Asteraceae Paradox: Chemical and Mechanical Protection of Taraxacum Pollen

Abstract

Excessive pollen harvesting by bees can compromise the reproductive success of plants. Plants have therefore evolved different morphological structures and floral cues to narrow the spectrum of pollen feeding visitors. Among “filtering” mechanisms, the chemical and mechanical protection of pollen might shape bee-flower interactions and restrict pollen exploitation to a specific suite of visitors such as observed in Asteraceae. Asteraceae pollen is indeed only occasionally exploited by generalist bee species but plentifully foraged by specialist ones (i.e., Asteraceae paradox). During our bioassays, we observed that micro-colonies of generalist bumblebee (Bombus terrestris L.) feeding on Taraxacum pollen (Asteraceae) reduced their pollen collection and offspring production. Bees also experienced physiological effects of possible defenses in the form of digestive damage. Overall, our results suggest the existence of an effective chemical defense in Asteraceae pollen, while the hypothesis of a mechanical defense appeared more unlikely. Pre- and post-ingestive effects of such chemical defenses (i.e., nutrient deficit or presence of toxic compounds), as well as their role in the shaping of bee-flower interactions, are discussed. Our results strongly suggest that pollen chemical traits may act as drivers of plant selection by bees and partly explain why Asteraceae pollen is rare in generalist bee diets.

Contribution of Extensive Farming Practices to the Supply of Floral Resources for Pollinators

“…Asteraceae are recognized to attract large array of insect visitors due to their copious nectar production [ 59 , 67 , 68 , 69 ]. However, Asteraceae pollen is not attractive and beneficial for generalist insect visitors due to its low protein content and detrimental secondary metabolites [ 70 , 71 ]. Pollen is the major resource in lipids (including sterols) and proteins (polypeptides and amino acids), but also certainly in secondary metabolites or pesticides that are harmful to pollinating insects [ 72 , 73 , 74 , 75 ].…”

“…Nevertheless, pollen resources are also vital for the development and survival of bee larvae [ 23 , 74 ]. Pollen quality, such as the protein content, amino acid profile, protein/lipid ratio, sterol profile [ 73 ], or the presence of secondary components [ 71 ], influences the floral preferences of pollinators [ 72 ]. Notably, bees are more selective in their choice of species for pollen resources than for nectar [ 76 ].…”

Pollen sterols are associated with phylogenetics and environment but not with pollinators

“…Indeed, pollen foraging bees on Asteraceae plants are mostly specialized oligolectic bees, while polylectic bee species avoid the pollen despite the ubiquitous distribution of Asteraceae species and their substantial amount of pollen provision (known as the Asteraceae paradox, Müller & Kuhlmann, 2008). While the reasons for this Asteraceae paradox remain unresolved, the abundance of Δ 7 -sterols we found in the pollen of Asteraceae species could provide a potential explanation (see also Vanderplanck et al, 2018Vanderplanck et al, , 2020b. Table S3.…”

Mechanisms of Nutritional Resource Exploitation by Insects

“…Many plant species therefore morphologically, mechanically and/or chemically protect pollen, to restrict its exploitation to the most effective pollinators [ 13 ]. Vanderplanck and colleagues [ 14 ] investigated how such restriction is achieved in Taraxacum (Asteraceae), which are known for their specialized pollinator interactions. The authors show that a Taraxacum diet strongly reduces offspring production and health in a generalist bumble bee ( B. terrestris ).…”