Fig. 3

Mechanisms underlying symbiont facilitation and parallels with free-living organisms. Blue circles: facilitating symbionts; green circles: facilitated symbionts; yellow shading: within-host environment; blue shading: between-host environment; grey or yellow circles: host; purple circles: resource; diamonds: host immune response; blue arrows: facilitation (solid: direct; dashed: indirect); black arrows: other types of effects (solid: enabled; dotted: cancelled). Traits facilitated are categorised as ‘development’ (i.e. symbiont’s growth or differentiation) in the within-host environment, ‘infection success’ (i.e. host entry or establishment) and ‘exit from the host’ at the interface between the within- and between-host environment, and ‘transmission’ in the between-host environment