Symbioses have shaped the evolution of life, most notably
through the fixation of heritable symbionts into organelles. The inheritance
of symbionts promotes mutualism and fixation by coupling
partner fitness. However, conflicts arise if symbionts are transmitted
through only one sex and can shift host resources toward the sex
through which they propagate. Such reproductive manipulators have
been documented in animals with separate sexes but not in other
phyla or sexual systems. Here we investigated whether the investment
in male relative to female reproduction differed between hermaphroditic
host plants with versus without a maternally inherited fungal
symbiont. Plants with the fungus produced more seeds and less pollen
than plants lacking the fungus, resulting in an ∼40% shift in functional
gender and a switch from male-biased to female-biased sex
allocation. Given the ubiquity of endophytes in plants, reproductive
manipulators of hermaphrodites may be widespread in nature
