Microbiome mediation of animal life histories via metabolites and insulin-like signalling.

The regulatory pathways by which gut microbiota potentially shape host life histories remain largely untested, however, a constellation of research suggests that gut bacteria likely have significant effects on their hosts via metabolites. In this article we review known and hypothesized pathways by which gut microbiota influence host life histories through interfacing with the neuroendocrine system, with a focus on the insulin-like growth factor (IGF) signalling pathway. Bacterially derived metabolites including short-chain fatty acids (SCFAs), polyamines, and peptides likely impact host life histories as metabolic substrates, essential nutrients, and via molecular signalling with well-studied neuroendocrine pathways. The hypothalamus-pituitary axis and insulin-like signalling (ILS) pathways are central regulatory networks for development, growth, reproductive maturity, reproduction, and senescence and are likely targets for tests of how gut bacterial metabolites shape host life histories. SCFAs in particular, as metabolites derived from bacterial fermentation, are implicated as significant microbiome signalling molecules shown to interface with the ILS pathway as well as to bind to receptors on neuroendocrine and peripheral nervous tissues. For example, experimental increases of SCFA production have been shown to affect IGF-1 levels in circulation and are associated with robust development, growth, reproduction, and delayed senescence. Finally, emerging -omics approaches are providing integrative ways to test and detail the potential diverse ways in which gut microbiota interact with their hosts and the likely important roles they play in shaping host life-history responses to varied environmental conditions.