Dennis WallDissertation Summary
The main objective of my dissertation is to reconstruct the history of Mitthyridium, a paleotropical endemic moss that has diversified across islands in the Pacific and Indian Oceans. This moss's genetic, developmental, morphological and ecological variation suggests its youth and status as a radiation. Further, the moss has a diversity in mode of reproduction (in a continuum from sexual to purely vegetative) that appears to vary within species rather than among them. As is well known, sex--gene flow--can have constraining, cohesive or creative forces on populations in nature. Thus, given its island distribution patterns (across many island chains in two oceans) and intraspecific variation in characters like mode of reproduction, Mitthyridium presents a unique system for testing alternative hypotheses of diversification. This group also presents a number of unique features that make it unlike other plant or animal systems. In short Mitthyridium is ideal for questioning radiations and may help bridge the gap between population divergence and evolutionary diversification.
Studies of diversification rate shifts require knowledge of both phylogeny and time. Yet, even with these components, the picture remains incomplete if the goal is to determine causal factors driving such shifts. In the present analysis I optimize biogeographic data and data on reproductive strategy to a large phylogeny of Mitthyridium, consisting of 140 population exemplars, in order to comprehend causal factors driving its relatively recent radiation. To reconstruct adequately this shallow level of phylogenetic divergence, I employ a single copy nuclear gene Glyceraldehyde-3 phosphate dehydrogenase (gpd). The gene’s utility for plant systematics is poorly known, and this represents the first study to use gpd for moss phylogeography. Using gpd, I reconstruct the relationships among 140 populations distributed across the known geographic range Mitthyridium. I optimized to this phylogeny the variation in reproductive strategy (Mitthyridium has a range of reproductive expression from fully sexual to vegetative) to test the influence of island isolation on reproductive mode and the consequent influence of reproductive strategy on diversification rate shifts. Results show a trend towards increased loss of sexual reproduction among island populations. These island groups were found sister to mainland populations that show significantly higher frequencies of sexual reproduction. This finding suggests a general evolutionary trend towards loss of sex on islands. Furthermore, the shift in reproductive mode is correlated with a shift in diversification rate. This correlation suggests a plausible cause of the rapid lineage radiation among island members of Mitthyridium.