Master of Science (MSc)
Faculty of Science
Cuscuta (dodder) is a genus of roughly 200 species of obligate stem parasites with sub-cosmopolitan distribution. The fruit, generally regarded as a capsule, has a thin pericarp containing one to four seeds and opening at the base (circumscissile dehiscence; DE), or remaining closed (indehiscent; IN). IN has evolved multiple times in Cuscuta from DE, and is most common in the North American clades of subgenus Grammica. In addition, some species produce fruits that open irregularly. Characteristics pertaining to the fruits of Cuscuta are important as their seeds contribute most to their distribution and prevalence across the globe, and their reduced vegetative organs limit the morphological variation available for species’ identification. In this thesis, I examined the structural mechanism behind DE to elucidate fruit types and their evolution. I surveyed fruit morphological traits to determine their systematic significance and functional correlations with dehiscence/indehiscence. Finally, I explored the putative evolutionary advantage(s) of fruit indehiscence by examining distribution, floatability, germination, and infructescence architecture. Pericarp structure revealed three distinct fruit types: DE fruits with an abscission zone (AZ), IN fruits with a uniform pericarp, and fruits that dehisce irregularly via the thinning of endocarp cell walls (IrA). IN fruits that break open irregularly (IrB) may also be an evolutionary fruit trait and were considered as such. Most qualitative fruit traits were polymorphic and their evolution involved multiple transitions to each state. Differences in quantitative traits were not consistent among fruit types, however IrB fruit species generally have a large interstylar aperture and large fruits with more seeds. IrB fruit species have a larger geographical range and more northern latitudinal limit than DE fruit species, and their infructescences slightly more compact than IN fruit species; which along with fruit traits may contribute to their irregular dehiscence. Capsules of C. gronovii were capable of floating for at least one week longer than their seeds. Seeds of C. gronovii exhibit a delayed germination when not removed from their IN fruits; a strategy known as bet-hedging. These results suggest that the evolution of IN in Cuscuta has provided certain species with heterodiaspory, and enhanced their dispersal and germination strategies.
Ho, Anna, "Diversity and evolution of fruits in Cuscuta (dodders; Convolvulaceae)" (2017). Theses and Dissertations (Comprehensive). 1979.
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