Bridget Bickner Thesis Defense (Elena Kramer and Robin Hopkins, Co-Advisor)

Title: Evolution of discrete reproductive strategies in Phlox (Polemoniaceae)

Abstract: Fitness is defined as an individual’s relative success in reproduction and survival. As populations adapt to changing ecological and environmental conditions, the fitness landscape can favor divergent reproductive strategies, leading to correlated and cascading effects on reproductive biology. In flowering plants, maternal fitness depends on both seed size—a proxy for offspring quality—and seed number, reflecting offspring quantity. However, a fundamental tradeoff between seed size and number constrains the potential for simultaneous fitness gains in both traits. In this thesis, I use an eco-evo-devo framework to investigate how and why variation in seed number arises within the wildflower genus Phlox. In Chapter 1, I characterize the developmental basis of ovule number variation across five independent origins of a novel multi-ovulate ovule packaging morphology from a single-ovulate ancestral state. We show that a dramatic shift at the earliest stages of development in the allometric relationship between ovule size and ovary size governs the transition from a single-ovulate to a multi-ovulate state. Specifically, this allometric difference reflects two distinct developmental trajectories: one in which a larger ovary yields a larger, single seed and the other in which a larger ovary yields an increased number of smaller seeds. The convergent evolution of this novel developmental response in every instance of the derivation of the multi-ovulate condition may reflect a previously unappreciated constraint on ovule development. Furthermore, this single mechanism appears to be the basis of variation in the size and number of both seeds and ovules. In Chapter 2, I created an F2 hybrid population using crosses between sister species Phlox drummondii and P. roemeriana, representing the extremes of not only ovule packaging strategies but also correlated variation in flower size and number. Most measured traits in the F2 population show approximately normal distributions, consistent with a genetic architecture involving many loci of small effect. Significant correlations between ovary size, ovule number, flower size, and flower number among F2 plants suggest that underlying genetic constraints may shape both the seed size/number and flower size/number tradeoffs, as well as the relationship between them. Finally, in Chapter 3, I characterize a natural polymorphism in Phlox longifolia involving the ancestral single-ovulate strategy and the derived multi-ovulate strategy. I show that genetic variation underlies the phenotypic differences observed across 102 populations spanning the species' range. While evidence for abiotic environmental factors as selective agents is mixed, I propose pollinator service as an alternative explanation for the geographic distribution of ovule packaging morphs. Collectively, these studies reveal the developmental and genetic underpinnings, as well as the ecological context, of seed packaging strategies in Phlox, offering novel insight into how changes in fundamental developmental pathways generate morphological diversity in natural populations and influence key components of the tradeoff between seed size and number.

Committee: Elena Kramer (Co-Advisor), Robin Hopkins (Co-Advisor), David Haig