The direct connection between this gene acquisition and the action of the Ps.

This gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.

We inferred the numbers of gene acquisitions and losses using the method described in Figure 1and in the Materials and methods section.

Such deletions are apparently irreversible in obligate mutualists, which lack recombination functions and genetic elements, such as prophages, transposons, and repetitive DNA that typically mediate gene acquisition.

In contrast, we cannot exclude the possibility that some inferred gene losses correspond to independent gene acquisition (although the probability of two independent acquisitions of the same gene is difficult to estimate).