Several hybrid animal species exist in the world. Cross a donkey dad and a horse mom, you get a mule. Switch those parent roles around (donkey mom, horse mom) and you get a hinny. Such hybrids are most often sterile, and in some pairings, the offspring will not even survive.
This phenomenon is known as ‘hybrid incompatibility’ and can involve a gene-gene mismatch between the parents, leading to sterility of the offspring, which preserves the integrity of the two species. In 2013, scientists discovered that the microbiome might also play a role in hybrid incompatibility. In two closely related wasp Nasonia species that had distinct microbiomes, hybrid crosses most often died. However, if those hybrid larvae were incubated in germ-free conditions (i.e., in the absence of a microbiome), then they survived! These data suggest that bacteria may provide a helping hand in the evolutionary process by contributing to the generation of new species via reproductive isolation. Well played microbiome, very well played.