dissertationAnimals have developed extraordinary capacities to maintain homeostasis in the face of severe osmoregulatory challenges from their environment. For instance, with respect to salt and water homeostasis, freshwater animals continuously eliminate excess water while conserving solutes, whereas land-dwelling organisms have to conserve water and solutes as much as possible. Comparative morphological studies suggest that animals have tackled the problems of excretion and osmoregulation by evolving a specialized structure: the excretory organ. Animal excretory organs are extremely diverse. Some are unicellular, such as the excretory cell in nematodes. Others are multicellular and highly specialized, such as the protonephridia/metanephridia in invertebrates or the kidneys in vertebrates. In light of such anatomical and functional diversity, the evolutionary origins of animal excretory systems pose an interesting question in biology. However, the hypotheses proposed thus far remain highly controversial for two main reasons. First, many evolutionary arguments are based solely on morphology in organisms for which no molecular data are available, precluding rigorous genetic comparisons. Second, while invertebrates are critical elements of this evolutionary puzzle, the molecularly tractable ones studied to date display highly derived excretory systems. C. elegans possesses a single excretory cell, while the ultrafiltration of nephrocytes is uncoupled from the absorption/secretion of Malpighian tubules in D. melanogaster. Comparative morphological studies have demonstrated the existence of more complex excretory organs amongst many other invertebrates, including planarians. Planarians have a protonephridial excretory system in which each protonephridial unit consists of a tubule, opening distally via a nephridiopore at the surface of the animal and ending proximally in one or more terminal structures called flame cells. Protonephridia are commonly found amongst many invertebrates. Since protonephridia combine ultrafiltration with filtrate modification, planarians close an "invertebrate gap" in the study of excretory system biology. Taking advantage of a rapidly expanding list of molecular tools in recent years, this dissertation project aims to perform a comprehensive molecular and functional study of planarian protonephridia in order to provide new insights into the longstanding question on the evolutionary relationship between vertebrate and invertebrate excretory systems and gauge planarians' potential as a novel invertebrate model for human kidney development and disease
