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.

Analysis of evolving lysogenised products of spontaneous zygogenesis in Escherichia coli.

Subclonal analysis of spontaneous zygogenesis (Z-mating) products of Escherichia coli K12 was undertaken to grasp the extent of vertical and horizontal evolution in unstable strains expressing one parental or recombinant genome. Isolates were obtained following serial cultures or serial intercrosses. A high diversity of strains was obtained, among which some resumed the phenotype of the partners of the initial or subsequent Z-matings. When non-complementing diploids are infected with a mixture of distinct temperate bacteriophages, lysogenisation occurs at the expenses of the active chromosome only. This event is associated with an alternate expression of prophage and chromosomal genes. Present work provides further evidence for the existence of non-complementing diploidy and opens a novel route for virus research in general.

Spontaneous zygogenesis (Z-mating) in mecillinam-rounded bacteria.

In Escherichia coli the process of spontaneous zygogenesis (Z-mating), i.e. complete genetic mixing in the absence of a conjugative plasmid, was investigated further. Spontaneous-zygogenesis-promoting (Szp+) cells displayed strong clustering with each other and with ordinary F* cells in the optimal cell density range for Z-mating. When induced to rounding by the drug mecillinam, they aggregated into large, dense, stainable syncytium-like cells leaving giant ghosts upon lysis. In Z-mating mixtures of mecillinam-treated cells, these giant cells co-purified with mating products as other cells died. Giant cells recovering from mecillinam treatment yielded monstrous, branching forms, whereas non-Szp+ coccal cells reverted to rods in one step, and some 29% of the colonies formed were identified as deriving from entities possessing two distinct genomes. Z-mating was examined between E. coli and a distantly related Serratia marcescens strain. In the presence of calcium, mecillinam-rounded cells of a stable non-complementing diploid hybrid with the E. coli phenotype segregated normally dividing cells of the Serratia form.

Spontaneous zygogenesis, a wide-ranging mating process in bacteria.

Spontaneous zygogenesis (or Z-mating) in Escherichia coli, unlike F-mediated conjugation, promotes formation of complete zygotes. Most Z-mating products have proven to be phenotypically unstable, losing part of the phenotype for which they were selected. Persistent Z-mating products appear as stable genetic recombinants or yield subclones of two types: either both parental types or one of them plus a recombinant type, and the ability to promote spontaneous zygogenesis can appear in all emerging types. Here it is shown that spontaneous zygogenesis-promoting E. coli strains can Z-mate with strains of Shigella flexneri, Salmonella enterica Serovar Typhimurium and Serratia marcescens, giving rise to products that behave like those of homospecific Z-matings. When genetic recombination was prevented in products of Z-mating between E. coli and S. enterica or S. marcescens, phenotype switching was observed in purified isolates exhibiting a single phenotype, thus providing further evidence of interspecies genotypic mixing. The presence of two distinct replicons within a cell introduces a new situation that contrasts with classical conjugation between heterologous strains, where the partially transferred chromosomal segment is subject to restriction, is unable to replicate and is limited in genetic recombination.

Noncomplementing diploidy resulting from spontaneous zygogenesis in Escherichia coli.

With the aim of understanding sexual reproduction and phenotypic expression, a novel type of mating recently discovered in Escherichia coli was investigated. Termed spontaneous zygogenesis (or Z-mating), it differs from F-mediated conjugation. Its products proved phenotypically unstable, losing part of the phenotype for which they were selected. Inactivation of a parental chromosome in the zygote is strongly suggested by fluctuation tests, respreading experiments, analysis of reisolates, and segregation of non-viable cells detected by epifluorescence staining. Some phenotypically haploid subclones were interpreted as stable noncomplementing diploids carrying an inactivated co-replicating chromosome. Pedigree analysis indicated that the genetic composition of such cells consisted of parental genomes or one parental plus a recombinant genome. Inactivation of a chromosome carrying a prophage resulted in the disappearance of both the ability to produce phage particles and the immunity to superinfection. Phage production signalled transient reactivation of such a chromosome and constituted a sensitive test for stable noncomplementing diploidy. Chromosome inactivation thus appears to be a spontaneous event in bacteria.

Spontaneous zygogenesis in Escherichia coli, a form of true sexuality in prokaryotes.

A new type of mating, differing from classic conjugation and previously observed in a certain strain of Escherichia coli K-12, has also been found in strains derived from ordinary F- cells of E. coli K-12 exposed to an exogenous factor originating in an E. coli clinical isolate. Immunofluorescence and electron microscopy after single and double labelling of DNA were used to produce evidence in favour of a novel mating mechanism by cell contact at the poles of the bacterial rod. These findings are supported by genetic analyses indicating complete genetic mixing. Unstable complementing diploids were formed, which throw off phenotypically haploid cells, of which some showed a parental phenotype and some were true genetic recombinants. Recombination was observed even when one parent was a UV-inactivated F- RecA- strain. The name 'spontaneous zygogenesis' (Z-mating, for short) is proposed for this kind of mating.