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Fishing for the Origins of Genome Complexity

Biologists at Georgia Tech have provided scientific support for a controversial hypothesis that has divided the fields of evolutionary genomics and evolutionary developmental biology, popularly known as evo devo, for two years. Appearing in the December 2005 issue of Trends in Genetics, researchers find that the size and complexity of a species' genome is not an evolutionary adaptation per se, but can result as simply a consequence of a reduction in a species' effective population size.

"As a general rule, more complex organisms, like humans, have larger genomes than less complex ones," said J. Todd Streelman, assistant professor in the School of Biology at the Georgia Institute of Technology and co-author of the study. "You might think this means that animals with the largest genomes are the most complex - and for the most part that would be right. But it's not always true. There are some species of frogs and some amoeba that have much larger genomes than humans."

To help explain this paradox, a pair of scientists from Indiana University and the University of Oregon published a hotly-contested hypothesis in 2003. It said that most of the mutations that arise in organisms are not advantageous and that the smaller a species effective population size (the number of individuals who contribute genes to the next generation), the larger the genome will be.

"We agreed with some of the criticisms of the hypothesis - that one had to remove the effects of confounding factors like body size and developmental rate," said Streelman. "We were able to remove the effects of these confounding factors and test whether genome size is adaptive."

Their test consisted of analyzing data from 1,043 species of fresh and saltwater ray-finned fish. Previous data on genetic variability had established that freshwater species have a smaller effective population size than their marine counterparts. If the hypothesis was correct, the genome size of these freshwater fish would be larger than that of the saltwater dwellers. It was.

Then they matched the data with estimates of heterozygosity, a measure of the genetic variation of a population. Again they found that species with a smaller effective population had larger genomes.

"We see a very strong negative linear relationship between genome size and the effective population size," said Soojin Yi, assistant professor in the School of Biology and lead author of the study. "This observation tells us that the mutations that increase the genome tend to be slightly deleterious, because population genetic theories predict such a relationship."

"The interesting thing here is that biological complexity may passively evolve," said Yi. "We show that at the origins, it's not adaptive mutations, but slightly bad ones that make the genome larger. But if you have a large genome, there is more genetic material to play with to make something useful. At first, maybe these mutations aren't so good for your genome, but as they accumulate and conditions change through evolution, they could become more complex and more beneficial."

Georgia Institute of Technology. December 2005.

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