| dc.description.abstract | With ~64,000 species of extant vertebrates spanning a tremendous array of morphologies, attempting to understand how organisms adapt and evolve phenotypically across environments has become a major focus of biology. Genomic information along with genetic manipulation of model organisms allows for a genotype to phenotype correlation to be made. Though progress has been made at understanding how traits are determined and regulated, genes associated with changes in phenotype continue to be discovered. One morphologically diverse vertebrate group which has received little attention during the genomic era, with an increase in the last 5 years, are squamate reptiles (lizards, snakes and amphisbaenians). As much developmental genetic insight has come from human cases of congenital malformations, this analogy has dominated my research program to study the evolution of body form in squamate reptiles by studying the highly divergent (i.e., `mutant' in the analogy) chameleon body plan relative to the generalized lizard body plan. I hope to understand molecular and cellular mechanisms involved in altering morphology with the ultimate goal of understanding the underlying genomic changes. Chameleons differ from other lizards (and tetrapods) due to the presence of several divergent and novel morphologies associated with their arboreal lifestyle, such as: a ballistic tongue for feeding, rapid and complex color/pattern change, their eyes rotate and independently, tails are prehensile, cranial skeletons are modified topographically and architecturally and the wrist skeletal elements on the hands and feet are highly derived for arboreality along with their cleft hands and feet. I chose to focus on the veiled chameleon (Chamaeleo calyptratus) not only for its morphology but because it overcomes a major constraint for studying squamate reptile embryos in the lab: unlike most squamates, at oviposition embryos are at an early gastrula stage whereas in other taxa species are at an advanced limb bud stage. Thus, studying early patterning and subsequent stages are now amenable, similar to that of chick and mouse. The first chapter/study was conducted not only to provide a very detailed ontogenetic description of embryogenesis for the veiled chameleon, desert grassland whiptail and the African house snake, but to serve as the foundation for future developmental studies. To understand how a particular morphology arises, one must know when it first appears (or appears to diverge from the `typical' morphology) ontogenetically. I characterized aspects of the extraembryonic and embryonic tissues along with using fluorescence imaging to provide a detailed account of the slow development (i.e., gastrulation and embryogenesis) for the chameleon in comparison to the two ecologically and morphologically divergent lizard and snake also in this study. The second portion of the dissertation focuses on craniofacial development of the lizard skull, focusing on the derived casque in the veiled chameleon relative to the `normal' cranial vault of the desert grassland whiptail (Part A). Skeletal development and detailed morphogenesis of the casque is described with a discussion on homology of parietal elements and how the casque evolved. Consequently, as the majority of the cranium is of neural crest cell origin, I characterized the cranial population which contributes to the skull to see if there were major differences in the pattern of migration into the embryonic head which may give rise to the unique chameleon morphology (Part B). The last chapter of my research focuses on the development of the second most obvious trait, unique to all chameleons: the cleft hands/feet. As this phenotype is present in many congenital malformations in humans, I thought it may serve as a unique model to study this disease. I found that the formation of the chameleon hand/foot is multistep, with clefting being the first step which is morphogenetically obvious and an integrative study is conducted using morphology and molecular methods (relative to the development of the whiptail autopodia). The three chapters should provide a large foundation for future work in squamate developmental genetics and will serve as a guide for comparative embryology and morphological novelties. With future sequencing of the Veiled chameleon genome (among others) along with a community working together to develop tools for genetic modification in this system, I hope that lizards and snakes will play a more central role in the study of body form evolution. | |
