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The evolution of seed dispersal syndromes in Prunus
AuthorBeck, Maurie J.
AdvisorVander Wall, Stephen B.
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ABSTRACT: There are two fruit types in the genus Prunus. The majority of species have fleshy-fruited drupes, which are considered the ancestral phenotype. In the deserts of Eurasia and North America there are also species that produce dry fruits and large nuts, suggesting this fruit type has originated independently on numerous occasions in response to dry conditions. Fleshy-fruited Prunus are dispersed by frugivorous animals, primarily birds and some mammalian carnivores. In this dissertation, by documenting complete seed fate pathways, I demonstrate that desert peach (Prunus andersonii), a dry, nut-producing species in the western Great Basin of North America, is only dispersed by scatter-hoarding rodents. Additionally, I demonstrate that western chokecherry (P. virginiana var. demissa, Rosaceae) is also dispersed by scatter-hoarding rodents, following primary dispersal by endozoochorous frugivores. This type of two-phased seed dispersal is a form of diplochory, a process that employs different modes of dispersal during sequential dispersal phases and usually offers unique benefits during each phase. Although not well documented, frugivory followed by scatter hoarding is believed to be more common than previously thought. In this case I show that phase I dispersal by frugivores transports seeds, often long distances, away from the parent plants. Scatter-hoarding rodents then harvest the seeds from feces and bury them in soil during phase II dispersal. Caching chokecherry seeds not only moves them away from the parent plant, but constitutes directed dispersal, a form of seed dispersal that disproportionately enhances seed and seedling survival. The transition in Prunus from primary dispersal by fruit-consuming animals to nut dispersal by scatter-hoarding rodents is difficult to envision. However, if the ancestor to desert peach or other dry-fruited species utilized diplochory, then the most parsimonious explanation is that the transformation from fleshy fruits to dry nuts was accompanied by the loss of frugivory and the reliance only on dispersal by scatter-hoarding rodents.In chapter 3 I review to similarities and differences between frugivory and dispersal by scatter-hoarding animals.