Novo tipo de variação genética pode fortalecer a seleção natural

New Type of Genetic Variation Could Strengthen Natural Selection

ScienceDaily (Feb. 18, 2010) — The unexpected discovery of a new type of genetic variation suggests that natural selection -- the force that drives evolution -- is both more powerful and more complex than scientists have thought.

Scanning electrograph image of Saccharomyces cerevisiae. (Credit: Image courtesy of NASA)

"We have discovered that natural selection can act not only on whole organisms and individual genes, but also on gene networks," says Antonis Rokas, assistant professor of biological sciences at Vanderbilt University and senior author of the paper reporting the discovery that was published in the February 18 issue of the journal Nature.

This finding may help explain how some organisms, including bacterial pathogens, maintain high levels of diversity and adapt rapidly to new stresses.

Working with colleagues at the University of Colorado School of Medicine (UCSM) and the Universidade Nova de Lisboa in Portugal, Rokas found that a close relative of brewer's yeast, Saccharomyces kudriavzevii, exists in two very different states: one that can efficiently digest the sugar galactose and one that cannot. Galactose is a natural sugar found in milk and many fruits and legumes. The variant found in Portugal that consumes galactose uses a network of six genes to convert the sugar into energy. What is surprising is the fact that a variant found in Japan that cannot process galactose has nevertheless preserved a non-functional version of the network of galactose genes for millions of years.

"This level of genetic divergence is normal between distantly related species, like human and mouse. Instead, we find it being maintained within a single species of yeast," says coauthor Mark Johnston from UCSM.

Normally, natural selection and recombination work jointly within an individual species to actively maintain a single version of the genes that perform critical functions or that give organisms a competitive advantage. However, this has not happened with the galactose genes in S. kudriavzevii. When the scientists compared the genomes of the Japanese and Portuguese populations of the yeast, they were surprised to find that the divergence between the galactose genes was a hundred times greater than the divergence between the two genomes as a whole. This indicates that the two states have co-existed for millions of years, which the scientists conclude is convincing evidence that the two have been actively maintained by natural selection.

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