Do hosts and their microbes evolve as a unit?

Do hosts and their microbes evolve as a unit?

Jyoti Madhusoodanan

A group of evolutionary biologists sees evidence for a hologenome whereas others dismiss it entirely. One thing’s certain: the debate remains heated.

But which is actually responsible for the adaptation—a change in the animal’s gene expression, or a change in its microbiome? According to one theory of evolution—which proposes that hosts and their resident microbes function as an evolutionary unit—the answer might be both.

This unit, dubbed the holobiont, carries what some have termed a hologenome, meaning the genetic information encoded by both a host and its microbes. The hologenome theory suggests that evolutionary pressure acts on holobionts, not hosts or microbes alone, and so the two should be considered a single unit of selection.

Studies of fish, wasps, corals, and several other animals provide evidence to support the provocative idea that creatures and their microbial inhabitants are linked as holobionts through evolutionary time. Some researchers endorse the concept because it offers a better way to represent the importance of microbes to plant and animal evolution. But others question whether the idea is more confusing or distracting than useful, suggesting that concepts such as ecological filtering, in which the environment (or in this case, the host) selects for or against certain microbial species, can already account for the dynamics researchers have documented.