The lion's mane: sexual and natural selection on pollen morphology in Taraxacum

Premise of the study: Spiny pollen has evolved independently in multiple
entomophilous lineages. Sexual selection may act on exine traits that
facilitate male mating success by influencing the transfer of pollen from
the anther to the body of the pollinator, while natural selection acts to
increase pollen survival. We postulated that relative to sexual congeners,
apomictic dandelions undergo relaxed selection on traits associated with
male mating success. Methods: We explored sexual selection on exine traits
by measuring the propensity for Taraxacum spp. pollen to attach to hairs
of flower-visiting bumblebees (Bombus spp.) or flies (Diptera: Syrphidae
and Muscoidea), and assessed natural selection by testing whether pollen
traits defend against consumption. Key Results: Pollen picked up by
bumblebees exhibited a narrower subset of spine spacing phenotypes,
consistent with stabilizing selection. Flies picked up larger pollen from
flowers than expected at random. Surveys of corbiculae (pollen basket)
contents from foraging bumblebees and feces of flies showed that pollen
consumed by both kinds of visitors is similar in spine characteristics and
size to pollen produced by the donor. When bees visit inflorescences of
apomictic T. officinale, they pick up pollen with spine spacing phenotypes
above the mean and shifted towards those of sexual T. ceratophorum.
Conclusions: We demonstrate that traits under sexual selection during
pollen pickup vary among pollinators, while natural selection for pollen
defense is nil in T. ceratophorum. In hybrid zones between apomictic and
sexual dandelions, pollen traits place apomictic donors at a dispersal
disadvantage, potentially reinforcing reproductive isolation.