Partners for life ... and perhaps death.

Talk about a long-lasting relationship. Some 100 to 250 million years ago, bacteria infected an aphidlike insect, says Nancy A. Moran of the University of Arizona in Tucson. They apparently never left. All modern- day aphids now carry these bacteria, which belong to the genus Buchnera. In fact, the two organisms are virtually inseparable. The bacteria, transmitted to future aphid generations within the insect's eggs, can no longer grow outside of symbiosomes, the cellular vesicles created by their hosts. As for the aphids, they apparently need the bacteria to supplement their diet of plant sap, which lacks essential amino acids and other nitrogen-carrying compounds.

The symbiosis has lasted so long that Buchnera have even evolved to meet the nutritional needs of their host aphids, says Paul Baumann of the University of California, Davis. He and other scientists have recently found that the Buchnera in many species of aphids contain extra copies of genes that encode the enzymes used to make amino acids such as tryptophan.

Moran, who studies the evolution of genes from the aphids and their bacteria, warns that this symbiosis may not last forever. Her data suggest that the populations of Buchnera in each aphid are too small for natural selection to weed out harmful mutations. The fitness of the bacteria may eventually decay to the point where neither they nor the aphids that depend upon them can survive, she speculates.