Location: Vegetable Crops Research
2023 Annual Report
Objectives
Objective 1: Identify pollinator behaviors, pollinator management strategies, and crop production strategies that together mitigate unintended gene flow.
Sub-objective 1.1: Pollinator behavior and plant reproductive strategies affect gene flow risk.
Sub-objective 1.2: Visual and Olfactory cues that attract pollinators can guide the development of pollinator or crop management strategies that reduce gene flow and increase yield.
Objective 2: Determine the impacts of cultivated carrot genes on the genomic landscape of wild carrot.
Approach
Objective 1. This objective is divided into two sub-objectives, each with three hypotheses to be tested.
Sub-Objective 1.1. We will use a combination of field and greenhouse experiments to test the hypotheses within this subobjective. For example, the rules bees use when moving between patches or fields will be tested using patches of distinct sizes and isolation distances and measuring the number of transitions made by bees from a center glyphosate-resistant patch to the different conventional patches. The number of gene flow events in the different conventional patches, identified by the presence of glyphosate-resistant seeds, will also be used to test the decision making process of bumble bees. Greenhouse experiments will examine the pattern of seed deposition on flowers visited in succession by three bee species, honey bees, leafcutting bees and bumble bees. We will use glyphosate-resistant pollen donor and conventional pollen recipients and examine the number and proportion of glyphosate-resistant seeds on flowers visited in succession to determine the seed curve for each bee species.
Sub-Objective 1.2. To determine the preference of each of three bee species to visual and/or olfactory cues, we will perform greenhouse experiments and quantify approaches and landings to different visual and/or olfactory cues. To identify a blend derived from nest cells that attract leafcutting bees, we will capture and identify the chemicals present in the bee cell using Gas Chromatography-Mass Spectrometry (GC-MS); determine whether there is a behavioral response and then use couple gas chromatography – electroantennographic detection (GC-EAD) to identify physiological responses. Finally, the electrophysiologically active constituents will be tested using a behavioral assay.
Objective 2. We will use genotyping by sequencing on both cultivated carrots used in a breeding program and wild carrots in close proximity to the breeding area and far away to detect the presence of cultivated carrot genes in wild carrot populations. The presence of cultivated genes in wild populations represents introgression. We will determine the extent of introgression of cultivar genes in wild carrot populations.
Progress Report
This is the final report for this project; the replacement project title has not yet been established.
Objective 1, Sub-objective 1.1. ARS and Oakridge Institute for Science and Education (ORISE) scientists in Madison, Wisconsin, established the rules used by leafcutting bees and bumble bees when selecting patches. Both bee species use information about distance and patch size when selecting the next patch to visit but while bumble bees can estimate the total resources available in a patch, leafcutting bees can only estimate partial resources available. These results were published in the journals ‘Scientific Reports’ (Fragoso et al. 2021), and ‘Biology Letters’ (Fragoso and Brunet 2023a). For honey bees, attempts to observe transitions over two years remained unsuccessful, so a different experiment was set up to understand patch selection by honey bees. Individual bees were marked and the level of fidelity to a patch was quantified. Results indicated high patch fidelity for honey bees (76% of bee reobservations occurred in the same patch a bee was originally marked), much greater than for bumble bees (47%). The dance language of honey bees was hypothesized as influencing patch fidelity. The high degree of patch fidelity of honey bees helped explain the difficulty in observing transitions between patches for that species. A manuscript summarizing these results will be published in ‘Ecosphere’ (Fragoso and Brunet, 2023b).
ARS scientists in Madison, Wisconsin, along with University of Wisconsin-Madison cooperators, developed and contrasted models describing bee movement in continuous landscapes for honey bees, leafcutting bees, and bumble bees (Brunet et al. 2021, Scientific Reports). Moreover, the pattern of pollen deposition and decay of pollen over successive flowers visited during a foraging boutwas quantified and contrasted for leafcutting bees and bumble bees, with bumble bees moving pollen further distances (Santa Martinez et al. 2021, AJB). ARS and ORISE scientists in Madison, Wisconsin, quantified the probability of finding a glyphosate resistance (GR) gene in a seed when a bee moved from a glyphosate resistant alfalfa plant to flowers of conventional alfalfa plants. Bumble bees moved the most GR genes the furthest, followed by honey bees, and least leafcutting bees. The ranking of bee species based on movements within a foraging bout corresponded to their ranking in gene flow experiments in the field, suggesting how small-scale bee movements translate into large scale gene flow events (Fragoso and Brunet 2023c, PLoS ONE). In addition, ARS scientists and collaborators examined how factors other than distance affected gene flow in alfalfa seed-production fields (Kesoju et al. 2021, PlOS ONE).
Objective 1, Sub-objective 1.2. ARS scientists in Madison, Wisconsin, examined and contrasted the response of honey bees, bumble bees, and leafcutting bees to visual and olfactory components of alfalfa plants. ARS scientists in Madison, Wisconsin, along with University of Wisconsin cooperators, quantified the behavioral response of Lygus hesperus to the floral scent of 17 host plants. They identified the compounds eliciting an electroantennogram (EAG) response in Lygus hesperus, and tested its behavioral response to these individual compounds, and to a blend. Together with collaborators, the blend was tested using traps in alfalfa and strawberry fields. The scented traps did not increase the capture rate of L. hesperus in the field, and the blend was therefore not tested on bees.
Objective 2. ARS and ORISE scientists in Madison, Wisconsin, detected gene flow from cultivar to wild carrot populations (Palmieri et al. 2019). The cultivar genes, however, only persisted (introgressed) in the wild carrot populations in some regions of the United States: parts of California, and Nantucket Island in Massachussetts, as determined by ARS scientists in Madison, Wisconsin, and collaborators (Fernandez et al. 2022 BioRxv). No introgression was observed in Wisconsin, despite the presence of a long-term carrot breeding site, and similarly in other regions with harsh winters. The authors hypothesized how the environment, mainly harsh winters, may limit the survival of the hybrids and some evidence supports the negative impact of cold weather on hybrid survival. The lack of introgression in Wisconsin was further confirmed by a smaller scale study that examined the presence of cultivar genes at increasing distances from a source (Brunet et al., Acta Horticulturae, accepted).
University of Georgia collaborators and ARS scientists in Madison, Wisconsin, generated selfed and outcrossed seeds that will be used to quantify cumulative inbreeding depression in alfalfa.
Land Institute collaborators, and ARS and ORISE scientists in Madison, Wisconsin, collected two years of data on flowering, seed set, and pollination in feral alfalfa populations in Wisconsin and Kansas and one year in California and Washington. In addition, they started determining the proportion of feral populations that carry the glyphosate resistance gene, together with the number of individuals with the GR gene per population.
Accomplishments
1. Patch Fidelity is greater for honey bees relative to bumble bees, which limits their role in gene flow among plants. Bee species can vary in their fidelity to previously visited patches, and high patch fidelity translates into lesser gene flow. ARS scientists in Madison, Wisconsin, marked individual honey bees and bumble bees in specific patches and examined the likelihood of these bees revisiting these patches. While 76% of honey bee reobservations occurred in the patch the bees were originally marked, this number was only 47% for bumble bees. Moreover, honey bees were as likely to return to the original patch irrespective of its size, while bumble bees were more likely to be faithful to larger patches. Higher patch fidelity of honey bees, relative to bumble bees, could result from their highly developed communication system in the form of waggle dance. Understanding bee species patch fidelity can help design strategies to reduce gene flow and the escape of genetically modified genes in agriculture
2. How leafcutting bees decide which patch to visit. Change in land configuration is an important driver of pollinator decline and knowledge of patch selection (patches of land selected by bees for vititation) in fragmented landscapes can guide the design of habitats to ensure their pollinator conservation. ARS scientists in Madison, Wisconsin, compared the observed transitions between patches for the alfalfa leaf cutting bee against predictions derived from four models of patch attractiveness that differed in the role of patch size, resources and distance between patches as predictors. The alfalfa leafcutting bee used both patch size and distance between selected patches, but could only evaluate partial resources within a patch. Like previous studies with bumble bees, this solitary bee did not prefer nearest neighbor patch nor did it move randomly between patches. Unlike bumble bees, which have been shown to estimate the total resources available in a patch, the leafcutting bee could only evaluate small sections of a patch for resources. Bee species differ in their patch selection process. This information can guide habitat design in fragmented landscapes to optimize resource acquisition of bee species and facilitate their conservation.
3. Commercial cultivar genes that transfer to wild carrot populations do not persist in colder environments. Crops and their wild relatives often hybridize but it remains unclear whether the crop genes persist in wild populations. This question is key to accurately assessing the risk of escape and spread of cultivar genes into wild populations. ARS scientists in Madison, Wisconsin, along with collaborators,used genomic methods to track the incorporation of cultivated carrot genes into wild carrot in the United States. Cultivar genes persisted in wild carrots in two regions: California and the Nantucket Island, in Massachusetts. However, there was no evidence of persistence in wild carrots bordering those geographic regions. The populations with evidence of introgression were located in areas with milder winters, suggesting a potential role of the environment, where harsher winters may prevent the persistence of crop alleles. Cultivar genes found to persist in wild carrot populations were found over all nine carrot chromosomes. The results of this study should be considered before the release of genetically modified genes in carrots.
Review Publications
Brunet, J., Jiang, Q., Zhao, Y., Thairu, M.W., Clayton, M.K. 2023. Bee species perform distinct foraging behaviors that are best described by different movement models. Scientific Reports. Article 71.. https://doi.org/10.1038/s41598-022-26858-9.
Fragoso, F., Brunet, J. 2023. Differential ability of three bee species to move genes via pollen. PLOS ONE 18(4). https://doi.org/10.1371/journal.pone.0271780.
Fragoso, F., Brunet, J. 2023. The decision-making process of leafcutting bees when selecting patches. Biology Letters. 19 (2). https://doi.org/10.1098/rsbl.2022.0411.
