An endangered monkey has given scientists new insights into evolution. The leaf-eating douc langur, native to East and Southeast Asia, has a "duplicated" gene that started as an extra copy of a gene for a particular enzyme but mutated into a gene for another enzyme with a different purpose. By recreating the gene's path from one enzyme to the other, scientists have addressed an important debate over how natural selection shapes such duplications.
Throughout evolutionary history, extra copies of genes have popped up spontaneously in the genomes of many organisms. This gene duplication is thought to be a major source of new genes and adaptations. But scientists disagree about what method natural selection uses to modify the extra copies. Some say positive selection favors duplicated genes that take on a new function. Others credit the lack of negative selection; because the new gene is redundant, it is free to mutate until it finds a new job. But most studies are based on statistical analyses that cannot distinguish definitively between the alternatives.
Now, evolutionary geneticist Jianzhi Zhang of the University of Michigan, Ann Arbor, says both theories are right. He and his colleagues discovered a duplicated gene for an enzyme called ribonuclease in the douc langur. In humans and other primates, ribonuclease attacks double-stranded RNA; it may help defend against viruses, some of which contain such genetic material. But in douc langurs, the duplicate ribonuclease gene evolved into a gene for a digestive enzyme. This enzyme helps supply protein by digesting the single-stranded RNA of leaf-fermenting bacteria that live in the langur's intestine.
To find out how the duplicate evolved, the researchers created nine designer mutant proteins, each with one of the nine amino acid changes that separate the duplicate from the original. Every change reduced the enzyme's ability to degrade double-stranded RNA--the enzyme's original job. This hints at a lack of negative selection. But statistical analysis showed that the duplicate gene evolved much faster than would be expected from random change, suggesting that positive selection was at work too, the team reports in the March 2002 issue of Nature Genetics.
Shozo Yokoyama, an evolutionary biologist at Syracuse University in New York state, says Zhang's functional tests are a difficult and essential step toward understanding the role of duplication. But "the final answer is still not there," he says. More research is needed to show not only how the amino acid changes reduced the ribonuclease's old function, but also how they helped it reach its new function.
Jianzhi Zhang's home page