Molecular and developmental approach to understanding diversification in mediophyte diatoms

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Diatoms (microscopic marine plants) are ecologically and economically important; indeed, the oxygen in every fifth inhalation originates from diatoms, but some produce neurotoxins harmful to humans. They are also the most species-rich (~200,000) group of microalgae, thus rivaling the diversity of flowering plants on land. Evolutionarily, and like flowering plants, diatoms are relatively new arrivals, suggesting an exceptionally high speciation rate. My long-term research objective is to evaluate the evolutionary significance of the processes and structures associated with diatom reproduction in order to understand what role various types of sexuality might have played in the spectacular diversification of diatoms since their first appearance in the Jurassic Period fossil record. The immediate objectives specific to this proposal are to apply reproductive, developmental and molecular approaches to representatives of two major transitions in diatom evolutionary history to untangle their relationships to each other and to their closest relatives. I propose to investigate two orders of diatoms that currently seem to poorly or uncertainly fit in both traditional and molecular phylogenies ("family trees"). Thalassiosiroids differ dramatically from their closest relatives, and biddulphioids are likely to hold evidence that indicates the closest living relative of the most advanced of the diatoms, the pennates. We will search for developmental indicators of how these ill-fitting or uncertain characters came to be. We will use advanced microscopical techniques (light and epifluorescence microscopy, scanning electron microscopy, X-ray microanalysis, silica-specific stains) to detect and localize opaline silica (the building blocks of diatom cell walls) and describe the fine structure of the cell-wall components during their developmental stages. We will also use a heretofore underutilized molecular tool, ITS2 consensus secondary structure models, in our phylogenetic inferences. Our reconnaissance work has shown that these models are as useful in untangling diatom phylogenetic relationships as they have proven to be in green algae and other plants. Phylogenetic analysis of these diatoms combined with the ontogenetic approach we propose here is novel in diatom evolutionary biology. We pioneered this integrative approach only about a decade ago and are the only laboratory in the world to systematically add reproductive data to the wider body of diatom evolutionary biology. The major obstacle to further advancement is the lack of reproducible sex-induction protocols, which we now have developed for the diatom groups involved. The results of this work will provide a multi-pronged framework to better our understanding of major evolutionary transitions in diatoms. The proposed work includes many new training opportunities, including microbial culturing, advanced microscopy, and molecular and bioinformatics projects for junior collaborators at all levels.