Gut bugs affect mating

Drosophila seem to prefer to mate with other Drosophila raised on the same diet as a result of the bacteria that live in their guts, according to a study published this week in Proceedings of the National Academy of Sciences (PNAS).

Image: Wikimedia commons,
Muhammad Mahdi Karim

These apparent mate preferences, which arose after just one generation, suggest that an organism's microbiota can facilitate rapid evolution and speciation."It's an interesting paper," said linkurl:Patty Gowaty;http://www.eeb.ucla.edu/indivfaculty.php?FacultyKey=8418 of the University of California, Los Angeles, who did not participate in the study. "The thought that these gut bacteria could be associated with the reproductive outcomes for individuals is fascinating.""There's a lot of emerging research these days about the physiological effects of microbiota, and changes in microbiota in response to environmental conditions," added evolutionary geneticist linkurl:Paul Hohenlohe;http://people.oregonstate.edu/%7Ehohenlop/ of Oregon State University, who was also not involved in the research. "This study ties that into mating preference, too."Twenty years ago, Diane Dodd of Yale University raised Drosophila melanogaster on different media for more than 25 generations and found that those raised on starch media were more likely to mate with other starch-raised flies, while those raised on maltose were more likely to mate with maltose-raised flies."Nobody understood the mechanism for this, but they understood it was important because mating preference is an early stage of sexual isolation and speciation," said microbiologist linkurl:Eugene Rosenberg,;http://www.tau.ac.il/lifesci/departments/biotech/members/rosenberg/rosenberg.html a professor emeritus at Tel Aviv University in Israel and coauthor of the PNAS paper. "And nothing is more fundamental to evolution than the origin of species."To investigate why these apparent mating preferences arise between flies raised on different diets, Rosenberg and his colleagues first repeated Dodd's experiment using molasses and starch media. Placing together two males and two females, one from each group, the researchers confirmed Dodd's finding that flies tended to mate with other flies raised on similar diets, and found that this nonrandom mating occurred after as little as one generation on the different medium types. The team then treated the flies with antibiotics to rid them of the microorganisms that reside in their guts. Immediately, the flies lost their apparent mating preferences, randomly choosing between molasses-raised flies and starch-raised flies. The results suggested that something in the gut microbiota was driving Drosophila mate preference.It is not clear whether or not these data represent actual mating preferences, however, Gowaty said. "That's one [possibility], but it's not the only one," she said. "When there's more than one male and one female, there's also the possibility of within-sex interactions" that affect which flies mate, such as male-male competition. To determine which bacteria might be driving the nonrandom mating among the flies, the team sampled the microbiota of flies raised on each type of media. One striking difference between the groups was the proportion of a bacterium called Lactobacillus plantarum. While this bacterium made up 26 percent of the microbiota of starch-raised flies, molasses-raised flies contained only 3 percent L. plantarum. When the researchers infected sterile (antibiotic-treated) flies with the bacteria, the mating preferences returned, indicating L. plantarum was indeed influencing the flies' mating patterns.How exactly the bacterium has this effect is uncertain, however, Rosenberg told The Scientist. The team detected significant differences in the composition of the flies' sex pheromones, known as cuticular hydrocarbons, but "this is just a hint that [microbiota] may be working through these sex hormones," he said. "Much more work has to be done to understand the mechanism" by which these bacteria are influencing the flies' sexual behavior.Another important unanswered question that remains is how these mating outcomes affect individual fitness, Gowaty said. Rosenberg and his coauthors suggest that their results fit with a theory of evolution called the hologenome theory, where the unit of natural selection is the "holobiont" -- the organism plus all of its microorganisms, as opposed to just the individual itself. "In order to get variation, which is the raw material for evolution, you can change either the microorganisms or the host," he explained. And because microorganisms can change in response to the environment much more rapidly than the host, evolution of the holobiont may be "an important, driving force, especially for rapid evolution," Rosenberg said. G. Sharon, et al., "Commensal bacteria play a role in mating preference of Drosophila melanogaster," PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1009906107, 2010.
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[ 2nd October 2008]