Bacterial transmission by pollinators to flowers – Impacts on floral biology and fruit development in blueberries (Vaccinium corymbosum ‘Duke’)
A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
(Lincoln University, 2026) Lignon, Valeria Aiko
Pollination involves a tripartite interaction among insect pollinators, plants, and their associated microorganisms. Insect pollinators come into contact with the nectary of blueberries (Vaccinium corymbosum ‘Duke’), potentially facilitating bacterial transmission from pollinator to flower. Following pollination, flowers develop into fruit, the final product of blueberry reproduction. However, whether pollinator-associated bacteria transferred between the pollinator and the flower also colonise the fruit remains unexplored.
The aims of this thesis were to 1) characterise bacterial communities on the external surfaces of various pollinators (bumble bee (Bombus terrestris), drone fly (Eristalis tenax), honey bee (Apis mellifera) and native bee (Lasioglossum sordidum)), 2) assess whether pollinator-associated bacteria are transferred to blueberry flowers and fruits by different pollinators, 3) assess whether different pollinators alter the floral volatile organic compound (VOC) profiles of blueberry flowers after pollination, and 4) assess whether pollinators influence blueberry fruit quality. To answer these objectives, caged experiments were conducted where individual ‘Duke’ blueberry plants were exposed to different pollination treatments and bumble bee densities. Both pre- and post-pollination stages were sampled to analyse floral VOCs using dynamic headspace volatile sampling and to characterise the bacterial community through 16S rRNA Illumina sequencing for the external surface of pollinators, blueberry nectary, fruit surface, berry skin and berry pulp. Fruit quality traits, including diameter, weight, total soluble solids, pH, phenolic content, and sugar content, were also measured.
This was the first study to characterise the external surface of insect pollinators in relation to blueberry in New Zealand. Each pollinator species exhibited distinct bacterial community composition and diversity, with gut-associated genera such as Gilliamella, Lactobacillus, and Snodgrassella detected on external surfaces. Pollination treatments did not significantly affect bacterial composition or diversity in blueberry nectaries or fruits; however, pollinator-associated genera appeared as minor taxa in these samples. Shared amplicon sequence variants (ASVs) belonging to pollinator-associated genera were detected across pollinators, blueberry nectaries, and fruits, suggesting potential bacterial transmission and that blueberry nectaries and fruits may serve as alternative hosts for these bacteria outside their typical habitat. Floral VOC composition was unaffected by pollinator species or densities; the only change observed was between pre- and post-pollination stages, indicating that pollination itself drives VOC shifts. Similarly, fruit quality was influenced by pollination rather than pollinator species or density, affecting only fruit size (diameter and weight), while the other traits appeared to be controlled by fruit maturation and ripening.
This thesis advances our understanding of plant–pollinator–bacteria interactions in blueberry systems. It demonstrates that while pollinator-associated bacteria rarely establish in blueberry nectaries or fruits, there is a transmission pathway from pollinators to the flowers and then fruits, which bacteria can potentially utilise. Furthermore, pollination itself, not pollinator identity or density, drives changes in floral VOCs and fruit size, while other fruit quality traits remain unaffected. These findings refine our perspective on the ecological and agricultural significance of pollinator-mediated bacterial exchange.