Zhang's initial analysis showed that the duplication occurred about four million years ago, some nine million years after the two main groups of colobines---Asian and African---split into separate lineages. To confirm that the duplication occurred after the split, he analyzed DNA samples from an African colobine known as the guereza or colobus monkey.
"We sequenced the gene, and to our surprise we found not one, not two, but three RNASE1 genes," Zhang said. "Further analysis showed that the duplications in African monkeys and Asian monkeys were separate, independent events." Next, Zhang wanted to know if the duplications resulted in similar functional changes in the enzyme. Just as in the Asian colobine, the duplicated genes in African colobines functioned more efficiently at the typical acidity level of the colobine small intestine, he found.
"Then our question was whether the similar functional changes were due to identical amino acid changes at the protein sequence level," Zhang said. "Indeed, we found three amino acid changes that were identical in the two lineages. They occurred independently, but they were identical." Additional experiments confirmed that the three, independent, parallel amino acid changes were responsible for the change in enzyme function.
In both Asian and African colobines, the original, less efficient, gene is not discarded after duplication. But why, Zhang wondered.
"The guess is that the old copy is still doing something important," he said. "RNASE1 has another function, which is to degrade double-stranded RNA. Double-stranded RNA is not normally found in food, but it's found in some viruses, so the old gene may be useful in defending against viral infection." Zhang checked the new and old genes in both lineages and found the same pattern: the new genes have lost the ability to degrade double-stranded RNA, but t
Source:University of Michigan