Horizontal transfer (HT) alters the repertoire of symbiosis genes in rhizobial genomes and may play an important role in the on-going evolution of the rhizobia-legume symbiosis. To gain insight into the extent of HT of symbiosis genes with different functional roles (nodulation, N-fixation, host benefit, and symbiont fitness), we conducted comparative genomic and selection analyses of the full genome sequences from 27 rhizobial genomes. We find that symbiosis genes experience high rates of HT among...