Data from: Source-sink plasmid transfer dynamics maintain gene mobility in soil bacterial communities

Horizontal gene transfer is a fundamental process in bacterial evolution
that can accelerate adaptation via the sharing of genes between lineages.
Conjugative plasmids are the principal genetic elements mediating the
horizontal transfer of genes, both within and between bacterial species.
In some species, plasmids are unstable and likely to be lost through
purifying selection, but when alternative hosts are available,
interspecific plasmid transfer could counteract this and maintain access
to plasmid-borne genes. To investigate the evolutionary importance of
alternative hosts to plasmid population dynamics in an ecologically
relevant environment, we established simple soil microcosm communities
comprising two species of common soil bacteria, Pseudomonas fluorescens
and Pseudomonas putida, and a mercury resistance (HgR) plasmid, pQBR57,
both with and without positive selection [i.e., addition of Hg(II)]. In
single-species populations, plasmid stability varied between species:
although pQBR57 survived both with and without positive selection in P.
fluorescens, it was lost or replaced by nontransferable HgR captured to
the chromosome in P. putida. A simple mathematical model suggests these
differences were likely due to pQBR57’s lower intraspecific conjugation
rate in P. putida. By contrast, in two-species communities, both models
and experiments show that interspecific conjugation from P. fluorescens
allowed pQBR57 to persist in P. putida via source–sink transfer dynamics.
Moreover, the replacement of pQBR57 by nontransferable chromosomal HgR in
P. putida was slowed in coculture. Interspecific transfer allows plasmid
survival in host species unable to sustain the plasmid in monoculture,
promoting community-wide access to the plasmid-borne accessory gene pool
and thus potentiating future evolvability.