Fig. 1

Holobiont phenotypes and selection. Holobionts A, B and C elaborate the same, positively selected phenotype ‘X’. Each multicellular host denoted by the outermost rectangle harbors four types of microbiota members (smaller shapes), with colour indicating a specific function. Hosts A and B have identical genetic backgrounds (indicated by the yellow fill color), but there is a redistribution of functions within the same components, symbolized by identical shapes but with changed colors within the outer rectangle. Host C differs from both A and B in having an entirely different genetic background (green fill color) as well as components (different shapes) but crucially retains all the functions required to produce favorable phenotype (colors are ‘conserved’). Thus, even though a holobiont phenotype (identical in all three cases) may be positively selected, the functional elements that produce this ‘favorable’ phenotype need not be identical. The findings of Lozupone et al. [24] support such phenotypic convergence under selection