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Brain genes evolve more slowly for advanced species

Human brain genes are evolving more slowly than those of other species, according to a study from an international team of researchers in the December 26, 2006 issue of PLOS Biology.

The research team sequenced about 5,000 genes expressed in the brains of macaque monkeys and compared those to sequences of mouse, chimpanzee, and human brain genes. Overall, the more advanced a species is, the faster its genes tend to change, but among brain samples the team found some surprising results.

“The trend appears to be contrary to what most people might think about the evolutionary rate of brain-expressed genes. We observed that the more advanced the brain is, the more slowly its genes evolve,” said Chung-I Wu, a professor in the Department of Ecology and Evolution at the University, in an e-mail interview. “Genes expressed in human brains have been evolving at the slowest pace.”

The findings were surprising in another sense as well. Normally, tissue-specific genes evolve faster than others because they have more limited effects. For example, genes expressed only in male reproductive organs evolve very quickly.

On the other hand, if a gene affects processes in multiple organs, a mutation to that gene is more likely to be harmful to some action in the body. Thus, that gene will tend to change only a little over time.

But the researchers found just the opposite effect in brain genes. Genes expressed solely in the brain evolve more slowly than those expressed both in the brain and elsewhere in an organism, which in turn change more slowly than genes not associated with the brain at all.

The researchers think the structure of human brain processes might account for these results. “We suspect that the complexity of a brain depends on the networking of its genes,” Wu said. “In an advanced brain, genes interact extensively. As a consequence, each gene is constrained in how fast it can evolve.” Genes are “freer” to change in a simpler brain with fewer interactions.

This left scientists wondering how the human brain has advanced so quickly, even though its genes have evolved more slowly since breaking off from chimpanzees millions of years ago. The researchers suspect this may be because changes to individual genes play only a small role in brain development and function.

“Evolution in the function of brains may depend more on how the component genes are assembled than how fast the components themselves change. The components themselves may not change very much as the brain evolves,” Wu said.

The study was co-authored by Wu, C. K. James Shen, Huan-Chieh Chien, and Hurng-Yi Wang of Academica Sinica in Taiwan; the University of Tokyo’s Sumio Sugano; Naoki Osada and Katsuyuki Hashimoto of Japan’s National Institute of Infectious Diseases; Chen-Kung Chou of Taipei Veterans General Hospital in Taiwan; Takashi Gojobori of Japan’s National Institute of Genetics; and Shih-Feng Tsai of the National Health Research Institute in Taiwan.

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