Forms follow functions: Exploring the evolution of morphological diversity in seed plant reproductive structures.

摘要

This dissertation uses conifers to explore how patterns of reproductive character evolution are driven or constrained by the suite of functions that different reproductive structures must perform. I begin by exploring how a specific functional relationship between pollen morphology and seed-cone morphology has influenced the evolution of gymnosperm reproductive biology. I present studies in living trees demonstrating that flotation can effectively concentrate pollen with air bladders, or sacci, during pollination in conifer taxa with downwards-facing ovules. I then use both fossil and extant conifer data to show that saccate pollen in both ancient conifers and ancient non-coniferous gymnosperms is physically and morphologically similar to that of modern conifers. Coupled with the prevalence of downwards-facing ovules in many of the fossil gymnosperm groups that produced this pollen, the functional correlation between saccate pollen and ovulate-cone morphology likely has a deep history in gymnosperm reproduction and therefore has significantly influenced the evolution of gymnosperm reproductive morphology. I then explore larger-scale relationships between the evolution of conifer pollen and seed-producing cones in general and the functional roles that these structures perform. Combined results from multivariate morphological analyses and traditional morphometric data reveal that seed cones display a greater range of morphological diversity over their history than pollen cones, and that this pattern was driven by a radiation of forms during the Jurassic and Cretaceous. This radiation is characterized by the appearance of robust seed cones with larger, more tightly packed cone scales and more simplified seed cones with fleshy tissues. These seemingly contradictory patterns are best explained as responses to the increasing importance of the cone structure in protecting seeds from specialized vertebrate predators in some species and attracting vertebrate seed dispersers in others. In contrast, pollen cone morphology and their relative amount of tissue have stayed similar throughout conifer evolutionary history, which is consistent with the idea that these structures have functioned only in aerial pollen dispersal. This work provides a novel framework for interpreting patterns of character evolution in reproductive structures, and suggests that biotic interactions have been some of the most important drivers of reproductive diversification in seed plants.