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 (non-fragmentary) 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.

💡 Research Summary

The authors set out to quantify not only how often lateral genetic transfer (LGT) occurs in prokaryotes but also what the typical “unit” of transfer is—whether whole genes or merely fragments of genes are exchanged. Using a rigorously phylogenetic framework, they examined 144 fully sequenced bacterial and archaeal genomes, extracting families of single‑copy orthologous genes to avoid complications from paralogy. For each gene family they reconstructed a maximum‑likelihood gene tree and compared it to a reference species tree derived from concatenated core genes. Significant topological incongruences were interpreted as LGT events, and the authors further distinguished between whole‑gene transfers and fragmentary transfers by analysing the length, boundaries, and homology of the transferred segments.

Their analysis revealed that 30.3 % of the examined single‑copy gene families bear signatures of LGT. Strikingly, about two‑thirds of these events involve only fragments of genes rather than complete open reading frames, indicating that previous surveys that focused mainly on whole‑gene exchanges have substantially underestimated the true magnitude of LGT. Functional annotation using COG categories showed a broadly even distribution of transferred material across metabolic, informational, and cellular processes, suggesting little functional bias in the choice of fragmentary versus whole‑gene transfer. However, whole‑gene transfers were significantly more common in pathogenic strains than in non‑pathogenic counterparts, implying that pathogens may preferentially acquire entire functional modules (e.g., virulence factors or antibiotic‑resistance genes) to gain immediate adaptive benefits.

Because the study deliberately excluded gene families that likely contain paralogs, the reported LGT frequency is probably a conservative estimate; inclusion of paralogous families would likely raise the proportion even further. Methodologically, the work combines high‑confidence phylogenetic reconstruction, bootstrap‑supported detection of incongruence, and a systematic approach to delineate transfer boundaries, providing a robust pipeline that can be applied to larger metagenomic datasets. The authors conclude that fragmentary gene transfer is a dominant mode of genetic exchange in prokaryotes, reshaping our understanding of microbial evolution and highlighting the need to consider both whole‑gene and fragmentary events when assessing the impact of LGT on genome innovation.