The evolution of a pleiotropic fitness tradeoff in Pseudomonas fluorescens

Abstract

The evolution of ecological specialization is expected to carry a cost, due to either antagonistic pleiotropy or mutation accumulation. In general, it has been difficult to distinguish between these two possibilities. Here, we demonstrate that the experimental evolution of niche-specialist genotypes of the bacterium Pseudomonas fluorescens that colonize the air-broth interface of spatially structured microcosms is accompanied by pleiotropic fitness costs in terms of reduced carbon catabolism. Prolonged selection in spatially structured microcosms caused the cost of specialization to decline without loss of the benefits associated with specialization. The decline in the cost of specialization can be explained by either compensatory adaptation within specialist lineages or clonal competition among specialist lineages. These results provide a possible explanation of conflicting accounts for the cost of specialization.T he evolution of a diverse community in a heterogeneous environment requires that no single type, whether it be a sexual species or an asexual genotype, be able to simultaneously outcompete all other types in all patches of the environment (1-3). According to the pleiotropy hypothesis of niche evolution, the evolution of such a universally superior type is unlikely, because alleles that increase fitness in one patch will inevitably cause a regress of fitness in other patches (2, 4-7). In contrast, recent theoretical work has demonstrated that pleiotropic fitness costs do not need to be invoked to explain the evolution of a diverse community of specialists. The evolution of a universally superior type could be prevented by demographic constraints on the ability of broadly distributed types to rapidly adapt to a variety of patches or conditions (8). In the absence of pleiotropy, a cost of specialization is likely to evolve as conditionally deleterious mutations that reduce fitness in rarely encountered or unproductive patches stochastically accumulate in specialist populations (9-11). In summary, both the antagonistic pleiotropy and mutation accumulation theories predict the evolution of tradeoffs in fitness across patches of a heterogeneous environment; they also predict that specialization will be accompanied by a cost, but they provide very different accounts of the genetic basis of this cost. According to the pleiotropy theory, the same mutations that create a specialist phenotype create a cost of specialization. The mutation accumulation theory assumes that mutations that create specialist phenotypes are not directly associated with a cost, but that specialist populations will stochastically accumulate mutations at other loci that carry a fitness cost.As a result of the relative ease with which organisms and environments can be manipulated in the laboratory, selection experiments have become a powerful tool for testing alternative hypotheses concerning ecological and evolutionary causes and consequences of specialization (12-15). The general protocol for these experiments is as follows: replicate s...

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A design-constraint trade-off underpins the diversity in ecologically important traits in species Escherichia coli

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