3D pollination biology using micro-computed tomography and geometric morphometrics in Theobroma cacao

Authors: WOLCOTT, KATHERINE - University Of Miami; WHITLOCK, BARBARA - University Of Miami; WUCHTY, STEFAN - University Of Miami; Gutierrez, Osman; STANLEY, EDWARD - Florida Museum Of Natural History

Submitted to: Applications in Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2023
Publication Date: 10/17/2023
Citation: Wolcott, K.A., Whitlock, B.A., Wuchty, S., Gutierrez, O.A., Stanley, E.L. 2023. 3D pollination biology using micro-computed tomography and geometric morphometrics in Theobroma cacao. Applications in Plant Sciences. 11(5): e11549. https://doi.org/10.1002/aps3.11549.
DOI: https://doi.org/10.1002/aps3.11549

Interpretive Summary: Cacao (Theobroma cacao L.) is an important perennial crop in the tropical regions of the world. Insects are mainly responsible for its pollination. However, the small size of the cacao flowers limits the size of the insects that can participate in the pollination process. Modern three dimensional imaging technologies as well as micro and nano scale resolutions are readily available in the market. To investigate the cacao pollination biology of this globally important crop, scientists from the University of Miami and USDA-ARS Subtropical Horticultural Research Station in Miami, FL collaborated to study the floral structural size of cacao flowers. They utilized micro-computed tomography to estimate floral dimensions to quantify functional size limits of pollinators. Cacao flowers’ size data from local plants and pollinator data size from museum specimens of potential insect pollinators was collected using micro-computed tomography equipment. A comparison analysis of the geometry of the pollinator’s path was created using a camera and an insect trap. Data analyses results indicate if a possible pollinating insect could fit in the dimension of the flower path. This resulting model could be applied to other plant species with unknown pollinators and determine if the size of the insects could fit in the dimension of the path.

Technical Abstract: Premise: Imaging technologies that capture 3 - dimensional variation in floral morphology at micro - and nano - resolutions are increasingly accessible. In herkogamous flowers, like Theobroma cacao, structural barriers between anthers and stigmas represent bottlenecks that restrict pollinator size and access to reproductive organs. To study the unresolved pollination biology of cacao, we present a novel application of micro - computed tomography (micro - CT) using floral dimensions to quantify pollinator functional size limits. Methods: We obtained micro - CT datasets from field - collected flowers and museum specimens of potential pollinators. To compare floral variation, we used 3D Slicer to place landmarks on surface models and performed geometric morphometric (GMM) analysis using geomorph R. We identified the petal side door (opening between petal hoods and filament) as the main bottleneck for pollinator access. We compared its mean dimensions to proposed pollinators to identify viable candidates. Results: We identified three levels of likelihood for putative pollinators and found floral reward microstructures along pollinator paths whose presence and location was unclear. Discussion: Using micro - CT and GMM to study 3D pollination biology of cacao provides new evidence for predicting unknown pollinators. Incorporating geometry and floral rewards will strengthen plant - pollinator trait matching models for cacao and other species.