Why does lateral transfer occur in so many species and how?

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Lateral gene transfer (LGT) or horizontal gene transfer (HGT) refers to the process of genetic movement between distantly related organisms. By introducing novel genetic information, LGT may spread the evolutionary success across distantly related organisms, allowing the recipient to access new niches or resources. Once a highly debated topic among scientists, LGT is now considered to be the driving force in prokaryotic evolution. Examples of LGT in prokaryotes include the spread of pathogenicity islands and antibiotic resistance. It has been estimated that almost all prokaryotic genes have been transferred at least once in their histories of evolution. Frequent LGT events may not only obliterate the evolutionary relationships among prokaryotic lineages, but also lead to the origin of major groups. Furthermore, pervasive LGT events among prokaryotes were also largely responsible for the concepts of net of life and pan-genomes. Investigations of horizontally acquired genes in eukaryotes can be complicated by multiple scenarios such as differential gene losses, independent gene acquisitions, and intracellular gene transfer from mitochondria and plastids. In particular, because of the prokaryotic origin of mitochondria and plastids, genes transferred from these intracellular organelles can be difficult to distinguish from those independently acquired from other bacteria. This issue can be more serious in view of the widespread occurrence and various levels of endosymbioses in eukaryotes. Nevertheless, it is generally agreed that LGT does occur frequently in unicellular eukaryotes. In complex multicellular eukaryotes (e.g., animals and plants), although it has been suggested that the differentiation of somatic and reproductive cells may constitute barriers to LGT, reports of horizontally acquired genes are increasingly common in these groups, suggesting that no barriers to LGT are insurmountable. It also becomes increasingly clear that LGT plays an important role in the formation and maintenance of global biodiversity. Mechanisms of LGT are better understood in prokaryotes than in eukaryotes. Transformation, transduction and conjugation are commonly responsible for gene transfer in prokaryotes. Other mechanisms, such as gene transfer agents (GTAs) and cell fusion, can also lead to the transfer of genetic material between prokaryotic cells. GTAs are structurally similar to bacteriophages, but carry mostly random DNA fragments from host bacteria. Mechanisms of LGT in eukaryotes remain largely elusive. Nevertheless, it is generally accepted that physical association or contact between donor and recipient organisms (e.g., endosymbiosis, feeding, parasitism) may facilitate the occurrence of gene transfer. Several mechanisms proposed for LGT in eukaryotes, such as gene ratchet and week-link model, are discussed in this review.