Gut microbes may put barrier between species

SNOWBIRD, Utah — The bazillion microbes teeming inside bigger creatures may be an overlooked but vital part of what divides the big organisms into species.

Two species of jewel wasp (Nasonia giraulti and N. vitripennis) stay separate in large part because most male larvae die when the species crossbreed, says Seth Bordenstein of Vanderbilt University in Nashville. Biologists have long blamed the demise on lethal incompatibilities in DNA. Yet using antibiotics to kill off the gut bacteria in the supposedly doomed hybrids rescued many of them, he reported June 23 at the Evolution 2013 conference. In this lab test, the germ-free hybrid larvae survived about as well as purebred germ-free larvae.

In a further test, colleague Robert Brucker gave microbe-free hybrid larvae two kinds of gut bacteria from regular hybrids. Survival rates plummeted.

“I would never have predicted that,” said Corrie Moreau, an ant taxonomist at the Field Museum in Chicago, who was in Bordenstein’s audience. “We were blown away.”

Susan Perkins, microbiology curator at the American Museum of Natural History in New York, agreed. “It’s the first time I’ve heard of good proof of the microbiome playing a role in maintenance of species,” she said.

The experiments don’t mean genetics is irrelevant for separating these species, Bordenstein said. Maybe biologists just missed a part of the story. Two species may split not only because their genes diverge but also because their communities of resident microbes diverge, he said.

To see how those communities might have diverged, Brucker and Bordenstein compared the kinds of microbes flourishing on four species of jewel wasp. They used the comparison to create an evolutionary family tree, and the relationships it showed are the same ones that scientists had deduced from the wasps’ genes.

The fatal mismatch of microbes divides species that diverged long ago in the family tree, but not a more recently separated pair, the researchers found. Both genetic and microbial trees show that N. giraulti’s closest relative is N. longicornis. Lab-bred hybrids of the two don’t usually die off in great numbers. And rendering hybrids of the more recently divided species germ-free didn’t make a noticeable difference in survival. The genetic and microbial barriers are both lower in this more-recent species split than across the ancient wasp divide.

Just how wasp genetics and microbes interact to kill hybrids remains to be seen. Bordenstein pointed out that the dying larvae darken in color, as if secreting melanin — a common response to inflammation. Some of the parts of animal genomes that evolve most rapidly are those dealing with immune systems, so perhaps genes growing apart between species end up welcoming or repelling different microbes.

The experiment presents a good reason to look at the hologenome, the combined DNA of an organism and its many microscopic residents, Bordenstein said. “That’s kind of a controversial term right now,” he said, because there isn’t much evidence regarding when the host-microbe combination matters. Looking at combined DNA certainly expands the notion of which genes matter. A human has roughly 20,000 genes, Bordenstein said, but resident microbes add some 8 million.