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The Weberian apparatus is a novel assemblage of modified vertebral elements linking the swim bladder to the inner ear, facilitating enhanced hearing in a large group of teleost fishes, the otophysans. Otophysans comprise one of the largest assemblages of vertebrates on the planet, with nearly 10,000 species currently recognized (Nelson 2006). Despite the vast size of the group, the Weberian apparatus represents one of few synapomorphic characters uniting the group. The largest and most basal otophysan group, the cypriniforms, comprises nearly 3,000 diverse species (Nelson 2006), and dominates freshwaters on most continents except South America and Australia. Within these freshwaters, the elements and supportive structures of the Weberian apparatus vary greatly. Using an integrative approach uniting analyses and approaches from the fields of morphology, development, evolution, and mutant analysis, the goal of this study is to gain unique insight into the evolution and development of the Weberian apparatus, an evolutionary novelty in otophysans. Within Cypriniformes, the morphology of the Weberian apparatus varies significantly among families within the order. Cyprinidae, the most basal and diverse family in the order, maintains the proposed ancestral morphology with limited exception (Rosen and Greenwood 1970; Chardon and Vandewalle 1997). More derived families, such as Balitoridae, exhibit expansive ventral bony encapsulations surrounding the swim bladder. These encapsulations coincide with specific environmental conditions, such as benthic life in fast moving streams. Growth of the Weberian apparatus exploits the inherent plasticity of the vertebral elements. Growth rates demonstrated negative and positive allometry as well as isometry in the zebrafish, Danio rerio, which was analyzed as a representative cypriniform. However, this variation in growth rates was not limited or unique to the zebrafish, as other teleosts also exhibited variation in vertebral growth without vertebral modifications. However, the extent of this variation was greater in the zebrafish than all other species examined. Development of the Weberian apparatus likely utilizes both changes in growth paired with novel spatial or temporal patterns of gene expression. Gene expression patterns of candidate genes, such as Sox9 and Pax9, were examined to determine differences between anterior and more posterior vertebrae, which exhibit unmodified elements. Development appears extremely constrained, as effects of the absence of expression of certain genes, as observed in selected zebrafish mutants, had little to no effect on the morphology of the Weberian apparatus, even while more caudal thoracic vertebrae were greatly affected.

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