Adaptive evolution can mitigate the negative effects of temperature stress on plant-pollinator interactions

Temperature stress negatively affects various aspects of plant fitness,
including plant-pollinator interactions, but whether plants can overcome
these adverse effects through adaptive evolution is largely unknown. Here,
we conducted a six-generation evolution experiment using fast cycling
Brassica rapa plants at ambient and elevated temperatures, with
bumblebee-pollination. At the end of the experiment, we re-grew the
evolved genotypes at different temperatures. We phenotyped the plants and
conducted pollinator bioassays to assess adaptive evolution, evolutionary
trait divergence, and the evolution of heat-mediated phenotypic
plasticity. We found that plants that had evolved with
bumblebee-pollination in both temperature regimes had higher seed set than
control plants, which suffered lower seed set when evolved under elevated
temperatures. We also showed that the number of flowers, a trait that
largely determined plant attractiveness to bumblebees and seed set, was
increased in bumblebee-pollinated plants, and so was heat-induced
phenotypic plasticity in flower number. Plants that evolved with high
temperature showed increased UV-reflection, and a stronger association
between flower size and nectar content (honest signalling), and reduced
scent emission. Our results show that plants that evolve under
pollinator-mediated selection can mitigate at least some of the negative
effects of temperature stress through adaptive evolution.