Chapter 1
Chapter 1
Domestication Reduces Floral Volatile Richness in Squash (Cucurbitaceae: Cucurbita) But Conserves Key Compounds Critical for Pollinator Attraction
Abstract
The domestication of crops leads to profound changes on plant phenotypes, yet its effects on floral traits mediating plant-pollinator interactions remain poorly understood. Floral volatile organic compounds play a crucial role in pollinator attraction by signaling the presence and quality of available floral resources. Here, we characterize VOC composition in Cucurbita, a genus containing multiple wild and domesticated species, to investigate how domestication affects floral volatiles. Then, we combined electrophysiology, blue vane trap field assays and pollinator visitation experiments to assess how the squash specialist pollinator, Xenoglossa pruinosa, responded to existing VOCs across different wild and domesticated plant species. Our results reveal significant compound losses within domesticated species blends. Combined gas chromatography-electroantennography identified ten electrophysiologically active compounds across wild and domesticated squash VOCs. Field assays assessing bee attraction to individual antennally active compounds using modified blue vane traps with compound lures identified one, four-dimethoxybenzene (a dominant volatile in domesticated squash blends) as an attractant for bees in isolation. We also found significant associations between bee visitation (assessed as floral approaches and nectaring behaviors) and increased emissions of one, four-dimethoxybenzene, dihydro-beta-ionone, and E-nerolidol as well as reduced emissions of linalool and methyl salicylate. Our findings provide novel insights into the chemical ecology of crop-pollinator interactions, demonstrating that domestication can reshape plant-pollinator communication mechanisms.
Introduction
Introduction
In the Anthropocene, the agriculturalization of landscapes is a dominant phenomenon with approximately forty percent of terrestrial land mass being occupied by crop plants that have been domesticated throughout the past ten thousand years. Domestication involves breeding plants for human-beneficial traits (e.g., larger fruit and higher sugar content) or traits adaptive to agroecosystems (e.g., loss of dormancy and shifts in annual life cycle). This process of artificial selection leads to rapid evolution that results in large phenotypic changes over a short period of time. Due to multiple, complex genetic processes such as pleiotropy, epistasis, linkage, and environmental factors, artificial selection often has unintended effects on plant phenotypes, including changes in traits involved in communication with other organisms. For instance, domesticated plants often look, smell, and taste different from related wild species, all of which can impact how they are perceived by insects. Flowers function as lures to attract pollinators by producing cues that may advertise the availability of nutritious resources (mainly pollen and nectar), mating sites, or even emulate potential mates. Floral cues are diverse and include visual, olfactory, humidity, and electromagnetic signals that pollinators often detect by integrating inputs from multiple sensory systems. Disruption of floral cue signals due to crop domestication can have profound impacts on plant-pollinator interactions and pollinator evolution. However, our knowledge of how crop domestication impacts floral cues and the communication channels between plants and their pollinators is limited.
Flowers produce a complex blend of volatile organic compounds (hereafter, VOCs) that allow foraging insects to locate them over larger spatial scales than through visual cues. Many insect pollinators possess highly sensitive olfactory systems tuned to detect small amounts of these floral volatile cues. Further, floral VOC blends also vary across individual plants and may reflect underlying stress that they may be experiencing, acting as honest cues of the quality of the resources offered by a particular plant, helping the pollinator to hone in on high-quality resources while foraging. There is evidence that domestication alters plant volatile profiles in cotton, but these changes do not impact herbivore preference. Although comparisons of the chemical composition of wild and domesticated blueberry nectar and pollen showed that domestication had significantly reduced chemical diversity, potentially impacting the health of visiting bumble bees, less is known about the ways in which domestication affects floral volatile profiles.
The most important and common group of insect pollinators of crops are bees, a clade of obligate pollinivores whose evolutionary history is closely intertwined with that of flowering plant species. Bee pollinators include both generalists-that collect pollen from a spectrum of plant families-and specialists-that collect pollen from a limited set of plant species. The importance of floral volatiles to generalist and specialist foraging bees has been well established. For instance, generalist honey bees (Apis mellifera) learn odor cues associated with specific pollen sources and the specialist Andrena vaga is highly sensitive to specific behavior-modulating odorants produced by host flowers. Generalist and specialist bees may use olfactory cues differently, as foraging involves unique challenges for each. Specialist bees tend to use specific blends emitted by their host plants to accurately locate flowers in heterogeneous environments. In contrast, generalists tend to show sensory biases towards volatile compounds present across floral blends, but see.
The chemical ecology of specialized plant-pollinator interactions is a rapidly advancing field of study. Specialist bees have been shown to be particularly dependent on olfactory cues during foraging, using volatiles to locate host plants over long distances. In multiple systems, specialist pollinators show sensory biases for the dominant volatile compounds or blends emitted by their host plants. For instance, Andrena vaga, a species that specializes on pollen from the plant genus Salix, uses four-oxoisophorone, a floral volatile produced by many Salix species, to accurately detect the plants it needs. Further explorations of the neurophysiology and neuroecology of this species have shown that the peripheral olfactory system is sensitive toward this compound. For all these systems, electrophysiological approaches have been particularly useful in identifying individual compounds and blends that may drive insect behavior and sensory evolution. These approaches allow us to directly identify individual compounds detected by the peripheral nervous system and test their behavioral relevance, thus linking species ecology with underlying neurophysiological mechanisms.
Here, we use the plant genus Cucurbita and its associated specialist pollinators as a model to explore how plant domestication has impacted floral detection and behavior by specialist pollinators. This genus, native to North and South America, contains all the cultivated gourds and squash species and their wild ancestors. The first domestication event of Cucurbita occurred approximately ten thousand years ago in Mexico, and an additional five domestication events took place throughout the Americas with subsequent cultivation of these species worldwide where these plants did not previously grow. The phylogeny and biogeography of Cucurbita is well-resolved and wild species occupy both xeric and mesic habitats in the Americas. The mesophytic clade contains five monophyletic lineages, each comprising either only extant domesticated species or domesticated and wild species. The xerophytic clade consists of six wild perennial species only found in dry regions of North America within the United States and Mexico.
The pollinator community of domesticated Cucurbita species in what is now the Northeastern United States and Southeastern Canada is dominated by the specialist pollinator species, Xenoglossa pruinosa, and the generalists Bombus impatiens and Apis mellifera. Both generalist pollinators are social and often introduced in the system as managed pollinators. In contrast, the wild bee X. pruinosa is a solitary, univoltine bee that is an obligate pollinivore on Cucurbita pollen. Xenoglossa pruinosa originated in central Mexico and has experienced a large, recent range expansion concurrent with the domestication and widespread cultivation of squash in North America. Previous studies have shown that these bees are attracted to squash floral volatiles that are also attractive to the specialist herbivore Acalymma vittatum. However, whether domesticated and wild Cucurbita species use similar compounds to attract these specialist pollinators remains unknown.
In this study, we investigated the effect of domestication on the volatile profiles of wild and domesticated Cucurbita flowers, how these chemical compounds were detected by specialist pollinators, and how they were associated with their attraction to traps and flowers. Specifically, we used one, gas chromatography-mass spectrometry to characterize floral volatiles, two, electrophysiological techniques to identify electroantennogram-active compounds, and three, behavioral assays, field attraction assays to traps and floral pollinator visitation experiments, to infer the behavioral relevance of identified compounds. We hypothesized that domestication impacts pollinator behavior in regard to squash species mediated by changes in floral volatile profiles. Specifically, we predicted that domestication leads to reductions in the floral volatile organic compound richness of Cucurbita species resulting in the losses of floral volatile compounds and fewer compounds detected by bee antennae. Additionally, we predicted that bee approaches and nectaring behaviors would correlate with emissions of electroantennogram-active compounds, with stronger associations to volatile organic compounds compounds emitted by wild squash species that the bees coevolved with. We found that domesticated and wild plants produced significantly, P is less than zero point zero five, differing floral volatile organic compound blends, with domesticates showing a marked reduction in the richness of compounds produced. Using combined gas-chromatography electroantennography, we identified ten compounds across the floral volatile blends of five species of Cucurbita that elicited electroantennogram responses and found that xerophytic and mesophytic squash produced different electroantennogram-active compounds. However, only one compound, one, four-dimethoxybenzene,
elicited squash bee attraction to traps when used in the field in isolation. Finally, experiments quantifying bee visitation to squash flowers showed that emissions of several electroantennogram-active compounds were associated with both pollinator approaches to flowers and nectaring behavior to flowers. Our results provide new insights into the chemical ecology of squash pollination and the impact of host-plant domestication on specialist pollinator behavior.