The bacterial species having the smallest genomes are typically those that spend their entire lives inside a host, exchanging nutrients that the host cannot synthesize in return for free room and board. Because the host supplies nourishment for these so-called endosymbionts, they are able to get by with fewer genes of their own. This reduction seemed to have limits, though. By randomly damaging bacteria, researchers had found about 300 genes essential for their growth. Accordingly, endosymbionts had never really dipped below a genomic sequence length of 500,000 to 600,000 base pairs.
Enter Carsonella rudii, which lives inside a special collection of cells in the abdomen of a certain psyllid, a flying insect that feeds on amino acid-deficient plant sap. Carsonella's genome is a mere 160,000 base pairs long, report Nancy Moran of the University of Arizona and her colleagues in the October 13 Science. The streamlined microbe contains at most 182 genes, which are smaller and overlap more with one another than is usual for bacteria. C. rudii is rich in genes for protein synthesis but lacks many genes for membrane synthesis, energy metabolism and DNA replication. Some of these genes may have been transferred to the host, which could be supplying its symbiont with essential enzymes, Moran notes. "If so, it would be very much like an organelle," she says. The mitochondrion and chloroplast are presumed endosymbionts that were assimilated into nascent plant and animal cells long ago. "This has never been shown to happen in an animal cell, because it's more difficult, but it's possible that Carsonella has basically done the same thing," Moran says.
A second endosymbiont, found in aphids, illustrates another possible fate for the genes of a hanger-on. A species of Buchnera aphidocola, which comes in a distant second in genome size at 420,000 base pairs, has lost the ability to synthesize the essential amino acid tryptophan, according to researchers from the University of Valncia in Spain, whose report appears in the same issue. The group speculates that another symbiont abundant in aphids is compensating for the loss. Given this backup, the B. aphidocola should be under less pressure to persist and may dwindle to nothing, notes evolutionary biologist Siv Andersson of Uppsala University in Sweden in an accompanying editorial.