A comparative approach to understanding selection on extracellular secretion

Many bacterial genes encode proteins that are secreted extracellularly. These proteins can be considered cooperative because all surrounding cells can benefit from their production. Therefore, it has been hypothesized that these cooperative genes would more frequently lie on mobile elements, such as plasmids, which can transfer to other cells. This could stabilise cooperation, leading to the prediction that plasmids should carry proportionally more cooperative genes than the less mobile chromosome. However, it is unknown whether this prediction holds across the bacterial tree of life. To address this, we analysed the gene content of the chromosome and plasmid(s) of 1620 genomes comprising 51 diverse bacterial species. We find that across species analysed, plasmids do not carry proportionally more cooperative genes than the chromosome. Contrary to prediction, the role of mobile elements in promoting cooperative behaviour is highly variable across bacterial species.