Character displacement in Burkholderia biofilm communities and the effect of adaptive history on the evolution of niche breadth

Ecologists agree that biodiversity is essential for maintaining highly productive and stable ecosystems, yet the mechanisms that generate and preserve diversity are unknown in many habitats. Competition for shared resources may cause selection to favor niche specialization, which reduces competition and reinforces diversity. If the environment is significantly altered, the ecological history of the remaining colonizer may influence fitness and capacity for diversification in new environments. In many cases, specialists have limited adaptive potential due to trade-offs generated by niche-specific adaptation; however, generalists may be adaptable in many habitats, which would make them good pioneer species for colonization. We investigated the role of character displacement and facilitation in maintaining diversity in a synergistic, biofilm-adapted population of Burkholderia cenocepacia, an opportunistic pathogen of cystic fibrosis patients that infects by producing diverse biofilms. Additionally, we examined effects of adaptive history on the ability of biofilm generalists and specialists to adapt to altered environmental conditions. Our model biofilm population consisted of phenotypically and genetically distinct ecotypes each fulfilling a separate ecological role. We found that diversification in the biofilm was associated with altered resource, as each ecotype occupied distinct spatial niches. We then evolved each ecotype in isolation in planktonic conditions to study whether adaptive history determined the potential to revert to a planktonic lifestyle. Following adaptation, each ecotype evolved into morphologically uniform populations phenotypically resembling the biofilm generalist type, yet fitness of these populations was constrained by prior niche specialization. Lastly, we modeled habitat colonization by evolving each ecotype in isolation in the biofilm environment and tested whether specialization limited the capacity for niche expansion in the absence of competitors. Each biofilm specialist evolved considerable functional diversity, yet the generalists expanded their niche without diversification, which suggested that their adaptive potential exceeded that of the specialists. In summary, diversification within biofilm communities generates competition that favors character displacement and facilitation. Furthermore, when environmental conditions are altered, generalists evolve more productive populations than specialists with less diversification and pleiotropic cost