New Insights into the Effects of Several Environmental Parameters on the Relative Fitness of a Numerically Dominant Class of Evolved Niche Specialist.

Anna Kuśmierska,Andrew J Spiers

doi:10.1155/2016/4846565

Abstract

Adaptive radiation in bacteria has been investigated using Wrinkly Spreaders (WS), a morphotype which colonises the air-liquid (A-L) interface of static microcosms by biofilm formation with a significant fitness advantage over competitors growing lower down in the O2-limited liquid column. Here, we investigate several environmental parameters which impact the ecological opportunity that the Wrinkly Spreaders exploit in this model system. Manipulation of surface area/volume ratios suggests that the size of the WS niche was not as important as the ability to dominate the A-L interface and restrict competitor growth. The value of this niche to the Wrinkly Spreaders, as determined by competitive fitness assays, was found to increase as O2 flux to the A-L interface was reduced, confirming that competition for O2 was the main driver of WS fitness. The effect of O2 on fitness was also found to be dependent on the availability of nutrients, reflecting the need to take up both for optimal growth. Finally, the meniscus trap, a high-O2 region formed by the interaction of the A-L interface with the vial walls, was also important for fitness during the early stages of biofilm formation. These findings reveal the complexity of this seemingly simple model system and illustrate how changes in environmental physicality alter ecological opportunity and the fitness of the adaptive morphotype.

Highlights

Adaptive radiation requires organismal evolvability, ecological opportunities, and diversifying selection which create new niches in terms of physical space and new interactions between organisms and environment or allow empty ones to be colonised by adaptive lineages [1,2,3,4]
As WS fitness is negatively frequency dependent [9], competition between Wrinkly Spreaders and the non-biofilm-forming P. fluorescens SBW25 competitor used in this work will vary depending on the relative starting ratio of strains
Experimental populations of P. fluorescens SBW25 in simple microcosms have proved to be a useful model system for investigating bacterial adaptive radiation and allowed mechanistic links to be made between mutation, the Wrinkly Spreader morphotype, and WS fitness advantage

Summary

Introduction

Adaptive radiation requires organismal evolvability, ecological opportunities, and diversifying selection which create new niches in terms of physical space and new interactions between organisms and environment or allow empty ones to be colonised by adaptive lineages [1,2,3,4]. One successful system has used Pseudomonas fluorescens SBW25 populations grown in small glass vials containing nutritionally rich King’s B medium [9] These microcosms can be incubated statically to produce a heterogeneous environment with spatial structure and, in these, P. fluorescens SBW25 populations rapidly diversify over 3–10 days and accumulate mutants such as the Wrinkly Spreader. This class of adaptive mutant or morphotype [8] produces distinctive wrinkled colonies on agar plates and a robust, well-attached biofilm at the air-liquid (A-L) interface of static microcosms (sometimes referred to as pellicles, but see [10]) (Figure 1). A range of mutations associated with diguanylate cyclases (DGCs) result in the Wrinkly Spreader (WS) phenotype through upregulation of c-di-GMP levels and the International Journal of Evolutionary Biology (a)

Methods

Results

Conclusion