Scientists first saw the microbiome when doing ribosomal RNA sequencing and getting so much of what many thought was contamination. As it turned out, they were revealing the beginnings of a field that would rapidly explode on the scene with tremendous significance. It’s now known that humans carry as many bacterial cells as they do cells of their own - the old 10:1 myth has been overturned; but those gut bacteria still contribute significantly to health. Some researchers are now showing that the microbiome has an impact on evolutionary changes.
The idea that the symbiotic relationship between a host and it’s associated microorganisms - a holobiont - could be influencing evolution is not new. First coined by Richard Jefferson at a Cold Spring Harbor symposium in 1994, the
Hologenome Theory of Evolution was reported independently by Eugene Rosenberg and Ilana Zilber-Rosenberg in 2008 using coral reefs as a model system. It was shown that the reefs were adapting to higher temperatures in their environment and developing pathogen resistance through a relationship that selected for the best holobiont. There has been some controversy about these findings, and it continues to be a subject of debate in the scientific community. The research into the hologenome concept of evolution has since expanded into many organisms, and is gaining steam.
In 2015,
Ten Principles of Holobionts and Hologenomes were proposed by Bordenstein and Theis in PLOS Biology to clarify the discussion surrounding this burgeoning field. In their paper, they seek to provide a conceptual framework so that the research integrates it into our understanding of how biology works and refines it instead of being a challenge to well established theories that are widely accepted. Several of these principles are applicable to evolution, including number ten: Holobionts and Their Hologenomes Do Not Change the Rules of Evolutionary Biology. They invite the community to “consider an expansive but not revolutionary extension of evolutionary genetics in light of the heritable and inherited microbiome.”
However, it is becoming more likely that our microbiome is influencing evolution. Seth Bordenstein, whose lab investigates how the microbiome contributes to speciation as well as behavior, had demonstrated with Brucker in 2013 that gut bacteria had contributed to hybrid death in a species of wasp. Twenty years ago, work with drosophila demonstrated diet induced mating preferences - raised on a diet of either starch or sugar, flies preferentially mated with flies raised on the same diet. Recently, it was shown that gut bacteria were responsible for these preferences. Especially because this behavior involves mating, it has implications for future generations and the gut bacteria of those progeny.
In the July edition of Trends in Ecology and Evolution, Michael Shapira, a UC Berkeley assistant professor of integrative biology who investigates that evolutionary relationship in C. elegans, is slated to review the most recent findings in the field. He has published several papers in this area, including using diets to investigate the microbiota of worms. Worms tend to have a core set of bacteria, though the balance shifts with different diets, and may be able to adapt to sudden changes in environmental conditions or to help deal with toxins. He also proposes that hosts and their microbiota evolve together and that portions of it can be passed on to offspring. "With the growing understanding that all animals are in fact in a symbiotic relationship with complex microbial communities, the framework to consider how symbiotic interactions shape host evolution should be expanded," he said.
Clearly an area ripe for research, it will be interesting to see what’s to come for this hot new field of study.
Sources:
Berkeley News,
Trends in Ecology and Evolution,
PLOS Biology,
Cold Spring Harbor Laboratory
