Résumé
Sexual selection may contribute to the evolution of plant sexual
dimorphism by favoring architecture traits that improve pollen dispersal
to mates in males. In both sexes, larger individuals may be favored if
large size allows the allocation of more resources to gamete production (a
‘budget’ effect of size). In wind-pollinated plants, large size may also
benefit males if it allows them to liberate pollen from greater heights,
fostering its dispersal (a ‘direct’ effect of size). To assess these
effects and their implications for trait selection, we measured selection
gradients on plant morphology in both males and females, as well as pollen
production and dispersal in males, of the wind-pollinated dioecious herb
Mercurialis annua. In two separate experimental common gardens established
at different densities, selection strongly favored plants dispersing their
pollen further. Selection for pollen production was observed in the
high-density garden only and was weak. In addition, male morphologies
associated with increased mean pollen dispersal differed between the two
gardens as elongated branches were favored in the high-density garden
while shorter plants with longer peduncles bearing inflorescences were
advantaged the low-density garden. Larger females were selected in both
gardens. Our results point to the importance of selection on male traits
that affect pollen dispersal, and to a lesser extent pollen production.