MICROBIOLOGY AND EVOLUTION
By Davide Castelvecchi
If you are looking for spectacular discoveries of missing links, forget about crawling fish. The key word is symbiosis, without which we would never have gotten past the stage of bacteria. This year we have practically seen this evolutionary engine in action.
In a way, we are a lot more similar to yeast than yeast is to E. coli. The most dramatic leap in biological complexity in all of natural history — apart from the origins of life itself — is arguably the one from bacteria to the more advanced types of cells we are made of, along with all animals, plants, and many single-cell organisms such as yeast and amoebae. The main feature that distinguishes our cells from bacteria is that, in addition to enclosing our DNA in a nucleus, our cells contain several highly specialized “organelles,” some possessing their own DNA and replicating almost like independent organisms.
Plant cells have organelles called plastids, which tap into the energy of the sun by performing photosynthesis. Scientists have long believed that plants originated from single-cell organisms that fed on cyanobacteria, the first bugs capable of photosynthesis. Over eons, the cyanobacteria-eaters figured that it was much more advantageous to keep their meals alive and use them as power sources rather than digesting them.
The guest cells in this living arrangement eventually lost most of their genes, concentrating on photosynthesis and not worrying about most of the day-to-day chores that independent cells need to perform. (A similar fate is believed to have happened to mitochondria, organelles found in our cells as well as in plants’.) But until recently, this was just theory. The plastids in present-day plants don’t look anything like cyanobacteria.
In September, a team of microbiologists from the University of Iowa and Northwestern University reported in Current Biology that they sequenced the DNA of a photosynthetic amoeba called Paulinella chromatophora. The amoeba’s plastids turned out to have thousands of genes, and seem to be close relatives of cyanobacteria. This proved that the plastids once were free-roaming bacteria, and that P. chromatophora acquired them relatively recently.
Then, in October, a Japanese-U.S. team of microbiologists revealed in Science that a bacterium called Carsonella ruddii, a symbiotic guest of insects called jumping plant lice, appears to be on its way to becoming an organelle. C. ruddii, which lives inside the insects’ cells, has turned out to have the shortest genome of any living organism, with only about 180 genes. (For my fellow computer nerds out there: its genetic information is the equivalent of 40 kilobytes, meaning it could fit in the RAM of a Commodore 64. By comparison, the human genome is the equivalent of about 750 Megabytes.) Until now, the simplest known bacterium had more than 500 genes, and scientists believed that the minimal number of genes that would guarantee the survival of an organism was around 300. Maybe that’s still true — it’s just that C. ruddii shouldn’t be considered a bacterium anymore, and should instead be seen as a true missing link — no longer a bacterium, but not yet an organelle either.