Units of genetic transfer in prokaryotes

The transfer of genetic materials across species (lateral genetic transfer, LGT) contributes to genomic and physiological innovation in prokaryotes. The extent of LGT in prokaryotes has been examined in a number of studies, but the unit of transfer has not been studied in a rigorous manner. Using a rigorous phylogenetic approach, we analysed the units of LGT within families of single-copy genes obtained from 144 fully sequenced prokaryote genomes. A total of 30.3% of these gene families show evidence of LGT. We found that the transfer of gene fragments has been more frequent than the transfer of entire genes, suggesting the extent of LGT has been underestimated. We found little functional bias between within-gene (fragmentary) and whole-gene (nonfragmentary) genetic transfer, but non-fragmentary transfer has been more frequent into pathogens than into non-pathogens. As gene families that contain probable paralogs were excluded from the current study, our results may still underestimate the extent of LGT; nonetheless this is the most-comprehensive study to date of the unit of LGT among prokaryote genomes.