Genetic recombinational events in prokaryotes and their viruses: insight into the study of evolution and biodiversity

The exact meaning of sexual reproduction and the precise evolutionary period at which recombination first took place remains the subject of intense debates. Despite some unity in biochemical organisation of genetic recombination, a plethora of mechanisms are found to exist in microbes and their viruses. Some routes used by viruses bypass barriers to genetic heterology and provide bacteria with genes conferring a selective advantage, and some contribute to genome enlargement. The present review aims at highlighting the diversity of such mechanisms with a particular focus on spontaneous zygogenesis (or Z-mating). The latter mode of genetic recombination, which was recently discovered in Escherichia coli, resembles gamete fusion in eukaryotes in that it involves complete genetic mixing. Vertical and horizontal evolution through mutations and homo- or heterospecific Z-mating can be monitored to some extent, providing a mean to interrogate the mechanisms of evolution in a way similar to introgression and symbiogenesis. The question arises as to whether Z-mating might represent a remainder of what happened in the very first organisms appearing on earth, as well as recombination events among viruses.