Human reproductive system microbiomes exhibited significantly different heterogeneity scaling with gut microbiome, but the intra‐system scaling is invariant

Maintaining sexual reproduction in a highly competitive world is still one of the major mysteries of biology given the apparently high efficiency of asexual reproduction. Co-evolutionary theories such as the Red Queen hypothesis would suggest that the microbiomes in human reproductive systems, specifically the microbiomes contained in semen and vaginal fluids, should reach some level of homogeneity thanks to arguably the most conspicuous microbiome transmission between two sexes. The long-term sexual coevolution should favor the dynamic homogeneity or stability, which should also be beneficial for sexual reproduction such as sperm survival or fertilization on physiological/ecological time scale. We present a piece of quantitative evidence in the form of microbial community spatial heterogeneity to support the stability notion by analyzing three big datasets of the human vaginal, semen and gut microbiome. Methodologically, we applied a recent community-level extension to the classic Taylor's power law, which reached the rare status of ecological law and has found applications beyond biology. Both ecological and evolutionary theories, such as hologenome/holobiont and Red Queen, as well as consideration of first principles, would predict that microbiome transmissions between two sexes should have homogenizing effects on the composition and stability of the microbiomes in human reproductive systems, and therefore have similar variance structures. This is supported by the finding that the power law analysis revealed human vaginal and semen microbiomes exhibited the same scaling parameter size in their community spatial (inter-individual) heterogeneities, while the both exhibited significantly different heterogeneity scaling parameter size with the human gut microbiome.