Sexual and natural selection on pollen morphology in Taraxacum

Abstract

Premise 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. 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 grains consumed by both kinds of visitors are similar in spine characteristics and size to those 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 toward 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.

The Scope for Postmating Sexual Selection in Plants

“…Pollen morphology more generally has been shown to affect the efficacy of attachment of pollen to pollinator bodies (Fig. 1d) [26]. Yet, despite these revealing examples, we still have a surprisingly poor understanding of whether enhanced pollinator attraction and/or pollen removal actually corresponds to an increased number of mates, the most relevant parameter in theories of sexual selection.…”

“…Dispersal structures containing pollen grains, called pollinaria (c.), evolved 'horns' in various milkweed species which decrease the pollination success of rival pollen donors by preventing the attachment of a subsequent pollinarium ([25] courtesy of Andrea Cocucci). Pollen morphology (d.), including their size and spine density, affects the probability of attachment on pollinator's bodies [26] (pollen of Catananche caerulea, courtesy of Agnès Mignot, ISEM reference palynological database). Male plant architecture of the wind-pollinated annual mercury (Mercurialis annua) (e.), including longer branch size and longer peduncles of male-flower bearing inflorescences, was shown to enhance pollen dispersal to a greater number of mates [18,21].…”

Mechanisms of Male-Male Interference during Dispersal of Orchid Pollen

“…Most pollen of outcrossing angiosperms fails to reach conspecific stigmas (Harder 2000;Harder and Johnson 2008), so selection should favor traits that promote pollen removal, transport and/or deposition on stigmas Lynn et al 2020).…”

Pollen morphological analysis of papaya (Carica papaya L.)

“…Microscopical techniques using a scanning electron microscope (SEM) have become popular to assess micromorphology of plants (Ayaz et al 2020, Lynn et al 2020, Silva-Fourny et al 2020, Gul et al 2021. Erdtman (1952) first described the pollen grains of papaya.…”