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Title: Ecology and Evolution of the Ferns of Moorea and Tahiti, French Polynesia
Abstract: Ferns are the only major clade of land plants with haploid (gametophyte) and diploid (sporophyte) stages that can grow separately from each other for extended periods. The ecology of fern gametophytes, which represent the sexual part of the life cycle, is very poorly known due to their small size and cryptic morphology. In contrast, the conspicuous sporophytic phase has been much better characterized in general. In my dissertation, I use the ferns of Moorea and Tahiti, French Polynesia as a study system to investigate the relative roles of fern sporophytes and gametophytes in community assembly and evolution.
In Chapter 1, I use DNA sequences to identify field-collected gametophytes to species and compare patterns of phylogenetic community diversity between fern sporophytes and gametophytes on Moorea and Tahiti. I find that phylogenetic diversity of sporophyte communities declines with elevation, but gametophytes lack such a correlation. Furthermore, I observe several species with gametophytes that are distributed beyond the range of conspecific sporophytes, and at least one species that may lack sporophytes on the islands completely. My results suggest that the transition from gametophyte to sporophyte functions as a filter restricting phylogenetic diversity of fern communities.
In Chapter 2, I use the filmy ferns (Hymenophyllaceae) of Moorea to investigate how changes in physiology between fern gametophytes and sporophytes are correlated with ecological niche. I find that the gametophytes of filmy ferns are not more stress-tolerant than sporophytes. Rather, they appear to be adapted for microhabitats that are strongly buffered relative to those of sporophytes. I infer that the gametophytes of filmy ferns rely on mechanisms such as asexual reproduction via gemmae to achieve dense population sizes over a wide range.
In Chapter 3, I conduct a phylogenetic comparative investigation of traits related to epiphytic growth in the ferns of Moorea. I find strong correlations with life history: epiphytes tend to have shorter stipes, smaller rhizomes, and gametophytes with non-cordate morphotypes relative to terrestrial species. Furthermore, I demonstrate that epiphytic communities are clustered phylogenetically and functionally relative to terrestrial communities, supporting a scenario of extreme environmental conditions in the forest canopy acting as a filter on epiphytic ferns. Taken together, these results clarify the morphological and physiological basis for the remarkable radiation of ferns into the angiosperm canopy.
In Chapter 4, I describe a new hybrid fern species, Microsorum × tohieaense, from Moorea based on morphological and molecular phylogenetic analysis. I also find genetic evidence supporting hybrid origins of Microsorum × maximum, which had previously been suspected on the basis of morphology. I discuss the significance of my findings for the taxonomy of the microsoroid ferns.