Contrary to current understanding, a new study has provided direct evidence that an individual’s microbiome can contribute to the origin of new species by reducing the viability of hybrids produced between males and females of different species.

These findings provide evidence for the controversial hologenomic theory of evolution, which proposes that the object of Darwin’s Natural Selection is not just the individual organism, but the organism plus its associated microbial community.

Seth Bordenstein, Associate Professor of Biological Sciences at Vanderbilt University, who performed the study with graduate student Robert Brucker, said: “It was a high-risk proposition. The expectation in the field was that the origin of species is principally driven by genetic changes in the nucleus. Our study demonstrates that both the nuclear genome and the microbiome must be considered in a unified framework of speciation.”

Bordenstein and Brucker conducted their research using three species of the jewel wasp Nasonia which are useful for biological control. The wasps have a microbiome of 96 different groups of microorganisms.

Two of the species studied (N.giraulti and N. longicornis) diverged at similar times, around 400,000 years ago so they are closely related in genetics and microbiomics. On the other hand, the third species (N. Vitripennis)emerged about a million years ago so there are much greater differences in both its genome and microbiome.

When the researchers interbred the three species, offspring of the two closely related species had a low mortality (around 8%), but if either were interbred with the third species, a much higher mortality rate in offspring was seen (>90%).

“The microbiomes of viable hybrids looked extremely similar to those of their parents, but the microbiomes of those that did not survive look chaotic and totally different,” said Brucker.

The researchers were able to determine that the mortality had a microbial basis by raising the wasps in a germ-free environment. Microbe-free hybrids actually survived just as well as purebred larvae but when their gut microbes were introduced, their survival rate plummeted.

“Our results move the controversy of hologenomic evolution from an idea to an observed phenomenon,” said Bordenstein. “The question is no longer whether the hologenome exists, but how common it is.”