  Phylogenetic studies show that horizontal gene transfer (HGT) is a
significant contributor to genetic variability of prokaryotes, and was perhaps
even more abundant during early evolution. Hitherto, research of natural HGT
has mainly focused on three mechanisms: conjugation, natural competence, and
viral transduction. This paper discusses the feasibility of a fourth such
mechanism - cell electroporation and/or electrofusion triggered by atmospheric
electrostatic discharges (lightnings). A description of electroporation as a
phenomenon is followed by a review of experimental evidence that
electroporation of prokaryotes in aqueous environments can result in release of
non-denatured DNA, as well as uptake of DNA from the surroundings and
transformation. Similarly, a description of electrofusion is followed by a
review of experiments showing that prokaryotes devoid of cell wall can
electrofuse into hybrids expressing the genes of their both precursors. Under
sufficiently fine-tuned conditions, electroporation and electrofusion are
efficient tools for artificial transformation and hybridization, respectively,
but the quantitative analysis developed here shows that conditions for
electroporation-based DNA release, DNA uptake and transformation, as well as
for electrofusion are also present in many natural aqueous environments exposed
to lightnings. Electroporation is thus a plausible contributor to natural HGT
among prokaryotes, and could have been particularly important during the early
evolution, when the other mechanisms might have been scarcer or nonexistent. In
modern prokaryotes, natural absence of the cell wall is rare, but it is
reasonable to assume that the wall has formed during evolution, and at least
prior to this, electrofusion could also have contributed to HGT. The concluding
section outlines several guidelines for assessment of the feasibility of
lightning-triggered HGT.
