Here's how to create a new species. Put animals—say finches—from the same species on separate islands and let them do their thing for many, many generations. Over time, each group will adapt to its new environment, and the genomes of the two populations will become so different that if you reintroduce the animals to the same habitat, they can no longer breed successfully. Voilà, one species has become two. But a new study suggests that DNA isn't the only thing that separates species: Some populations diverge because of the microbes in their guts.
The paper is "important and potentially groundbreaking," says John Werren, a biologist at the University of Rochester in New York. "Scientists have studied speciation … for many years, and this opens up a whole new aspect to it."
The new work involves three different species of parasitic jewel wasps, tiny insects that drill into the pupas of flies and lay their eggs, letting the offspring feed on the host. Two of the species, Nasonia giraulti and N. longicornis, are closely related, whereas the third species, N. vitripennis, diverged from the other two about 1 million years ago. When N. giraulti and N. longicornis mate in the lab, most of their offspring survive, but when either mates with N. vitripennis, almost all male larvae in the second generation die.
Seth Bordenstein and Robert Brucker, biologists at Vanderbilt University in Nashville, wondered if the reason for this mortality went beyond incompatible DNA. They knew that the gut microbes in N. vitripennis differed from those in the other two species, and they suspected that these microbes could play a role in the offspring deaths. Indeed, when they raised all three species of Nasonia without gut microbes—by rearing them on sterile food—almost all the second generation offspring of matings between N. vitripennis and N. giraulti wasps survived. And when the scientists reintroduced bacteria into the germ-free wasps, most of their second-generation offspring died , the duo report online today in Science.
Werren says that the work introduces a whole new way to look at what sets species apart. Instead of just thinking about genes of the parents not meshing in hybrids, he says, biologists could now think about how the parents' genes are incompatible with the offspring's microorganisms. Some parental genes could enable the immune system to keep certain gut bacteria in check, for instance, and without them the gut microbes might sicken the animal and kill it.
Bordenstein goes one step further. The genes of microbes harbored by an organism are just as important for evolution as the genes in its own cells, he says, calling both together a "hologenome." Werren disagrees. Microbes in the gut interact with the bigger organism, just like other organisms in nature interact, for instance predator and prey, Werren says. "They are not co-evolving as a single unit, so why would we call them a single genome?"
Axel Meyer, an evolutionary biologist at the University of Konstanz in Germany, is skeptical about whether gut microbes have a big effect on speciation. The paper is "exciting," he says, "but there might be huge biological differences between species in how the microbiome [the community of microbes in an organism] is established." As a result, he says, it is an open question whether there will be many more examples of gut microbes separating species. Werren thinks there will be. "No pun intended," he says, "but my gut tells me that this is going to be common."