The Hologenome Theory

A hologenome is defined as the sum of genetic information from both the host and its microbiota. This recently proposed theory of evolution considers the holobiont (the organism and all of its associated symbiotic microbes) a unit of natural selection and is based on four well documented generalizations:

  1. All animals and plants establish symbiotic relationships with microorganisms.
  2. Symbiotic microorganisms are transmitted between generations.
  3. The association between host and symbiont affects the fitness of the holobiont.
  4. Under environmental stress, the symbiotic microbial community can change rapidly.

These points taken together suggest that the genetic wealth of diverse microbial symbionts can play an important role in both the adaptation and evolution of higher organisms.

Image: BODY MICROBES, by Bruno Vergauwen.

Researcher proposes that a host’s microbiota can steer its evolution in new directions

This has got to be the most exciting idea in biology that I’ve read about in months.

…In a recent paper in the journal Trends in Ecology and Evolution, Shapira, who studies the gut microbes of the nematode C. elegans, reviews evidence that demonstrates how microbiotas affect and contribute to host evolution, either by evolving along with the host, or by stepping in at critical moments to help the host adapt to a new environmental challenge.

These examples, he says, bolster the relatively recent concept of the hologenome, a term referring to the genomes of the host and its microbes together, encompassing perhaps thousands of different types of bacteria on the skin, in the gut and even in reproductive organs. In his recent paper, Shapira elaborates on a 2008 proposal by Tel Aviv University researchers that evolution can act on the hologenome, rather than on the genomes of the host and its microbiota separately. This implies that as the host evolves to suit a changing environment, its microbiota play a critical role in directing and participating in that evolution.

“When I came across the paper by Ilana Zilber-Rosenberg and Eugene Rosenberg describing the hologenome concept, it blew my mind,” Shapira said. “The idea that animals could undergo selection not based solely on their own genome, but with the help of many more, opens the door for previously unimagined evolutionary paths.”…

An experiment performed in fruit flies demonstrates this. When raised on different types of food, flies develop different gut microbiotas, presumably better at handling the available food.

The surprising outcome, however, was that “within one generation, the flies developed mate preference for their own group, ignoring the others, and that this was dependent on the microbes in the gut that helped them utilize the food,” he said. “This led to de facto reproductive isolation of two populations and could facilitate future speciation, that is, real reproductive isolation – a genetic barrier preventing members of the two groups from parenting viable or fertile progeny.”…

“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.

Researchers examining the genes of different species of aphids and of their individual gut bacteria found that the emergence of new species of aphids during evolution was mirrored by speciation events in the insects’ Buchnera symbionts. This demonstrates how the linked fates of the two species lead to co-evolution. But Buchnera are not alone. Subsequent work showed that aphids also harbor other symbionts that are less important for, or dependent on, their host, but nevertheless help the insects adapt to new niches in a changing environment…