Integrating mechanical fit and signalling in a bimodal pollination system: intraspecific variation in the floral traits of a bat- and hawkmoth pollinated cactus

"Background: Plants with bimodal or generalist pollination systems interact with functionally distinct pollinator groups. However, how intraspecific variation in floral traits enables interactions with these contrasting pollinator groups remains poorly understood. We examined how intraspecific variation in floral traits underpins a bimodal pollination system in Pilosocereus pachycladus, a cactus pollinated by bats and hawkmoths.
Methods: We described the olfactory and visual signalling of flowers by assessing volatile organic compounds and spectral reflectance, respectively. Next, we used geometric morphometrics to quantify variation in the floral shape. Then, we explored relationships between these floral traits.
Results: Flowers primarily emit benzaldehyde, (E)-β-ocimene, and sulphur compounds (e.g. dimethyl trisulphide and dimethyl disulphide), whose relative proportions varied among individuals, and exhibit reflectance peaks from 400-700 nm. We found a gradient of flower shapes, some apparently better fitting bats (e.g. mask-like or bell-shaped) and other hawkmoths (e.g. tubular). Floral odour showed a relationship with morphology, with larger flowers emitting more dimethyl disulphide (attractive to bats) and narrower flowers emitting more (E)- β-ocimene (attractive to hawkmoths).
Conclusions: Bimodal or generalist pollination systems may be maintained by continuous intraspecific variation spanning the requirements of distinct pollinator groups, and relationships between mechanical fit and floral signalling within populations may enable these systems."