7 December 2010

Mating choices driven by bacteria?

by Kate Melville

In a discovery that has implications for our understanding of natural selection, Tel Aviv University scientists have shown that symbiotic bacteria can greatly influence a host body's choice of mate. The research, carried out by Eugene Rosenberg, Daniel Segel and Gil Sharon of Tel Aviv University's Department of Molecular Microbiology and Biotechnology, appears in the Proceedings of the National Academy of Sciences.

Rosenberg and his fellow researchers propose that the basic unit of natural selection is not the individual living organism, plant or animal, but rather a larger biological milieu called a holobiont. This milieu can include plant or animal life as well as their symbiotic partners. In the case of animals, these partners include microorganisms, such as intestinal bacteria.

"Up to now, it was assumed that the host organism undergoes evolution on its own, while its symbiotic bacteria undergo their own evolution," Rosenberg explained. "The mechanism that we discovered enables evolution to occur more rapidly in response to environmental changes. Since a generation is shorter for bacteria than for multicellular organisms, they genetically adjust more quickly to changes in the holobiont."

Working with the rapidly-reproducing fruit fly, the team first repeated an experiment carried out two decades ago by a Yale University researcher, in which a fly population was divided in half and fed different diets - malt sugar versus starch. A year later, when the flies were re-integrated as one group, those who had been fed starch preferred starch-fed mates, while the sugar-fed flies preferred mates of a similar nutritional background.

Importantly, the repeat experiment carried out by Rosenberg shows that this dietary influence takes effect within just a generation or two rather than over an entire year.

In their second experiment, the researchers repeated the first experiment, but with the addition of an antibiotic, which killed the majority of the flies' symbiotic bacteria. The mating process then became random.

In subsequent experiments, the researchers successfully isolated the bacterial species responsible for reproductive isolation in flies with diet-related mating preferences, and found the bacteria Lactobacillus plantarum to be present in greater numbers in starch-fed fruit flies than in sugar-fed flies. When L. plantarum was reintroduced into the antibiotic-treated flies, the preferential mating behavior resumed; proving that this bacterial species is at least partly responsible for the mating preference.

The team analyzed the sexual pheromones produced by the fruit flies. There turned out to be differences in pheromone levels between the two groups of flies - differences that again disappeared after administering antibiotics. "Pheromone alterations are a mechanism by which we can identify mating preferences. We therefore hypothesize that it is the bacteria that are driving this change," Rosenberg concludes.

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Source: Tel Aviv University