Evolution of the Contractile Vacuole

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The organelles of the membrane-trafficking system underlie the movement of material within eukaryotic cell, as well as exchange with the extracellular environment. The effective action of this organellar system mediates healthy human cellular function, as well as that of plants of agricultural importance and algae of economic and environmental impact. Evolving such an organellar system would have been a crucial step in the transformative process establishing the eukaryotic cellular configuration and giving rise to nearly all of the biological diversity that we see today. My research program involves understanding the patterns and processes that drive the evolution of the membrane-trafficking organelles. This provides insights into the events and fundamental forces that have shaped our cellular history. The comparative nature of this work also yields important cell biological data identifying novel cellular components and commonalities between well-studied model eukaryotes and lesser characterized organisms. A critical step to understanding the cell biology of modern cells and how it evolved is establishing homology between organelles in diverse eukaryotes. The endoplasmic reticulum (ER), Golgi, and plasma membrane have obvious direct organellar orthologues and are deduced to have been present in the Last Common Ancestor of Eukaryotes (LECA). However, while molecular phylogenetic and cell biological data have demonstrated homology between the various organelles of the endocytic system (recycling endosome, late endosome, lysosome), the relationships between them are unresolved. In particular, endocytic organelles also have secretory counterparts and it is unclear from where these organelles have evolved. These include secretory granules, trichocysts, and contractile vacuoles. The contractile vacuole (CV) is an osmoregulatory organelle found in ecologically important and diverse freshwater and soil-dwelling organisms (eg. ciliates, dictyostelids). Nonetheless, CV cell biology is only now being assessed using modern methods. As the name implies, the CV is presumed to be a vacuole/lysosome homologue. This project will use molecular evolutionary and transcriptomic analysis to examine the evolution and function of the CV, specifically, whether it is derived from a single homologous ancestor or derived independently from endolysosomal organelles. Based on morphological similarities, the starting hypothesis is that CVs in diverse eukaryotes are derived from a single common ancestor predating LECA. This hypothesis will be addressed in three short-term goals. Firstly, comparative genomics of known CV proteins will be undertaken yielding a possible conserved core CV complement, as well as lineage-specific features. Secondly, transcriptomics of three tractable, taxonomically disparate, model eukaryotes will be used determine genes that are differentially expressed upon induction of CV function. This will not only validate whether the ‘core CV complement’ are actually involved in CV function in diverse eukaryotes, it may also identify novel genes of functional CV importance. Finally, phylogenetic analysis of protein families associated with CV function (eg. SNAREs, stomatins, aquaporins) will test the hypothesis of ancient CV monophyly or determine from which other organelles the CV might have arisen. By identifying the genes of importance in CV function and their distribution in eukaryotes, we gain information about the workings of an organelle present in ecologically crucial lineages of eukaryotes. By understanding the evolution of the CV, we will be able to integrate an important organelle into the larger framework of membrane-trafficking evolution and gain a better understanding into the rise of eukaryotic life on Earth.