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.

Xenobiotic estradiol-17ß alters gut microbiota of hatchling American alligators (Alligator mississippiensis).

Environmental oestrogens pose serious concerns for ecosystems through their effects on organismal survival and physiology. The gut microbiome is highly vulnerable to environmental influence, yet the effects of oestrogens on gut homeostasis are unknown because they are poorly studied in wildlife populations. To determine the influence of environmental oestrogens (i.e., xenoestrogens) on the diversity and abundance of gut microbiota, we randomly assigned 23 hatchling American alligators (Alligator mississippiensis) to three ecologically relevant treatments (control, low, and high oestrogen concentrations) for 10 weeks. We predicted that xenoestrogen exposure would decrease microbial diversity and abundance within the digestive tract and that this effect would be dose-dependent. Microbial samples were collected following diet treatments and microbial diversity was determined using 16S rRNA gene-sequencing. Individuals in oestrogen-treatment groups had decreased microbial diversity, but a greater relative abundance of operational taxonomic units than those in the control group. In addition, this effect was dose-dependent; as individuals were exposed to more oestrogen, their microbiome became less diverse, less rich and less even. Findings from this study suggest that oestrogen contamination can influence wildlife populations at the internal microbial-level, which may lead to future deleterious health effects.

Lizard Embryos Prioritize Posthatching Energy Reserves over Increased Hatchling Body Size during Development.

Embryonic development in oviparous organisms is fueled by maternally allocated yolk, and many organisms hatch before that energy store is used completely; the resultant leftover (residual) yolk is internalized and may support early posthatching life. However, embryos that use most, or all, of their yolk supply before hatching should hatch at a larger size than those that do not exhaust those energy reserves, which could also have benefits for posthatching growth and survival. To examine the trade-off between residual yolk and offspring size, we experimentally reduced yolk quantity at oviposition in lizard eggs (Amphibolurus muricatus) and then quantified offspring size and the amount of internalized residual yolk. This design enabled us to determine whether embryos (1) exhaust yolk supply during development (thereby maximizing neonatal size) or (2) reduce neonatal size by retaining yolk reserves at hatching. Our data support the latter scenario. Eggs from the yolk-reduced treatment produced smaller offspring with a proportion of residual yolk similar to that of offspring from unmanipulated eggs, suggesting that the fitness benefits of posthatching energy stores outweigh those of larger neonatal size.