Evolution in a test tube: rise of the Wrinkly Spreaders

Abstract

Understanding evolutionary mechanisms is fundamental to a balanced biological education, yet practical demonstrations are rarely considered. In this paper we describe a bacterial liquid microcosm which can be used to demonstrate aspects of evolution, namely adaptive radiation, niche colonisation and competitive fitness. In microcosms inoculated with Pseudomonas fluorescens SBW25, evolved mutants such as the Wrinkly Spreader (WS) rapidly arise to form biofilms covering the air-liquid (A-L) interface. WS are readily isolated due to a distinctive colony morphology and reach ∼30% of the population within five days. When re-inoculated into static microcosms, WS preferentially colonises the A-L interface by producing a biofilm, demonstrating a niche preference distinct from the ancestral SBW25 which grows throughout the liquid column. This ability provides the WS with a ∼2.5× competitive fitness advantage over the non-biofilm forming ancestral SBW25. However, WS and SBW25 have similar fitness in shaken microcosms where biofilms cannot form. These practical demonstrations of WS evolution, suitable for secondary or tertiary-level classes, can be linked with a literature-based review of the underlying molecular biology of the WS phenotype to provide a true exemplar of the modern evolutionary synthesis, the current paradigm in evolutionary biology.

Predator Escape: An Ecologically Realistic Scenario for the Evolutionary Origins of Multicellularity

“…Most labs exploring macroevolutionary change have relied on comparisons with living lineages, fossils, or computer simulations (Dubowsky and Hartman 1986;Rodrígues et al Rodríguez et al 2006;Soderberg and Price 2003). While these are valuable teaching materials, inquiry based exercises that utilize living organisms can be more transparent, and offer a direct connection to the ecological theater in which evolutionary processes play out (Delpech 2009;Green et al 2011;Olson and Loucks-Horsley 2000;Plunkett and Yampolsky 2010).…”

Cellulose Expression in Pseudomonas fluorescens SBW25 and Other Environmental Pseudomonads

“…This work, described in the following section, ultimately showed that the evolutionary innovation was the use of cellulose to produce a physically robust and resilient biofilm which allowed the colonisation of the A-L interface. Competitive fitness experiments have demonstrated that the WS has a significant fitness advantage over non-biofilm-forming strains in static microcosms [5,11,49]. Simplistically, WS cells were able to intercept O2 diffusing across the A-L interface from the atmosphere before non-biofilm-forming competitors could do so lower down the liquid column, and as a result, WS populations could grow more rapidly than non-WS populations [53].…”

“…Simplistically, WS cells were able to intercept O2 diffusing across the A-L interface from the atmosphere before non-biofilm-forming competitors could do so lower down the liquid column, and as a result, WS populations could grow more rapidly than non-WS populations [53]. In contrast however, the WS do not enjoy a fitness advantage in shaken microcosms where the O2 concentrations are uniform or on agar plates where the WS phenotype is unstable [5,[48][49].…”

“…A-L interface biofilm formation appears to be an evolutionary deep-rooted ability amongst bacteria, presumably with significant ecological advantages. In experimental microcosms, increases in competitive fitness of biofilm-formers have been observed compared to non-biofilmforming strains, whilst the cost to being a biofilm-forming mutant in an environment not suited to these structures is also measurable [5,[47][48][49]. Biofilm attributes compiled from [15,46,73].…”

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

“…Colonisation of the A-L interface by the Wrinkly Spreader allows better access to O 2 diffusing into the liquid column from the air above, providing a competitive fitness ( W ) advantage over the ancestral wild-type P. fluorescens SBW25 and other non-biofilm-forming mutants whose growth is O 2 -limited deeper into the microcosm [9, 11, 14, 21]. This advantage may reflect a significant change of physiology, as biofilm-isolated cells growing in the high-O 2 zone can be differentiated from those recovered immediately below the biofilm by Raman spectral profiling [22] ( P. fluorescens SBW25 can form a different type of biofilm when induced with Fe 3+ which also provides a fitness advantage [23]).…”