Speaker
Description
Natural selection is the non-random differential reproduction of individuals in a population. Selection occurs upon a pool of already exiting variation. Thus, the outcome of selection depends on available phenotypic variation. Although mutations in the genome are random, generation of variant types is not random because of constraints and redundancy. Therefore, from all possible genotypes, only a fraction of phenotypes is likely to be realized. The bacterium Pseudomonas fluorescens SBW25 is a good model to investigate the biases in the generation of variation and its effects on selection. In microcosms, bacterial growth in unshaken conditions results in depletion of oxygen in the liquid. Mutants that arise that are able to grow at the air-liquid interface (ALI), where Oxygen is available, are selected for. There are multiple ways by which SBW25 mutants can grow at the ALI. Overproduction of polymers is one, overproduction of proteinaceous fibers is another. We always observe mutants taking the first solution and, despite the enormous efforts on understanding the genetics of adaptation of SBW25 ALI colonizer mutants, we have never observed mutants with the second solution. In this work we used a combination of transcriptomics, functional genetics and experimental evolution to show how the proteinaceous fiber Fap, although playing a subtle role in the ALI colonization by the canonical mutants using the polymer solution, are themselves sufficient for the ALI colonization but never realized. We later revealed that fap regulation is tightly regulated and shared with polymers regulation, because its activation depends on the same second messenger that regulates polymers, c-di-GMP. The initial mutation required for the Fap phenotype is one that elevates c-di-GMP. This mutation also results in polymer activation, which allows mutants to grow in the ALI. Consequently, selection is reduced, and the other Fap-activating mutations become detrimental. Therefore, the Fap-phenotype is not independently (from the polymers) accessible by mutation and therefore never realized. These results underscore the complexity of adaptive evolution, which involves several layers of explanation, including available genome potential, gene regulation, fitness effects, and the likelihoods of alternative pathways. While some phenotypes are fit and possible solutions to the same problem, some will never be seen by selection.
