Adaptive Divergence in Experimental Populations ofPseudomonas fluorescens. IV. Genetic Constraints Guide Evolutionary Trajectories in a Parallel Adaptive Radiation

Abstract

The capacity for phenotypic evolution is dependent upon complex webs of functional interactions that connect genotype and phenotype. Wrinkly spreader (WS) genotypes arise repeatedly during the course of a model Pseudomonas adaptive radiation. Previous work showed that the evolution of WS variation was explained in part by spontaneous mutations in wspF, a component of the Wsp-signaling module, but also drew attention to the existence of unknown mutational causes. Here, we identify two new mutational pathways (Aws and Mws) that allow realization of the WS phenotype: in common with the Wsp module these pathways contain a di-guanylate cyclase-encoding gene subject to negative regulation. Together, mutations in the Wsp, Aws, and Mws regulatory modules account for the spectrum of WS phenotypegenerating mutations found among a collection of 26 spontaneously arising WS genotypes obtained from independent adaptive radiations. Despite a large number of potential mutational pathways, the repeated discovery of mutations in a small number of loci (parallel evolution) prompted the construction of an ancestral genotype devoid of known (Wsp, Aws, and Mws) regulatory modules to see whether the types derived from this genotype could converge upon the WS phenotype via a novel route. Such types-with equivalent fitness effects-did emerge, although they took significantly longer to do so. Together our data provide an explanation for why WS evolution follows a limited number of mutational pathways and show how genetic architecture can bias the molecular variation presented to selection.

Founder niche constrains evolutionary adaptive radiation

“…Coexistence among WS genotypes has also been shown to be stabilized by negative frequency-dependent fitness interactions, indicating differences between their niches (41). The genetic changes underlying this model of adaptive radiation have been established for various niche specialists at the nucleotide level (40,(42)(43)(44)(45)(46)(47).…”

The distribution of fitness effects of new beneficial mutations inPseudomonas fluorescens

“…To address this, we sequenced 20 of the 100 WS mutants at four loci known to acquire WS-generating mutations: wspF, awsX, awsR and mwsR [16]. We found 13 mutations; five of these were unique, two occurred twice and one occurred three times (table 1).…”

Cellulose Expression in Pseudomonas fluorescens SBW25 and Other Environmental Pseudomonads

“…Interestingly, no naturally occurring WspR mutants have been identified yet, despite the fact that engineered WspR mutants like WspR19 were found to show the predicted phenotype [60,[69][70][71]. Mutations in other operons leading to the activation of the AwsR and MwsR DGCs can also induce the WS phenotype [47,72]. These different routes activating the WS phenotype can be seen as an example of parallel evolution leading to new A-L interface biofilm-forming genotypes in static microcosms [72].…”

“…Mutations in other operons leading to the activation of the AwsR and MwsR DGCs can also induce the WS phenotype [47,72]. These different routes activating the WS phenotype can be seen as an example of parallel evolution leading to new A-L interface biofilm-forming genotypes in static microcosms [72].…”

“…AwsXR was a previously unrecognised regulatory system, first identified in SBW25 where the mutational activation of the DGC AwsR results in the WS phenotype [47,72], though the normal means of regulating AwsR activity remains unknown. FleQ is a c-di-GMPresponsive transcriptional regulator, and in P. aeruginosa PA01 it controls the hierarchical regulatory cascade for flagella biosynthesis and the repression of the PEL biosynthesis genes [88][89].…”