By all accounts, they shouldn’t exist. But two bacteria are beating the odds, living despite the absence of genes considered to be essential for life.

In last week’s issue of Science, researchers reported the completion of two genome sequences for two bacterial organisms that have mutually beneficial relationships with their insect hosts. (The bacteria are known as endosymbionts, because they live inside their hosts’ cells.) The genomes of both bacteria—Carsonella ruddii and Buchnera aphidicola—are smaller than researchers previously believed possible.

“I think that both of these two papers show that clearly these [endosymbiont] genomes can lose genes, even genes that you might think beneficial or essential for the host,” said Siv Andersson, a molecular evolutionist at Sweden’s Uppsala University who wrote an essay for Science commenting on the studies. “There may not be a lower limit to how small an endosymbiont genome can be.”

Scientists had previously hypothesized that an organism couldn’t have a genome with fewer than 400,000 nucleotides, the units that compose a strand of DNA. But Carsonella‘s genome has evolved right past that theoretical limit, weighing in at a mere 160,000 bases. In fact, it is the smallest genome ever sequenced.

Scientists are still not entirely sure how these bacteria and the insects that house them continue to survive. The researchers hypothesize that these bacteria may not survive as discrete organisms for long: Carsonella may be transitioning from its own species into a mere organelle, or specialized cell structure, of its host, and B. aphidicola may be on the road to extinction.

University of Arizona biologist Nancy Moran, a coauthor of the Carsonella study, said that when she and her colleagues first sequenced the genome, it was so small that they assumed their measuring procedure wasn’t working properly. But when they outsourced the sequencing work to a lab in Japan, those scientists got the exact same results.

Moran said that Carsonella has genes missing for nearly every essential cell process, including those that are involved in DNA replication, the transcription of DNA to RNA, and the translation of RNA into proteins. The most obvious hypothesis for how the bacterium continues to survive, she said, is that some of its genes may have transferred from the symbiont’s genome to the host insect’s genome. The host, an insect called the psyllid, may now be expressing those bacterial genes and sending their protein products back into the Carsonella cells.

“That hypothesis could explain how this organism is still able to replicate and to persist,” Moran said. “And the reason that hypothesis suggests itself is that is what happens in the case of the cell organelles, like mitochondria and plastids.”

If this is indeed what has happened, according to Moran, Carsonella will never be independent again, and it may be rapidly evolving into an organelle of the psyllid. She added that the bacterium has retained its ability to make nutritional amino acids for the psyllid, so the relationship is truly symbiotic.

“The [Carsonella article] is fascinating not only because it reduces the minimal genome size of an endosymbiotic organism dramatically,” said University of Connecticut biologist Joerg Graf. “It’s like a snapshot into the progression of becoming an organelle.”

While Crasonella may be transitioning, B. aphidicola, however, is quickly losing its ability to help the aphids that host it.

In the second sequencing study, researchers found that B. aphidicola has lost some genes that code for the metabolic functions it provides for its host along with genes needed for its own survival. The researchers conclude that another endosymbiont inside the small insect hosts must have picked up where B. aphidicola left off and started providing the host itself with key nutrients. The researchers indicate that this may foretell the demise of B. aphidicola.

“If a second bacterium is capable to develop essential functions lost by B. aphidicola, this association might end up in an ongoing process of replacement of the latter by the former, without apparent consequences for the aphid,” said study coauthor Amparo Latorre, a University of Valencia geneticist, via email.

Originally published October 20, 2006

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