A little over a year ago, Bordenstein, a biologist at Vanderbilt University in Nashville, Tennessee, and his then-graduate student, Robert Brucker, mated two incompatible species of wasp in the lab, creating a hardy hybrid that lived when most others died. Normally, when members of two related species of parasitic wasps in the genus Nasonia, N. giraulti and N. longicornis, mate with their more distant relative N. vitripennis, the hybrid offspring die. Until recently, no one could figure out exactly why, but it was clear that this was one of the major barriers dividing the species. But when Bordenstein and Brucker treated the wasps with antibiotics, eliminating the millions of microbes that lived on their bodies, they found that many of the hybrid offspring unexpectedly could survive and thrive. By stripping off the wasps’ microbiomes — the microbial community inhabiting the insects — Bordenstein and Brucker had brought a totally new hybrid wasp to life.
The findings, published in Science in July 2013, highlight a surprising idea in biology: that symbiosis — a long-term, stable and often beneficial interaction between organisms — could drive two populations apart, the first step in the development of new species. Although the idea has been floating around for nearly a century, it has only recently begun to gain traction in biology. This idea contrasts sharply with the traditional picture of evolution, in which new species emerge either from geological isolation or from a relentless struggle for food and mates. According to this new hypothesis, a host organism’s microbes might trigger changes in mating and reproduction that begin to define two different populations.
Joshua Gibson, an evolutionary biologist at Purdue University in West Lafayette, Indiana, and a Nasonia expert, thinks it’s too soon to say conclusively that the microbiome can cause speciation based on Bordenstein’s work. “It’s not clear whether the microbiome led to the genetic changes or whether the genetic changes caused the microbiome to shift,” he said. Even if the microbiome did cause a new species of Nasonia to evolve, that doesn’t mean it is happening everywhere across the animal kingdom, Gibson added. The factors affecting each species will be different, as will the importance of the microbiome, he said.
In order to fully grasp the complexities of how symbiotic microbes might affect evolution, Bordenstein said, the fields of microbiology and evolutionary biology will have to form a symbiotic relationship of their own. “I see it as a chance to define the future of biology,” he said. “Will we have a genomic-centric view of life, in which the nuclear genome is the only way we look at evolution, or will we have a more unified view, in which the definition of an animal changes to include both the genome and the microbiome?”