First evidence of correlated rates of evolution between hosts and parasites

Genes of lice have evolved much faster than those of humans and chimps on which they feed, according to research that has just been published.

“Understanding differences between species in the rate of molecular evolution is of considerable interest in the fields of evolution, molecular biology, population genetics and systematics,” observed Kevin P. Johnson and his colleagues in a paper appearing this week in Proceedings of the Royal Society B.

Studying genetic differences that have arisen in humans, chimps and their lice offered a convenient way of estimating the relative rates of such evolution. Humans and chimps shared a common ancestor about 5 million to 6 million years ago. Their lice too would have evolved across the same period of time from a louse that lived off that ancestral primate. With a common period of evolution, the rate of genetic change in the primates could be readily compared with that of the lice.

Dr. Johnson of the University of Illinois at Urbana-Champaign in the U.S. and his colleagues examined 1,534 genes that humans, chimps and the two species of lice (the human body louse, Pediculus humanus, and the chimpanzee louse, Pediculus schaeffi) had in common. They computed the extent of differences in each of those genes for humans and chimps, and then compared that to the differences found in genes in the two lice.

This comparison revealed that lice have been evolving nearly 15 times faster than their hosts at the DNA sequence level.

“Interestingly, the genetic divergence between lice was correlated with the genetic divergence in these same genes between humans and chimpanzees,” the scientists noted in their paper. “That is, genes that evolve more rapidly in humans and chimpanzees also evolve more rapidly in their parasitic lice, even though lice and primates are separated by more than 600 million years of evolution.”

This was the first evidence of correlated rates of evolution across the genome between hosts and their parasites, they added. The explanation for such a correlation appeared to be the relative level of functional constraints on different genes.

In humans and chimps, however, a greater proportion of gene sequence changes resulted in changes to the protein structure. That means that even though louse genes have been changing at a faster rate, most of those changes were ‘silent’ and had no effect on the protein they coded for, a press release issued by the University of Illinois pointed out.

Since those changes made no difference to the life of the lice, they were tolerated, Dr Johnson was quoted as saying in the press release. Those sequence changes that actually change the structure of proteins in lice were likely to be harmful and were being eliminated by natural selection.

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