Do hosts and their microbes evolve as a unit?

Abstract

A group of evolutionary biologists sees evidence for a hologenome whereas others dismiss it entirely. One thing’s certain: the debate remains heated . Tilapias like their baths balmy. These tropical fish are happiest in warm pools. But they can be made to adapt to tanks as cold as 12 °C, where they express a set of genes different from their warm-water-dwelling counterparts. Their gut microbes turn out to be different as well—and it may be that these unique microbes play a part in helping fish cope with frigid surroundings, according to the results of a recent study (1). Various research groups have suggested in multiple articles that wasps, aphids, tilapia, and coral (clockwise, top left to bottom left) are among the creatures that exhibit the hallmarks of a hologenome. But many researchers remain skeptical. Image credit (clockwise from top left): Wikimedia Commons/M.E. Clark and Shutterstock/Frances van der Merwe/Piriya Gutsch/Stephan Kerkhofs. 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 …