Retrotransfer or gene capture: A feature of conjugative plasmids, with ecological and evolutionary significance

The traditional view on bacterial conjugative gene exchange is a gene flow from the plasmid-containing donor strain into the plasmid-free recipient strain. When mobilization of non-conjugative plasmids is described, it is either a biparental mating with a donor containing both conjugative and a non-conjugative but mobilizable (Mob+) plasmids, and a plasmid-free recipient, or a triparental mating with a donor, containing the Mob+ plasmid, a helper strain harbouring a conjugative helper plasmid, and again a plasmid-free recipient strain. In these scenarios, mobilization was always considered as a gene flow from the original donor to the recipient strain. However, the first report of gene flow at high frequencies in two directions was published by Mergeay et al. in 1987R35 . By means of an IncP conjugative plasmid, chromosomal genes were not only mobilized from the original donor of the IncP plasmid towards the plasmid-free recipient, but also from the original recipient back into the donor. The terms ‘retrotransfer’, ‘shuttle transfer’, plasmid-mediated ‘gene capture’ or even a kind of bacterial ‘hermaphroditism’, were used to describe a conjugational biparental event that led to the inheritance (capture) by the original host of a conjugative plasmid, of genetic traits (either chromosomal markers or plasmids) from the mating partner, free of conjugative plasmids. The frequency of this retrotransfer can be very high, being in some cases of the same order of magnitude as the frequencies observed for the transfer of genetic traits promoted in the canonical donor–recipient direction (Mergeay et al., 1987R35 ). Although the phenomenon was first observed and analysed in most detail for broad-host-range (BHR) IncP1 plasmids (Mergeay et al., 1987R35 ), it was also reported for other plasmids such as some IncN (pULG14) and IncM plasmids (R69.2) (Thiry et al., 1984R46 ; Mergeay et al., 1985R34 ), the IncF plasmid pDE-FL54 and the IncW plasmid pSa322 (Heinemann & Ankenbauer, 1993aR24 ), and for the catabolic IncP9 plasmid pWW0 (Ramos-Gonzalez et al., 1994R39 ). This capture of new genetic traits could thus be of great significance for the evolution of microbial communities, especially where stress is selecting rare events, yielding individual populations with new combinations of genes, which may allow better survival or more rapid growth.