The Microbiome as a Maternal Effect: A Systematic Review on Vertical Transmission of Microbiota

The microbiome is an interactive and fluctuating community of microbes that colonize and develop across surfaces, including those associated with organismal hosts. A growing number of studies exploring how microbiomes vary in ecologically relevant contexts have recognized the importance of microbiomes in affecting organismal evolution. Thus, identifying the source and mechanism for microbial colonization in a host will provide insight into adaptation and other evolutionary processes. Vertical transmission of microbiota is hypothesized to be a source of variation in offspring phenotypes with important ecological and evolutionary implications. However, the life-history traits that govern vertical transmission are largely unexplored in the ecological literature. To increase research attention to this knowledge gap, we conducted a systematic review to address the following questions: (1) How often is vertical transmission assessed as a contributor to offspring microbiome colonization and development? (2) Do studies have the capacity to address how maternal transmission of microbes affects the offspring phenotype? (3) How do studies vary based on taxonomy and life history of the study organism, as well as the experimental, molecular, and statistical methods employed? Extensive literature searches reveal that many studies examining vertical transmission of microbiomes fail to collect whole microbiome samples from both maternal and offspring sources, particularly for oviparous vertebrates. Additionally, studies should sample functional diversity of microbes to provide a better understanding of mechanisms that influence host phenotypes rather than solely taxonomic variation. An ideal microbiome study incorporates host factors, microbe-microbe interactions, and environmental factors. As evolutionary biologists continue to merge microbiome science and ecology, examining vertical transmission of microbes across taxa can provide inferences on causal links between microbiome variation and phenotypic evolution.