The mystery of conjugation
The mechanism of bacterial conjugation, discovered by Joshua Lederberg and Edward Tatum in 1946, remained a mystery for nearly ten years. Stanford University Professor Stanley Falkow (1935-2018), observed: “I give lectures on the history of bacterial genetics, and I always said that the Americans had the wherewithal to discover sex in bacteria, but it took the Frenchmen to work out all the fine details.”
Catching bacteria in the act
In 1955, American microscopist Thomas F. Anderson, travelled to Paris to catch conjugating bacteria in the act. Anderson later recalled: “I documented with Francois Jacob and Elie Wollman the conjugal bliss of bacteria. Somehow, news of this reached the editors of the weekly picture magazine, Paris-Match,
and they used one of our pictures to illustrate a story by Ramond Cartier entitled simply, La Vie.
“The picture caption was, ‘Un Accouplement de Bacterie.’ When I objected that very few readers would understand what “Accouplement” meant, I was told to have no fear. They would. How many American readers of Life magazine in 1956 would have known the meaning of the equivalent English word, coupling?”
The French researchers and Irish physician William Hayes devised a clever experiment to clarify the steps of bacterial conjugation. Hayes had discovered a strain of supermales, so-called Hfr, that were 1000 times more potent than ordinary males. The researchers mixed together supermales and a female strain, quickly forming mating pairs. By vigorously beating the mixtures with a Waring Blendor, they interrupted mating at different intervals. The results showed that male genes were slowly transferred to females in a definite order. It took one gene 5 minutes to transfer, another 10, another 18, and so on; the entire male chromosome transferred after 100 minutes.
Sean B. Carrol wrote that Jacques Monod dubbed this discovery the spaghetti experiment. He compared the male chromosome to a strand of spaghetti that was sucked up by the female cell. The untransferred portion was broken off at different time points, like a pasta noodle breaks before getting completely slurped up.
The Impressive Male Appendage
Bacterial conjugation begins when the male bacterium sends out a long fiber, the “sex pilus” that ensnares the female and draws her close.
This triggers the mating bridge, establishing close contact and the formation of the mating bridge. Then the male begins to transfer his long strand of DNA to the female.
Science historian, Thomas D. Brock, noted the significance of the French discovery: “the studies of Jacob and Wollman established a new paradigm for bacterial genetics. No longer could the mating process in in bacteria be considered a normal, albeit somewhat “peculiar” example of conventional sexual reproduction. The bacterial process was strikingly different from the process in higher organisms.”
Stanley Falkow. 14 May 2015. Author’s interview at the Department of Microbiology and Immunology, Stanford University.
Carroll, Sean B.2013. Brave Genius: A Scientist, a Philosopher, and Their Daring Adventures from the French Resistance to the Nobel Prize. Crown/Archetype. Kindle Edition. Crown Publishing Group, New York.
Brock, T. D. 1990. The Emergence of Bacterial Genetics. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY.