Lateral (or horizontal) gene transfer (LGT) refers to the transmission of genes between individuals without direct vertical inheritance from parents to their offspring. In contrast to vertical inheritance, LGT can cross species barriers and may even allow transmission of genes across the kingdoms of life. In prokaryotes, LGT is well documented and the supporting mechanisms have been widely described [1].
The high prevalence of LGT in prokaryotes has even challenged the validity of a bifurcating Darwinian tree of life and led to the suggestion that an interconnected rhizome of life would be a more realistic representation of relations between species [2]. The model bacterium Escherichia coli is a good illustration of the plasticity of bacterial gene repertoires due to gene acquisition via LGT and differential loss. While a typical E. coli genome contains ~5000 protein-coding genes, the pan-genome of E. coli, taking into account more than 60 different strains, is estimated to contain more than 15,700 genes [3]. Interestingly, only 6 % of these genes are present in every strain and the variable portion of a typical E. coli gene set reaches 80 %. Furthermore, LGT plays crucial roles in acquisition of antibiotic resistance, adaptation to new environments, and pathogenicity of bacteria. Hence, it is commonly accepted that the biology, genome composition, and ecology of prokaryotes have been deeply impacted by LGT.
In eukaryotes, there is no clear description of a mechanism for straightforward horizontal gene exchange between species. Furthermore, there has been no report of abundant eukaryote to eukaryote LGT to date (Fig. 1). Although fungi are known to exchange conditionally dispensable whole chromosomes, and that this can affect their pathogenicity, this seems to be restricted to strains of the same or closely related species [4].
Prokaryote to eukaryote LGT intuitively appears to be even less straightforward. Prokaryotic genes are subject to transcriptional and translational controls that are quite different from those of eukaryotic genes. Hence, even if a prokaryotic gene were successfully integrated in a eukaryotic genome, functional assimilation would be difficult. These complications are amplified in metazoan species, in which the germline is usually separated from the rest of the cells. A gene acquired from a prokaryote would have no chance of being fixed at the level of a population or a species if it were not transmitted to the next generation through integration into the germline. Analysis of somatic human samples suggests the separate germline is actually a stringent barrier against LGT [5].