Given the prevalence of intermediate levels of outcrossing among angiosperms, a general mechanism explaining the evolution and maintenance of this condition is needed. Although numerous theoretical models predict mixed mating to be evolutionarily stable, the conditions favoring intermediate selfing are often very stringent and have limited applicability. Here I investigate the role of two plant antagonists, vegetative herbivores and intraspecific competitors, in influencing the mixed mating system of Impatiens capensis (Balsaminaceae). This species exhibits an obligate mixed mating system by producing heteromorphic flowers (i.e., individuals produce both selfing, cleistogamous and facultatively-outcrossing, chasmogamous flowers). Thus, these antagonisms may affect mating system at the level of relative heteromorphic flower production, outcrossing within chasmogamous flowers and whole-plant outcrossing. In a comparative study exploring how herbivory and intraspecific competition jointly affect mating system expression, I found that these antagonisms affect plant growth and mating system traits differently, and thus the mating system response could not be accurately predicted from plant growth response. Using surveys of wild populations and experiments manipulating herbivory under field conditions, I found that herbivory reduced outcrossing by increasing proportional cleistogamous reproduction. In the field, I found that herbivory increased outcrossing among chasmogamous flowers due to effects on flower display, pollinator visitation rate and pollinator fauna composition. Overall, herbivory slightly lowered whole-plant outcrossing.
To understand further the consequences of mixed mating, I manipulated herbivory in two wild I. capensis populations to explore the demographic consequences of mixed mating, herbivory, and the interactive effects of mating system and herbivory. I found that selfed individuals had higher rates of germination and survival and lower fecundity than did their outcrossed counterparts. Herbivory also had demographic consequences as it reduced population growth rate due to its effect on vital rates of selfed individuals.
Overall, the results presented in this dissertation offer important insight to the ecological factors that cause variation in mating system as well as the long-term consequences of variation in mating patterns. Furthermore, these findings have implications for population genetic diversity and structure and point to the role of natural enemies in contributing to the maintenance of a mixed mating system.
| Identifer | oai:union.ndltd.org:PITT/oai:PITTETD:etd-08162005-151927 |
| Date | 10 October 2005 |
| Creators | Steets, Janette Ann |
| Contributors | Dr. Tia-Lynn Ashman, Dr. Stephen Tonsor, Dr. Jeffrey Lawrence, Dr. Susan Kalisz, Dr. David Carr |
| Publisher | University of Pittsburgh |
| Source Sets | University of Pittsburgh |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.pitt.edu/ETD/available/etd-08162005-151927/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University of Pittsburgh or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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