May 19, 2010
Above are depictions of the seven pairs of CaNA gene found in Paramecium with the conserved functional domains indicated below in color. Intron locations are shown as straight lines in each gene and the connecting lines between genes represent regions that have undergone gene conversion.
WOOSTER, Ohio - In the microscopic world of Paramecia, some surprisingly big things are taking place, and at least one local scientist is paying close attention.
These unicellular ciliate protozoa commonly found in fresh water ponds are providing valuable snapshots of the evolutionary process, according to Dean Fraga, professor of biology at The College of Wooster. Fraga, who specializes in the study of molecular evolution, notes that the Paramecium's genome has duplicated three times over the past several hundred million years, the most recent example still retaining many of the duplicate copies produced from that event. The result has been an accumulation of more genes (slightly less than 40,000) than are found in the human genome. "This gives us a chance to see what happens after gene duplication and the extent to which duplicate genes are lost or adopt new functions over time," says Fraga, whose work will be published in the July issue of the journal Eukaryotic Cell.
Fraga and his co-authors have characterized one rather large gene family in Paramecium that encodes an important signaling molecule called protein phosphatase 2B (PP2B), or calcineurin (CaN). "Paramecium tetraurelia has seven subfamilies of the catalytic CaNA subunit and one subfamily of the regulatory CaNB subunit," explains Fraga in the abstract of his paper. "Each subfamily has two 'duplicates' from the most recent WGD (Whole Genome Duplication) for a total of 14 in the case of CaNA. The size of the CaNA family is unprecedented, and we hypothesized that the different CaNA subfamily members were not strictly redundant as previously suggested and that at least some fulfill different roles in the cell."
Fraga and his colleagues examined the physiological roles of three of the seven subfamily pairs and found that each pair had distinct but overlapping functions. Based upon comparisons with what similar genes do in other organisms they hypothesize that the family has expanded and retained the duplicates from previous WGD due to a tendency to specialize - a process by which the duplicate genes might subdivide the various tasks one gene handled before the duplication event.
"We hope to expand this analysis and test whether this is a general phenomena or not by looking at other gene families in Paramecium. This system affords us the opportunity to ask these questions on a large enough scale to perhaps derive some insight in the molecular mechanisms by which protein families evolve." said Fraga.
"This work may eventually help us provide an answer to the question many of us have, namely, why does such a simple organism need so many genes?" added Fraga. Some have proposed it is because the cell is so large and simply needs additional copies for gene expression purposes, but Fraga and his fellow researchers contend that the Paramecium may be accomplishing this by, in effect, sub-dividing the various cellular functions between the two genes that a single gene was responsible for prior to a duplication event. This would accomplish the same purpose but in a way that would open the door for continued innovation.
The implications of Fraga's research are significant because this tiny organism can shed light on how humans evolved. "Because the changes have occurred so recently, relatively speaking, with a simple organism like paramecium, we have an opportunity to get 'under the hood' like a mechanic and look at what's happening at the genetic level using the tools we have available in this system," he said. "For example, there is evidence in the human genome that we are the product of at least two WGD since our lineage acquired a backbone, and understanding what happens after such an event may help us better understand how we evolved as a species."
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