What traits are carried on mobile genetic elements, and why?

Abstract

Although similar to any other organism, prokaryotes can transfer genes vertically from mother cell to daughter cell, they can also exchange certain genes horizontally. Genes can move within and between genomes at fast rates because of mobile genetic elements (MGEs). Although mobile elements are fundamentally self-interested entities, and thus replicate for their own gain, they frequently carry genes beneficial for their hosts and/or the neighbours of their hosts. Many genes that are carried by mobile elements code for traits that are expressed outside of the cell. Such traits are involved in bacterial sociality, such as the production of public goods, which benefit a cell's neighbours, or the production of bacteriocins, which harm a cell's neighbours. In this study we review the patterns that are emerging in the types of genes carried by mobile elements, and discuss the evolutionary and ecological conditions under which mobile elements evolve to carry their peculiar mix of parasitic, beneficial and cooperative genes.

Figure 1

An illustration of the nested nature of plasmids and their bacterial hosts. Different sub-populations may harbour different densities of plasmids. If a gene involved in cooperation is carried on a plasmid, this will affect the relatedness between individuals in a patch (because of relatedness being measured at a focal locus, in this case on a plasmid). Thus, sub-populations A and D have high plasmid relatedness, whereas sub-populations C and B have low plasmid relatedness. Relatedness is influenced by local cell density and migration between sub-populations and, in the case of loci on plasmids, relatedness changed because of the degree of horizontal gene transfer (see Box 2 and Figure 3 for details).

Figure 2

General classification of social traits carried by MGEs and their effect on the bearer or a social neighbour. The x axis represents the effect of the trait on the fitness of the host bacteria, whereas the y axis represents the effect of the trait on the fitness of the host bacteria's neighbours.

Figure 3

The effect of horizontal gene transfer rate β on genetic relatedness between individuals within a patch, measured at loci with transfer rate β. The equation follows from that described in Box 2, where m=0.1, N=50 and s=0. In the absence of any HGT (that is, β=0) for the parameters chosen, r=0.079 (shown by the dotted line).