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

Abstract

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 elements of this microbiome. This has potential benefits, but it also brings with it some hazards for the host. We point out here that vertical transmission from both parents would allow host populations to be invaded by elements that are deleterious. In contrast, vertical transmission, when restricted to one parent, acts as a sieve preventing the spread of such elements. Because some transmission from mother to infant is unavoidable in placental mammals, uniparental transmission of symbionts, if it were to be selected, would require some separation of the father from the newborn infant. This paper therefore puts forward the hypothesis that the asymmetry in roles of mother and father, together with the hazards that come with biparental transmission, generate a selection pressure against male lactation in humans, and in mammals in general.

Gut microbiome disturbances of altricial Blue and Great tit nestlings are countered by continuous microbial inoculations from parental microbiomes

“…The direct and indirect transfer of maternal microbiomes is likely essential for naturally developing chick microbiomes, as they may lose some gut symbionts due to diet and habitat changes, and during infections with natural pathogens or ones associated with anthropogenic activities [16, 23, 38, 85]. Skewed maternal microbial transfer may reduce competition between parental microbial symbionts sharing the same niches within offspring guts, with potential deleterious effects to chicks [86, 87]. However, male microbial symbionts are not completely lost during generational transmission, indicating that colonization of males does not necessarily mean a dead end for microbes.…”