EVALUATING THE EFFECTS OF POLLINATOR MEDIATED SELECTION ON PATTERNS OF FLORAL VARIATION

Abstract

Angiosperm flowers astonish for their high morphological diversity. More importantly, flower shape variation has a significant reproductive and evolutionary role. Here, I studied the relationship between flower shape and pollination precision, i.e. the precise transfer of pollen across flowers by pollinators. In the first two chapters, I studied the role of one floral trait, corolla chirality, in three species of Hypericum (Hypericaceae). Unfixed corolla chirality is the presence of pinwheel arrangement of petals, both right and left rotated, within an individual. Specifically, I evaluated whether corolla chirality promotes disassortative mating between flower morphs through directed movement of pollen and pollinators between flowers. This precise pollination mechanism could increase outcrossing rates by reducing geitonogamous pollinations. Nevertheless, pollinators were indifferent to corolla chirality and thus pollination for unfixed corolla chirality is similar to radially symmetric flowers with a generalized (non-precise) pollination system. In chapter 3, I performed a macroevolutionary analysis on multiple key flower traits. I hypothesized that flower traits with precise pollinations due to precise fit with their pollinators or due to increased pollination specialization will be under uniform directional selective pressures and thus be less variable than flowers with less precise pollination system. I found that flowers with lateral orientation or bilateral symmetry were significantly less variable than their alternative states (vertical and radial, respectively). Thus, I demonstrate that traits that restrict pollinator landing and movement play an important role in pollination precision. In chapter 4, I quantified patterns of genetic variation available for pollination precision to evolve in a male reproductive trait (i.e. stamen height) using fast-cycling Brassica rapa. The match of anthers and stigmas to the contact area on the pollinator body conveys precise pollen transfer. My results suggest that individual mean stamen height can evolve to match the population mean pistil height (presence of additive genetic variation), but that some level of imprecision will remain due to lack of additive genetic variation for within-individual stamen height variation. In summary, floral traits vary in their role in pollination precision and the evolution of pollination precision may be constrained by the types and amounts of genetic variation.