It is well known that gut microbes play an important role in the health of many organisms. Seth Bordenstein and Robert Brucker, biologists at Vanderbilt University, were curious to see what other effects these microbes may have on an organism. They studied the role of microbes in three related species of parasitic jewel wasps. Two of the species, Nasonia giraulti and N. longicornis, are closely related, whereas the third species, N. vitripennis, diverged about 1 million years ago. Offspring of a cross between N. giraulti and N. longicornis generally result in surviving offspring. However, when either specie was breed with N. vitripennis, almost all male larvae in the second generation die.
With the knowledge that the gut microbes in N. vitripennis differed from N. giraulti and N. longicornis, Bordenstein and Brucker wanted to know if the different microbes played a factor in unsuccessful crossing of different species. To test this, scientists raised all three species of Nasonia without gut microbes by only feeding them sterile food. Interestingly, when the the gut microbe free N. vitripennis was breed with the N. giraulti wasps, almost all of the second-generation wasps survived. The surviving wasps where then reintroduced to bacteria and most of them died.
This experiment provides evidence that parental genes not meshing correctly in hybrids is not the only thing that separates species. It provides an entire new way to understand evolution, looking at the compatibility of parental DNA with the offspring’s microorganisms. For example, some parental genes might allow the immune system to keep certain gut bacteria in check, and without them the gut microbes might make the animal sick. This study provides a new and interesting way to look at the divergence of species.