A group of molecular biologists recently demonstrated that the symbiotic bacteria inside a fruit fly greatly influence its choice of mates. They 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 tend to be microorganisms like intestinal bacteria.
The first experiment repeated a study 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. The repeat experiment carried out by the researchers shows that this dietary influence takes effect within just a generation or two rather than over an entire year.
"Up to now, it was assumed that the host organism undergoes evolution on its own, while its symbiotic bacteria undergo their own evolution. 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," says Prof. Eugene Rosenberg of Tel Aviv University's Department of Molecular Microbiology and Biotechnology.
In their second experiment, the team repeated the first, but with the addition of an antibiotic, which killed the bacteria and eliminated the specific mate preference. The mating process became random, with no dietary influence.
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.
Finally, in cooperation with Prof. Avraham Hefetz of Tel Aviv University's Department of Zoology, 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.
"The finding indicates that 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 says. He states that these discoveries have implications for our entire understanding of natural selection, something which may even lead to the development of a new theory of evolution.
Of course, once a month someone is rewriting Darwin and natural selection so we won't get new biology text books printed just yet.