Redundant Evolution

arabidopsis_plant
The flowering plant, Arabidopsis.
Credit: INRA

Redundancy seems to be a factor in evolution, according to a new study published in the journal Nature. University of Georgia geneticist Andrew Paterson has found that blocks of genes in plants have duplicated themselves over time. These "carbon copies" may have led to the development of new plant species.

Scientists already knew that many plants contain two or more copies of most genes, but they don’t know why these copies exist, or when they became a part of the genome.

Paterson and his colleagues studied Arabidopsis thaliana, which has the smallest genome known among flowering plants. Because Arabidopsis is easily grown in the lab, it was the first plant to have its genetic sequence completely determined.

About 30 percent of the genes in Arabidopsis are duplicated. Paterson discovered that each time a duplication event occurred, the entire genetic sequence of Arabidopsis doubled. The copies became a functioning part of the plant, but over time, some of the copies were shuffled around or lost. This loss of genes led to many of the differences among modern plants.

The scientists compared Arabidopsis genes with genes from plants such as cotton, cauliflower, alfalfa, soybeans, tomatoes, rice, pine trees, and moss. After looking at 22,000 genetic comparisons, the results were pooled. "Breakpoints" indicated duplication events. The study shows that the genetic sequence of Arabidopsis has duplicated at least twice, and perhaps a third time.

Squeezing the Trigger

One of the duplication events occurred sometime between 170 and 235 million years ago. This event may have coincided with the time that dicots diverged from a common ancestor. Dicots are a class of flowering plants (most flowering trees, for instance, are dicots).

Paterson says the duplication event that occurred roughly 200 million years ago happened very early in the divergence of dicot plants from common ancestors. Because the event occurred so early in the development of dicots, it affected every plant that resulted from this divergence.

chicxulub_crater
Artist’s depiction of the Chicxulub impact crater.
Credit: NASA

What could have caused this genetic duplication to occur? This event appears to closely follow one of the most catastrophic events in life’s history: the P/T extinction. This event -marked in the geologic record as the Permian/Triassic boundary – occurred about 250 million years ago. Known as "The Great Dying," up to 96 percent of marine species and about 70 percent of land species were wiped out during this period in Earth‘s history. Scientists have not been able to determine what caused this cataclysm to life, although theories of asteroid impacts, climatic changes, and the greenhouse effect have all been suggested.

There was another duplication event sometime between 15 and 86 million years ago, but it did not affect as many plants – mostly just members of the Brassicales order. This is probably because fewer plants diverged from a common ancestor during this time.

This duplication event, too, occurred around the same time as a major extinction event. The Cretaceous-Tertiary (K-T) extinction occurred about 65 million years ago, and is famous for being the event that claimed the dinosaurs. The consensus at present is that this extinction event was triggered by an asteroid impact. Scientists even think they have identified the crater that resulted from this impact – the Chicxulub crater in the Yucatan.

It has long been understood that changes in the environment can lead to genetic changes and new species. Still, Paterson does not claim that the duplication of genes in plants has anything to do with mass extinction events.

"We’ve toyed with the possibility of links to P/T or K-T," says Paterson, "but to suggest that such a link exists based on these data would be very speculative at this point."

What’s Next?

A third possible duplication event may have occurred prior to the other two events. Paterson plans to do additional analysis of gymnosperms (such as pines) in order to clarify this event.

"We are trying to pin down more precisely the timing of the events," says Paterson. "We’re also looking at additional plant genomes, such as rice, to better understand how duplication has affected the history of plant evolution."


Related Web Pages

Monocots versus Dicots
Arabidopsis
First Plant Genome Sequenced