Tuesday, July 24, 2007

Genetic variation and Genetic stability

Scientists from the Max-Plank institute and from the Salk Institute and the university of Chicago have identified which genes are prone to variation and which are stable and do not have modification. They have developed a method to sift out whole genomes for all the environmental fixes and addendums accumulated over time. What the scientists are after is the regions that are currently targeted by natural selection or have been so during the evolutionary past.

This is an exciting prospect because it lends itself to a further accumulation of evidence about how evolution works. The entire genome isn't affected, only parts of it. Scientists studying the plant the mustard weed Arabidopsis thaliana have been able to identify genetic variations in 23 strains.

Why study arabidopsis?
About 10 years ago Arabidopsis was adopted by plant scientists as an easily manipulated model for other plants because it is simple to grow in the laboratory, has a short life cycle and a small genome. Arabidopsis only has about 120 million base pairs of DNA. Compared to corn, which might have as many as 2.5 billion base pairs of DNA and the human genome with roughly 3 billion pairs, one can study it more easily.

Effects on genes
Plants are under constant threat from heat, cold, high acidity or salinity, or pathogens such as viruses and leaf-munching insects. So how do plants survive? Plants mobilize physiological and biochemical defenses for their survival. Scientists expected certain classes of genes to be highly variable due to natural selection in different environments. And now two different studies revealed precisely which gene family members indeed were shaped by evolution. In general, genes that don't change over time are under strong negative selection because they perform important housekeeping functions, while genes that vary widely such as disease resistance genes are under strong positive selection.

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