Gut microbiomes of humans carry a complex symbiotic assemblage of microorganisms. As in all mammals, the special mode of feeding newborn infants through milk from the mammary gland enhances the opportunity for vertical transmission of the milk microbiome from parents to the gut microbiome of offspring. This has potential benefits, but it also brings with it some hazards for the host. Here we use mathematical and numerical models to demonstrate that vertical transmission from both parents would allow host populations to be invaded by microbiome elements that are deleterious. In contrast, vertical transmission, when restricted to one parent, acts as a sieve preventing the spread of such elements. We show that deleterious symbionts generate selection for uniparental transmission in host populations, and that this selective advantage is maintained in the presence of moderate horizontal transmission. Some vertical transmission from mother to infant is bound to happen in placental mammals. This paper therefore puts forward the hypothesis that the asymmetry between females and males, together with the hazards that come with biparental transmission of the milk microbiome, generate selection against male lactation in humans, and in mammals in general.

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Maternal transmission as a microbial symbiont sieve, and the absence of lactation in male mammals

Theoretical models are useful to investigate the drivers of community dynamics. In the simplest case of neutral models, the events of death, birth and immigration of individuals are assumed to only depend on their abundance-thus, all types share the same parameters. The community level expectations arising from these simple models and their agreement to empirical data have been discussed extensively, often suggesting that in nature, rates might indeed be neutral or their differences might not be important. However, how robust are these model predictions to type-specific rates? Also, what are the consequences at the level of types? Here, we address these questions moving from simple neutral communities to heterogeneous communities. For this, we build a model where types are differently adapted to the environment. We compute the equilibrium distribution of the abundances. Then, we look into the occurrence-abundance pattern often reported in microbial communities. We observe that large immigration and biodiversity-common in microbial systems-lead to such patterns, regardless of whether the rates are neutral or non-neutral. We conclude by discussing the implications to interpret and test empirical data. 

J. A. Piceno

Papers

The effect of microbial selection on the occurrence-abundance patterns of microbiomes