Abstract
Heriades truncorum (Megachilidae) is a specialist bee that forages on Asteraceae and collects pollen by tapping its abdomen on pollen-presenting florets which places the grains directly in the ventral scopa. We tracked pollen transfer by female H. truncorum between conspecific inflorescences of Inula ensifolia and Pulicaria dysenterica by labelling pollen with quantum dots. On average, bees transferred 31.14 (I. ensifolia) and 9.96 (P. dysenterica) pollen grains from the last visited inflorescence, 39% and 45% of which were placed on receptive styles. Pollen germination ratio is significantly lower for inflorescences of P. dysenterica visited by one H. truncorum (0.13%) compared with open control inflorescences (0.51%), which suggests that the bees mainly transfer self-pollen of these self-incompatible plants. Thus, a single visit by H. truncorum does not grant the plant high reproductive success, but the bees’ abundance and flower constancy might reduce this disadvantage.
Highlights
Many mechanisms have evolved in angiosperms to secure reproductive success
Tracking quantum dot–marked pollen grains from the previously visited inflorescence showed that a bee deposited on average 31.14 (± 25.49 SD, n = 21) pollen grains on an inflorescence of Inula ensifolia
Regarding the single-visit deposition (SVD) of marked pollen grains on specific structures of an inflorescence, we found no aggregation of pollen grains on particular florets in P. dysenterica (Friedman test, χ 2 = 3.02, df = 2, p = 0.221), whereas in I . ensifolia, more pollen per floret was placed on florets in the male phase of anthesis than on florets in the female phase of anthesis, disc floret buds, or ray florets (Friedman test, χ 2 = 7.72, df = 2, p = 0.021, post hoc Wilcoxon signed-rank test) (Figure 4A,B)
Summary
Many mechanisms have evolved in angiosperms to secure reproductive success (reviewed in Pacini and Hesse 2004; Pacini and Dolferus 2016). Entomophilous plants face the dilemma that bees and other flower-visiting insects act as pollinators and as pollen thieves, eating pollen or collecting it to feed their offspring (Thorp 1979). As the circularly arranged disc florets open successively, pollen is available continually and cannot be collected entirely in one flower visit. This pollen portioning strategy leads to more flower visitors collecting and possibly transferring pollen from one inflorescence. The adhesion of pollen grains to a flower visitor’s body is mainly attributable to pollenkitt—a lipid-based substance covering pollen of most angiosperms—which grants protection from adverse biotic and abiotic influences (reviewed in Pacini and Hesse 2005; Hesse 2010). Pollen grain size and shape affect the adhesion of entomophilous pollen grains (Lin et al 2013)
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