Sexual Selection in a Hermaphroditic Plant

Sexual selection acts to increase mating success in animals and plants. Compared with extensive studies in animals and dioecious plants, sexual selection has received less attention in hermaphroditic plants, despite its potential for explaining evolution of flora traits that primarily influence mating success. In my dissertation, I used Passiflora incarnata, a hermaphroditic plant with dichogamy, herkogamy, and andromonoecy, to address how these traits might have been the targets of sexual selection through male or female function by examining their effects on pollination success. In addition, I asked how sexual selection is acting on current floral traits under different ecological contexts. I conducted my research in a natural population and a common garden of P. incarnata in Virginia. First, I estimated how temporal (dichogamy) and spatial (herkogamy) separation of male and female reproductive function affect pollen removal and pollen deposition. Dichogamy assured that almost half of the pollen could be successfully exported without interfering with stigmas, indicating dichogamy may facilitate male pollination success. In contrast, increased herkogamy resulted in reduced pollen deposition and lower fruit set, yet, a higher seed number, suggesting variation in herkogamy chiefly influences female mating and reproductive fitness. Second, I studied morphological differences between male and hermaphroditic flowers in P. incarnata and estimated mating success and male fitness for both flower types. Although two flower types were similar in morphology, male flowers were more successful in siring seeds than hermaphroditic flowers due to higher mating success (pollen export). Thus, the production of male flowers in P. incarnata is likely an adaptive mechanism to enhance iii male fitness. Lastly, I quantified sexual selection on floral traits under unmanipulated, limited-pollination, and resource-addition environments. Sexual selection was detected on style deflexion. A significant Bateman's gradient was observed only under limitedpollination treatment, suggesting stronger sexual selection under such conditions. My results demonstrate that floral traits are under sexual selection via female fitness and the selection can change with ecological contexts. Taken together, these studies illustrate how floral traits influence pollination success in hermaphroditic plants and enhance our understanding of the role of sexual selection in floral trait evolution.

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