisms could reveal how organisms have or may respond to the increasing salinization of oceans and freshwater habitats."

Jianzhi Zhang, a University of Michigan professor of ecology and evolutionary biology, said that the Princeton-based study shows that certain traits have a limited number of molecular mechanisms, and that numerous, distinct species can share the few mechanisms there are. As a result, it is likely that a cross-section of certain organisms can provide insight into the development of other creatures, he said.

"The finding of parallel evolution in not two,
but numerous herbivorous insects increases
the significance of the study because such frequent
parallelism is extremely unlikely to have happened
simply by chance

It shows that a common molecular mechanism
is used by many different insects to defend themselves
against the toxins in their food, suggesting that perhaps
the number of potential mechanisms for achieving
this goal is very limited. That many different insects
independently evolved the same molecular tricks
to defend themselves against the same toxin
suggests that studying a small number
of well-chosen model organisms
can teach us a lot about other species.

Yes, evolution is predictable to a certain degree."

Jianzhi Zhang

professor of ecology and evolutionary biology
University of Michigan

Andolfatto and his co-authors examined the sodium-potassium pump protein because of its well-known sensitivity to cardenolides.

In order to function properly in a wide variety of
physiological contexts, cells must be able to control
levels of potassium and sodium.

Situated on the cell membrane, the potassium to sodium
ratio is controlled by "pumping" three sodium atoms
out of the cell for every two potassium atoms bought in.

Cardenolides disrupt the exchange of potassium and
sodium, essentially shutting down the protein pump.

The human genome contains four copies of the pump
protein, which is a candidate gene for a number of
human genetic disorders, including salt-sensitive hypertension and migraines.

In addition, humans have long used low doses
of cardenolides medicinally for purposes such as
controlling heart arrhythmia
and congestive heart failure.

The Princeton researchers used the DNA microarray facility in the University's Lewis-Sigler Institute for Integrative Genomics to sequence the expression of the sodium-potassium pump protein in insect species spanning three orders: butterflies and moths (Lepidoptera); beetles and weevils (Coleoptera); and aphids, bed bugs, milkweed bugs and other sucking insects (Hemiptera).

They found that the genes of cardenolide-resistant insects incorporated various mutations that allowed it to resist the toxin. During the evolutionary timeframe examined, the sodium-potassium pump of insects feeding on dogbane and milkweed underwent 33 mutations at sites known to affect sensitivity to cardenolides. These mutations often involved similar or identical amino-acid changes that reduced susceptibility to the toxin. On the other hand, the sodium-potassium pump mutated just once in insects that do not feed on these plants.

Significantly, the researchers found that multiple gene
duplications occurred in ancestors of several resistant
species. These insects essentially wound up with one
conventional sodium-potassium pump protein and one
"experimental" version. The newer, hardier versions
of the sodium-potassium pump are mostly expressed
in gut tissue where they are likely needed most.

"These gene duplications are an elegant solution to
the problem of adapting to environmental changes.

In species with these duplicates, the organism is free
to experiment with one copy while keeping the other
constant, avoiding the risk that the new version of the
protein will not perform its primary job as well."

The researchers' findings unify the generally separate
ideas of what predominately drives genetic evolution:
protein evolution, the evolution of the elements
that control protein expression, or gene duplication.

This study shows that all three mechanisms can be used
to solve the same evolutionary problem.

Central to the work is the breadth of species the researchers were able to examine using modern gene sequencing equipment, Andolfatto said.

Andolfatto: "Historically, studying genetic evolution at this level has been conducted on just a handful of 'model' organisms such as fruit flies. Modern sequencing methods allowed us to approach evolutionary questions in a different way and come up with more comprehensive answers than had we examined one trait in any one organism.

The power of what we've done is to survey diverse organisms facing a similar problem and find striking evidence for a limited number of possible solutions," he said. "The fact that many of these solutions are used over and over again by completely unrelated species suggests that the evolutionary path is repeatable and predictable."

Peter Andolfatto worked with lead author and postdoctoral research associate Ying Zhen, and graduate students Matthew Aardema and Molly Schumer, all from Princeton's ecology and evolutionary biology department, as well as Edgar Medina, a biological sciences graduate student at the University of the Andes in Colombia.

The paper, "Parallel Molecular Evolution in an Herbivore Community," was published Sept. 28 by Science. The research was supported by grants from the Centre for Genetic Engineering and Biotechnology, the National Science Foundation and the National Institutes of Health.

Original article: http://www.princeton.edu/main/news/archive/S35/06/74S40/