Evolution often connotes "progress," such as increasing complexity as organisms acquire new genes. But as a new analysis of the genome sequences of two bacteria shows, genes can also be lost over time. The sequences could shed light on why genes degrade and how that changes their functions.
For a tiny bacterium nestled in an animal cell, shedding genes might conserve energy and improve efficiency. Hints that this might be true came 3 years ago, when scientists in Sweden sequenced the genome of Rickettsia prowazekii, a typhus-causing bug that can survive only in the cells of its insect hosts and the animals it infects, like humans. Many genes in R. prowazekii functioned only partially, leading scientists to speculate that they might have once worked properly, but slowly degraded.
The new work backs up this hypothesis. For comparison with R. prowazekii, microbiologist Didier Raoult of the Marseilles School of Medicine in southern France and his colleagues sequenced Rickettsia conorii. Like R. prowazekii, the bug is confined inside cells. The two species diverged from a common ancestor 40 million to 80 million years ago.
To explore how the two grew apart, Raoult's team compared their genetic sequences. They report in the 14 September issue of Science that not only is R. prowazekii's genome smaller than R. conorii's (1.1 billion base pairs compared to 1.3 billion), but that it contains far fewer functioning genes, 834 compared to 1374. It's clear that these differences are due to gene loss, because the more rapidly evolving R. prowazekii contains remnants of 229 genes found in R. conorii, and the arrangement of this "junk" DNA mirrors the configuration of active genes in R. conorii.
"This [sequence] is telling us something about evolution that maybe we already should have known," says David Walker, a pathologist at the University of Texas Medical Branch in Galveston, referring to the fact that bacterial genes decay. By clarifying how lost genes may guide the bacterium's evolution, scientists can perhaps grasp the origin of its existing design.