Parthenogenesis-inducing Wolbachia bacteria are reproductive parasites that
cause infected female wasps to produce daughters without mating(1,2). This
manipulation of the host's reproduction enhances the transmission of Wolba
chia to future generations because the bacteria are passed on vertically on
ly from mothers to daughters. Males are dead ends for cytoplasmically inher
ited bacteria: they do not pass them on to their offspring. Vertical transm
ission of Wolbachia has been previously considered to be the main mode of t
ransmission. Here we report frequent horizontal transmission from infected
to uninfected wasp larvae sharing a common food source. The transferred Wol
bachia are then vertically transmitted to the new host's offspring. This na
tural and unexpectedly frequent horizontal transfer of parthenogensis-induc
ing Wolbachia intraspecifically has important implications for the co-evolu
tion of Wolbachia and their host.