New research has pointed out that symbiotic bacteria within a fruit fly has a huge influence on its choice of mates.
Based on a theory developed by Prof. Rosenberg and Dr. Ilana Zilber-Rosenberg of Tel Aviv University's Department of Molecular Microbiology and Biotechnology, the scientists 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.
In the case of animals, these partners tend to be microorganisms like intestinal bacteria.
"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," said Rosenberg.
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.
In their second experiment, the Tel Aviv University 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.
The researchers found the bacteria Lactobacillus plantarum to be present in greater numbers in starch-fed fruit flies than in sugar-fed flies. When it was reintroduced into the antibiotic-treated flies, the preferential mating behaviour resumed - proving that this bacterial species is at least partly responsible for the mating preference.
Finally, 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 said.
He added 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.
The study was recently published in the Proceedings of the National Academy of Sciences (PNAS).