Context-dependent adaptation in structured environments

Adaptive evolution often leads to niche specialization, but successful colonization of a new niche can depend as much on ecological context as on genetic change. This is especially true in spatially structured environments, where the construction of habitat and order of arrival of mutants shape evolutionary outcomes. In static broth cultures, the air-liquid interface (ALI) provides a high-oxygen niche typically colonized by mat-forming strategists. In Pseudomonas fluorescens, single mutations that upregulate cellulose production give rise to 'wrinkly spreader' (WS) types-canonical niche specialists that out-compete ancestral types at the ALI. Here, we show that this view can be refined: WS mutants often fail to establish when introduced alone at low density, but co-culture with ancestral types rescues colonization. Microscopy and simulations reveal that ancestral types physically scaffold the ALI, enabling attachment and facilitating the in situ emergence of diverse WS mutants. Ancestral types later disperse, allowing WS expansion. This transient asymmetric interaction shows that even strongly adaptive mutations may rely on ecological facilitation. More broadly, our findings highlight the reciprocal links between ecology and evolution, revealing how transient ecological dependencies can shape adaptive trajectories.