The human body harbors at least 10 times more bacterial cells than human cells. Collectively known as the microbiome, this community may play a role in regulating one's risk of obesity, asthma and allergies. Now some researchers are wondering if the microbiome may have a part in an even more crucial process: mate selection and, ultimately, evolution.

The best evidence that the microbiome may play this critical role comes from studies of insects. A 2010 experiment led by Eugene Rosenberg of Tel Aviv University found that raising Drosophila pseudoobscura fruit flies on different diets altered their mate selection: the flies would mate only with other flies on the same diet. A dose of antibiotics abolished these preferences—the flies went back to mating without regard to diet—suggesting that it was changes in gut microbes brought about by diet, and not diet alone, that drove the change.

To determine whether gut microbes could affect an organism's longevity and its ability to reproduce, Vanderbilt University geneticist Seth Bordenstein and his colleagues dosed the termites Zootermopsis angusticollis and Reticulitermes flavipes with the antibiotic rifampicin. The study, published in July 2011 in Applied and Environmental Microbiology, found that antibiotic-treated termites showed a reduced diversity in their gut bacteria after treatment and also produced significantly fewer eggs. Bordenstein argues that the reduction of certain beneficial microbes, some of which aid in digestion and in the absorption of nutrients, left the termites malnourished and less able to produce eggs.

These studies are part of a growing consensus among evolutionary biologists that one can no longer separate an organism's genes from those of its symbiotic bacteria. They are all part of a single "hologenome."

"There's been a long history of separating microbiology from botany and zoology, but all animals and plants have millions or billions of microorganisms associated with them," Rosenberg says. "You have to look at the hologenome to understand an animal or plant." In other words, the forces of natural selection place pressure on a plant or animal and its full array of microbes. Lending support to that idea, Bordenstein showed the closer the evolutionary distance among certain species of wasps, the greater the similarities in their microflora.

Researchers believe that the microbiome is essential to human evolution as well. "Given the importance of the microbiome in human adaptations such as digestion, smell and the immune system, it would appear very likely that the human microbiome has had an effect on speciation," Bordenstein says. "Arguably, the microbiota are as important as genes."

This article was published in print as "Backseat Drivers."

1 Comments

1. 1. jvkohl 07:18 PM 3/1/12

   It is great to see this addressed from perspectives on the microbiome, but it also seems more likely that viruses are driving bacterial and other speciation via their ability to alter intracelluar signaling pathways involved in gene expression. The effect of the viruses is on nutrient acquisition by the bacteria, which determines their species-specific pheromone production. Pheromones are responsible for self / non-self recognition across all species. Bacteria ingest the DNA of more heterospecifics that conspecifics. Well ‘fed’ bacteria reproduce; pheromone-dependent quorum-sensing prevents reproduction in starving organisms.
   
   Survival the fittest is initially determined by nutrient chemicals that cause changes in intracellular signaling. These changes lead to stochastic gene expression linked to reproductive success in asexual organisms. Evidence of a winning genetically predisposed virus-driven plan shows up in speciation from microbes to sexual reproduction in yeasts and mammals, with endogenous retroviruses clearly involved in our primate lineage.
   
   It now appears that viruses are driving the changes in the bacteria that are driving speciation via their effects on the chemical appeal of food, and its species-specific metabolism to pheromones. It is the chemicals that make food odors appealing and chemicals that make conspecifics appealing (or not), and this provides the sensory drive required for nutrient-driven species diversity. As more is learned about the epigenetic effects of odors on gene expression in the cells of all organisms, I think it will become clearer that the origin of the epigenetic effects is viral with involvement of bacteria at the next step up the ladder to speciation in organisms from microbes to man.
   

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