The Paradoxical Effects of Allelic Recombination on Fitness

Horizontal transfer (HT) plays a major role in bacterial evolution, providing a way for bacteria to take advantage of beneficial mutations found by other bacteria, possibly from other species. Within a given species, horizontal transfers allow bacteria to evade the clonal interference phenomenon (Hill and Robertson, 1966) through allelic recombination: when two different beneficial mutations are found concomitantly in two different lineages, horizontal transfer allows both mutations to be assembled into a single organism, thus speeding up evolution. Transfer also enables the isolation of the “ruby in the rubbish” (Peck, 1994): beneficial mutations being very rare compared to deleterious ones, it is likely that deleterious mutations will happen at the same time as a beneficial one, thus overwhelming the benefits of the latter. Transfer however, allows to solve this problem by breaking the linkage between the affected alleles. In this work, focusing on transfer involving recombination rather than simple plasmid exchange, we used the Aevol model to study the influence of HT on the evolution of both fitness and genomic architecture. The Aevol model is a digital genetics model which is realistic at the level of the genome but abstract at the phenotypic level: each individual has a double stranded genome upon which genes are detected through signal siquences and a transcriptiontranslation process. These genes are then interpreted in a mathematical formalism and combined to solve a curvefitting task (Knibbe et al., 2007).