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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Organismic and Evolutionary Biology

Year Degree Awarded

2018

Month Degree Awarded

September

First Advisor

R. Craig Albertson

Subject Categories

Evolution | Genetics | Integrative Biology | Life Sciences

Abstract

Phenotypic novelties are an important but poorly understood category of morphological diversity that are often associated with elevated rates of diversification and/or ecological success. The aim of this dissertation is to explore a phenotypic novelty at many levels to contribute to our understanding of how these unique traits can arise (e.g., genetically, developmentally, and evolutionarily) as well as their ecological consequences (e.g., trait function). The extreme snout of the Lake Malawi cichlid fish Labeotropheus is used as a case study. The first chapter establishes the Labeotropheus snout as a model of phenotypic novelty by characterizing the gross morphology, genetic architecture, and growth patterns of the snout. Growth data show snout size diverges at early juvenile stages between Labeotropheus and a closely related ecological competitor. The second chapter reveals the tissue-level developmental origins and proximate molecular basis of the snout, providing evidence of the co-option of an existing genetic pathway in the evolution of this novelty. Specifically, histological staining shows that the exaggerated snout is composed of hypertrophied intermaxillary ligament which forms a dynamic boundary with the surrounding loose connective tissue before anchoring to the epithelium. Further, protein manipulation, gene expression, and genetic mapping implicate the TgfβàScx pathway in the development of this trait. We also identify and confirm adam12 as a novel candidate regulating ligament development. The third chapter expands the scope of this dissertation by using phylogenetic comparative methods to investigate snout shape in distantly related cichlid lineages. These data show that three cichlid lineages have converged on the exaggerated snout phenotype and models of evolution suggest that snout shape is under selection for feeding behavior. The fourth chapter explores intraspecific variation in snout morphology and environmental sensitivity. Comparative anatomy between two distinct populations of Labeotropheus coupled with lab experiments to manipulate feeding behavior show that both snout shape and the plastic response differs between populations. Overall, by taking an integrative approach and using the Labeotropheus snout as an example, the data presented in this dissertation provide a holistic understanding of a morphological novelty from proximate genetic mechanisms to ultimate ecological consequences.

Available for download on Sunday, September 01, 2019

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