Biotic diversity is characterized by patterns of both divergence and similarity. Both natural selection and constraint may operate to conserve a trait across related species, depending on the ecology of the species. My dissertation investigates the roles of these evolutionary forces in maintaining a family-diagnostic stamen morphology. Flowers in the Brassicaceae (mustard plant family) are characterized by four long and two short stamens within a flower (Zomlefer 1994). Although found in >95% of the genera in the family (Endress 1992), the reason(s) for the widespread conservation of this morphology are not known.Existing adaptive hypotheses for the evolution and the maintenance of tetradynamy address how the trait could increase fitness for outcrossing species. While there is some evidence that tetradynamy is adaptive in self-incompatible members of the Brassicaceae (e.g. Kudo 2003; Conner et al. 2009), how it functions is not clear. Additionally, there have been multiple independent losses of self-incompatibility in the family (Lloyd 1965; Mable et al. 2005), followed in some cases by the evolution of high self-pollination rates (Preston 1986). In these highly selfing species, the maintenance of short stamens, which general appear too short to pollinate the stigma within a flower (Müller 1961), is particularly mysterious. My dissertation unites approaches from evolution, ecology, and genetics to understand the maintenance of short stamens in the Brassicaceae across the full range of mating systems. I investigate the function and morphology of short stamens in three species: the obligately outcrossing wild radish (Raphanus raphanistrum), the highly selfing model plant Arabidopsis thaliana, and A. thaliana's sister species, A. lyrata, which includes both outcrossing and selfing populations.In the outcrossing species Raphanus raphanistrum, I used experimental manipulations in arrays exposed to pollination in the field to show that having more stamens increases male fitness, and female fitness is also affected by stamen treatment. There were some indications that short stamens were more attractive to pollinators at high overall pollinator visitation rates. In the highly selfing model plant Arabidopsis thaliana, I showed that short stamens do not significantly increase fitness. I found many populations, particularly near the southern end of the geographic range, have partially lost the short stamens. Genetic mapping revealed three QTL controlling the number of short stamens, with strong epistasis greatly reducing their individual effects. Ongoing evolutionary loss of non-adaptive short stamens in Arabidopsis thaliana may be slowed by gene interactions, low genetic variance in the north, and an inbreeding mating system. Finally, I investigated the evolution of floral morphology in selfing and outcrossing populations of the mixed-mating Arabidopsis lyrata. I found that while relative investment in female fitness has increased as predicted in selfing populations, predicted changes in size and evolution of short stamens have not yet occurred. This may be consistent with recent evolution of selfing, continued facilitation of pollination by insects, and/or constraint on the evolution of short stamens.
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