Itsy bitsy genome.

Researchers have sequenced the smallest genome yet discovered, a string of DNA belonging to a species of bacterium that lives inside sap-eating insects' gut cells.

This species, Carsonella ruddii, has a genome of about 160,000 base pairs, the building blocks that make up DNA. In contrast, people's genomes are about 3 billion base pairs long.

Nancy Moran of the University of Arizona in Tucson and her colleagues began studying C. ruddii's genome to find out what functions these symbiotic bacteria might perform for their insect hosts. She notes that her team was "very surprised" when the sequence data showed that the microbe's genome was so tiny.

C. ruddii seems to be missing hordes of genes previously thought to be essential to life, says Moran. For example, this microbe isn't capable of making several enzymes important for replicating itself. Insect cells that house the bacteria appear to take up the slack, while C. ruddii reciprocates by manufacturing amino acids that aren't in their hosts' diets.

Her team writes in the Oct. 13 Science that because C. ruddii leans on its host's cells for so many functions, it might eventually evolve into an organelle like a mitochondrion or a chloroplast. Researchers suggest that those structures were once bacteria but now function as parts of cells.--C.B.