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Abstract
The mechanosensory lateral line is a crucial sensory system ancestrally present in all vertebrate lineages. Identifiable in the earliest vertebrate fossils, lateral lines have been evolving for half a million years through major morphological transformations in vertebrate evolution. The lateral line is essential for behaviors like schooling, predation, and escape. Developing from a series of ectodermal placodes, it consists of a series of receptor cells called neuromasts distributed over the heads and trunks of aquatic organisms. The cranial component of the lateral line in jawed vertebrates consists of lines around the orbit, on the cheek and on the lower jaw. While the trunk lateral line has been studied in detail as a model for cell migration in zebrafish, very little is known about the cranial lateral line. This dissertation takes an integrative approach combining data from palaeontology, comparative anatomy and developmental biology to examine the evolution and development of the cranial lateral line system in vertebrates. Contrary to previous hypotheses of lateral line evolution as a direct, linear transformation from conditions in jawless to jawed vertebrates, we propose a scenario where lateral lines evolved independently in agnathans and gnathostomes, and were marked by a radical phase of remodelling preceding the advent of jaws, resulting in a shift from a postorbital to a preorbital location of complex lateral lines. This shift may have been essential for colonization of the water column by jawed vertebrates. Focusing on the cranial lateral line system in zebrafish, I show that the neural crest, an important cell type, is closely physically associated with the lateral line and is required for normal lateral line development. Abrogation of the neural crest, through genetic or pharmacological manipulations results in stereotypical deficits to the lateral line system. I further describe the development of the lateral line system in postembryonic and larval stages of zebrafish and show that superficial neuromasts are added continuously through development via a variety of mechanisms including the migration of a new primordium, intercalation between existing neuromasts or budding from an existing neuromast. I also describe a novel cross- placodal mechanism involved in the development of a subset of cranial neuromasts. Using experimental ablations, I demonstrate that zebrafish undergo a developmental switch in ontogeny after which innervation becomes indispensable for lateral line development. Taken together, my findings shed new light on the evolution of vertebrate lateral line morphologies, show how interactions with other tissues may influence lateral line development and uncover new developmental mechanisms underlying the growth of the lateral line system. These findings lay the foundation for future studies on this ancient, crucial and complex sensory system.