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Planet Earth

Microcosm Winner #3: How Long Has E. coli Been So Sexy?

The LoomBy Carl ZimmerMay 9, 2008 8:00 PM


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Now we come to the third winning question about Microcosm. Kenatiod writes,

Long ago, in bacteriology class, the teacher (an ex-nun at an ex-Catholic college) was telling us about the type "F" pili that are used to pass DNA so coli can have sex. One of the students asked "Why do they call them type F?" The teacher started to answer, but stopped, and then she turned bright red. The class start laughing, and then she did as well, and then someone asked, "What other kinds of pili are there?" She pulled herself together, said "Thank you" and class continued. I would like to know both the answer to the original question, and also when in evolutionary history these tiny beings started having sex.

Read on for the erotic answer... A pilus is a hair-like growth that sprouts from the surface of E. coli and other bacteria. E. coli uses different kinds of pili for different jobs, such as sticking to one another to form what's known as a biofilm. But the most famous pilus was the one that Kenatiod's teacher spoke of, the F pilus. F stands for...wait for it...fertility. It's an especially long tube that joins one microbe to another, not to bind them but to let DNA flow from the tube-builder to the recipient. It took a while for scientists to discover this tube, and at first all they knew was that there was some factor that some E. coli had that let them donate DNA. At first they thought the donation took place the way it does between male and female animals--hence the name fertility. Now they know better. The discovery of this microbial version of a sex organ was, as I explained in my answer to the first winning question, a big event in the history of biology. At first, scientists were excited simply because it meant they could run powerful experiments on E. coli to learn how genes work. But today scientists recognize that it represents something profound about life outside the lab. E. coli and other microbes don't just pass down their DNA from one generation to the next, the way we do. They also carry out a kind of natural genetic engineering, injecting their DNA into other organisms. A pilus is just one tool for this injection. Viruses can also move from host to host, carrying with them genes that provide resistance to antibiotics or other threats. Some bacteria can just slurp up genes from dead organisms. The odds of any one E. coli experiencing this so-called horizontal gene transfer are very low. But in populations of billions or trillions of bacteria, it is occurs regularly. And so horizontal gene transmission has had a huge impact over the long run. It has helped make the evolution of antibiotic resistance in bacteria such a night mare, for example. And the genomes of E. coli strains turn out to be mosaics, with hundreds or thousands of genes having been inserted, rather than inherited. Sex even brings into question whether the history of life is shaped like a tree. It may be more like a mangrove. Because there's evidence of horizontal gene transfer across the most distantly related lineages of living things, this kind of microbial sex probably was taking place very early in the history of life--maybe from the start. But the sort of gene transfer carried out with an F pilus is probably much younger. Why it evolved--and why it endures--is a question scientists are exploring now with experiments. In a study that came out last fall, scientists compared how E. coli that could have F-pilus sex and ones that couldn't evolved over 1,000 generations. Both strains became faster growers, but the sexy microbes became better adapted than the sexless ones. Their results suggest that sex allowed new, beneficial mutations to come together in individual microbes, creating a much bigger advantage than either would alone. This idea goes back decades, but scientists first conceived of it to explain the familiar sex found in our animal kingdom. But, once again, what is true for E. coli appears to be true for the elephant.

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