Novel insights into pollen movement and floral evolution revealed by quantum dots

Minnaar, Corneile (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: To understand the evolution of flowers and mating systems in animal-pollinated plants, we have to directly address the primary function for which flowers evolved—the movement of pollen from anthers to stigmas. Yet, despite a long and distinguished history of making significant advances in understanding of natural selection and evolution, the field of pollination biology has largely studied pollen movement indirectly (e.g., pollen analogues or paternity assignment to seeds) due to a lack of suitable pollen tracking methods. Consequently, understanding of pollen export mechanisms and male reproductive strategies has been limited. In Chapter 2, I describe and test a novel technique to label and track the movement of pollen grains using quantum dots. I show that quantum dots can be attached to pollen grains of several different species and that their attachment to pollen appears not to affect pollen dispersal. In Chapter 3 I employ quantum dot pollen-labelling to test the placement and transfer of pollen in a unique population of Lapeirousia anceps (Iridaceae) with a bimodal distribution in floral tube length. I find that floral-tube length acts as a strong reproductive isolation barrier between plants with short-tubed flowers and longtubed flowers. In Chapter 4 I use quantum dots to explore the function of floral handedness in Wachendorfia paniculata. Based on pollen transfer experiments, pollen moves predominantly between left- and right-handed flowers, rather than between flowers of the same type. These experiments allowed the creation of the first map of anther-level pollen grain placement on the bodies of bees. Pollen placement maps revealed pollen quality heterogeneity across pollinator bodies, and that stigmas of W. paniculata aligned with areas on bee bodies where the capture of outcrossed pollen is most likely. This led to greater than expected outcross pollen movement. These findings underline the importance of studying micro-scale pollen landscape composition on pollinator bodies and how stigmas interact with them. The thesis concludes with a review which assesses the history of studying male function in plants and identifies critical gaps in our understanding of the ecology and evolution of pollen transport. I explore male reproductive function along the male fitness pathway, from pollen production to ovule fertilization. At each step of the pathway to paternity, I discuss evolutionary options to overcome barriers to siring success. In particular, I highlight a newly emerging idea that bodies of pollinators function as a dynamic arena facilitating intense male–male competition, where pollen of rival males is constantly covered or displaced by competitors. This perspective extends the pollen-competitive arena beyond the confines of the stigma and style, and highlights the opportunity for important new breakthroughs in the study of male reproductive strategies and floral evolution.


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