Plasticity in leakiness

In dioecious plants, males and females frequently show ‘leaky’ sex
expression, with individuals occasionally producing flowers of the
opposite sex. This leaky sex expression may have enabled the colonization
of oceanic islands by dioecious plant species, and it is likely to
represent the sort of variation upon which selection acts to bring about
evolutionary transitions from dioecy to hermaphroditism. Although
leakiness is commonly reported for dioecious species, it is not known
whether it has plastic component. The question is interesting because
males or females with an ability to enhance their leakiness plastically in
the absence of mates would have an advantage of being able to produce
progeny by self-fertilization. Here, we demonstrate that leaky sex
expression in the wind-pollinated dioecious herb Mercurialis
annua is plastically responsive to its mating context. We
compared experimental populations of females growing either with or
without males. Females growing in the absence of males were leakier in
their sex expression than controls growing with males, producing more than
twice as many male flowers. Our results thus provide a striking instance
of plasticity in the reproductive behavior of plants that is likely
adaptive. We consider how females might sense their mating environment as
a function of pollen availability, and we discuss possible constraints on
the evolution of plasticity in sex expression when the environmental
signals that individuals receive are unreliable.