|Leaf-eating monkey shows gene theory in action|
By Adam Marcus
March 15, 2002
Thousands of human genes were once copies of other genes that have over time developed mutations and drifted away from the originals, according to the theory of gene duplication. Genomics has provided circumstantial evidence for the theory, but scientists have not been able to show how gene duplication might lead to new functional genes. Now, researchers believe they have an example involving a species of leaf-eating monkeys, called douc langur.
Jianzhi Zhang, an evolutionary biologist at the University of Michigan in Ann Arbor, and colleagues say gene duplication is behind the uniquely efficient way douc langurs digest their dinner. After a gene for an enzyme was duplicated millions of years ago, the species developed a second gastric enzyme that ultimately turbo-charged its ability to draw nutrients from leaves.
"Duplication is believed to be important for generating new genes and important for adaptation of organisms to the environment, but it has not been shown clearly how this works," says Zhang. "This study provides a clear-cut case that gene duplication provides the opportunity for the evolution of new genes." The study appears online in Nature Genetics.
Duplication typically occurs when chromosomes pair and recombine sloppily in a process called 'unequal crossing-over.' Initially, the duplicated gene performs the same function as the original. But subsequent mutations can give the copy a unique role. If the mutation has a beneficial effect, the new gene might survive; if not, it might end up as a functionless 'pseudogene.'
Douc langurs belong to a subfamily of Asian primates called colobines, which split from other Old World monkeys about 15 million years ago. Most monkeys eat fruits or vegetables and even meat, but colobines primarily eat leaves, which are more difficult to digest. Their digestive system is similar to that of cows. In the gut of a monkey are bacteria that ferment leaves and take in the nutrients they release. Then, the animal digests the bacteria to obtain proteins and other nutrients.
All colobines have a key pancreatic enzyme, RNASE1, which helps them digest bacterial ribonucleic acid (RNA)a source of nitrogen. About four million years ago, douc langurs developed an extra copy of the RNASE1 gene, according to Zhang's group. The new gene, called RNASE1B, mutated, and its enzyme ultimately made the monkeys more efficient in extracting energy from leaves.
Douc langurs had been eating leaves for millions of years before the new gene appeared, says Zhang, so it was not the duplication that turned them into vegetarians. But it appears to have greatly improved their efficiency in digesting plants. "This gene was not necessary for the process of fermentation but it helps," Zhang says.
. . .