MOLECULAR AND DEVELOPMENTAL APPROACH TO TWO MAJOR TRANSITIONS IN DIATOM EVOLUTION.

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Diatoms (microscopic marine plants) are tremendously important ecologically (the oxygen in every fifth breath we take comes from diatoms) and economically (some produce neurotoxins harmful to humans). They are also the most species rich group of microalgae (~200,000 species) – thus rivalling the richness and diversity of flowering plants on land. Evolutionarily, like flowering plants, diatoms also are relatively new arrivals, suggesting a high rate of speciation. Unlike flowering plants, however, the mechanism(s) of speciation in diatoms is unknown.The long term objective of my research is to evaluate the evolutionary significance of diatom sex and related structures, and to better understand what role various types of sexuality play 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 developmental and molecular approaches to representatives of two major lineages of diatoms in key evolutionary positions: the basal non-polar centrics (earliest branches in diatom phylogeny) and basal araphid pennates (deepest split among the most derived diatoms, the pennates). We will use monoclonal cultures already at hand and establish many more from local and distant geographies to develop robust and densely sampled “family trees” (phylogenies) for these lineages.We will use advanced microscopical techniques (epifluorescence and deconvolution light microscopy, scanning electron microscopy, x-ray microanalysis, silica-specific stains) to detect and localise opaline silica (the building blocks of diatom cell walls) and describe the fine structure of the cell-wall components during their stages of development. Determination of mating compatibility and behavioural expression of sex (e.g., unisexual, polysexual) will provide new insights into the propensity for in- vs. out-breeding among the pennates, thereby facilitating predictions about the intra- and interspecific genetic variability of these diatoms, and ultimately testable predictions about mechanisms of diatom speciation. Results from this study of diatom sexual structures and mating behaviour will be integrated into phylogenies of the diatom groups delineated by analysis of selected DNA sequences.Phylogenetic analysis of these diatoms combined with comparative developmental approaches we propose here are novel in diatom evolutionary biology, which we pioneered only about decade ago. The major obstacle to further advancement was the lack of reproducible sex-induction protocols which we now have for the diatom groups involved. The results of this work will provide a multipronged framework to better our understanding of major evolutionary transitions in diatoms and allow us to characterize species boundaries at a biological (sex-compatibility) and molecular level. The proposed work includes numerous new training opportunities, including advanced microscopy and phylogenetic projects for undergraduate researchers and more advanced students.