Eastern Redbud (Cercis canadensis) is a horticulturally important pod-bearing tree in the legume family (Fabaceae), which is host to complex tritriophic interactions consisting of Redbud seeds, their bruchid beetle predator, Gibbobruccus mimus, and multiple species of wasp parasitoids of the bruchid beetle larvae. Along with native C. canadensis species, many cultivars are commercially available. Previous research on other legumes has found seed beetle predation varying among cultivars. Here, we test the hypothesis that seed predation by G. mimus may be higher in Eastern Redbud cultivars due to reduced genetic diversity when compared to the native population. To investigate this, Eastern Redbud pods were collected from 10 cultivars (60 pods per tree; 3 or 4 trees per cultivar) at the National Arboretum in Washington, DC. Seeds were classified as either intact, beetle exit hole, wasp exit hole, aborted seed, or irregular seed. We predict that phenotypic differences among cultivars will influence rates of predation and parasitoidism. We are also interested in whether seed predation by the beetle always leads to death of the plant embryo, as the label “predator” implies. Our initial findings show viable embryos can be found in seeds following the emergence of the beetle, suggesting the designation “seed predator” may not be accurate. To determine if seed predation by G. mimus causes embryo death, we will stain seeds sorted into the forementioned categories with tetrazolium chloride, which changes color if cellular respiration is present. Additionally, we will cold stratify and germinate seeds to determine if they could produce viable offspring, even after damage. If seed damage leaves an intact embryo and improves water imbibement, we expect to see an increase in germination rates for seeds that have bruchid beetle or wasp exit holes compared to intact seeds. Such findings could lead to a reclassification of these tritrophic interactions.
Unraveling the Tritrophic Interactions Between a Seed Predator (Gibbobruchus mimus) and Parasitoid Wasps Among Eastern Redbud (C. canadensis) Cultivars
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Student Abstract Submission