Cockroach guts may not sound like a promising hunting ground for clues to the origins of complex cells. But by plumbing the lower digestive tracts of the household pest, scientists have dug up the first genetic evidence that energy-producing organelles of some anaerobic cells may have once been free-living bacteria. The find, reported in today's Nature, suggests that these so-called hydrogenosomes may have evolved from the same ancestor as a more famous cellular energy pump, the mitochondria.
Most eukaryotic cells--cells with a nucleus--get their energy from organelles called mitochondria, which break down sugars with oxygen. Studies of mitochondrial genomes have convinced scientists that the organelles descended from ancient bacteria that somehow took up residence in an early eukaryotic cell. Another possible descendent of these infiltrating bacteria is an organelle called the hydrogenosome, which generates energy for some anaerobic eukaryotes. But despite a superficial resemblance to mitochondria, all known hydrogenosomes lacked DNA and for that reason could not be rigorously compared to mitochondria.
Now, after searching cockroach hindguts, a low-oxygen environment known to harbor cells with hydrogenosomes, Johannes Hackstein and his colleagues at the Catholic University of Nijmegen in The Netherlands report that they have finally hit pay dirt. They examined the hydrogenosome of a single-cell organism called Nyctotherus ovalis, which they painstakingly collected by hand from dissected roaches. "We had to kill a lot of cockroaches," says Hackstein, but the sacrifice was worth it. Electron micrographs revealed structures that looked like ribosomes, a key piece of protein-making machinery. And if the hydrogenosome were manufacturing its own proteins, the scientists reasoned, it must have a genome to provide the blueprint. After disemboweling dozens of the bugs, the team cloned a gene for ribosomal RNA that closely resembles the corresponding mitochondrial gene.
This gene provides "the first direct evidence for the evolutionary history of the hydrogenosome," says Miklos Muller of Rockefeller University, who thinks hydrogenosomes and mitochondria evolved from a common ancestor that was capable of both anaerobic and aerobic respiration (Science, 13 March, p. 1633 ). But other evolutionary biologists caution that the evidence is still circumstantial. "It's very important if it is true," says Michael Gray of Dalhousie University in Halifax, Nova Scotia, "but the data are very slim."