Embryonic developmental modules have been theorized to provide a mechanistic basis for speciation. This thesis describes how the yolk extension in cypriniform fishes represents a novel evolutionary and developmental module of the teleostean phylotypic period.
The yolk extension is an evolutionary module: it is a heritable cladistic trait characteristic of the order Cypriniformes, and is present in sister order Characiformes, and a more basal order, Anguilliformes. Moreover, the yolk extension satisfies six criteria proposed by Rudy Raff to define developmental modularity. These are that it: (1) has a physical location; (2) undergoes developmental transformation; (3) undergoes evolutionary transformation enabling comparison to a proposed homology; (4) forms autonomously using specific genes; (5) shows connectivity to other modules; and (6) is comprised of a hierarchy of components that may be part of a larger hierarchy.
The yolk extension is a cylindrical tube extending posteriorly from a spherical or elliptical yolk ball. It undergoes heterochronic changes in development, shown by comparing its ontogeny in zebrafish, Danio rerio (Cypriniformes), with other cypriniform species. A comparative literature analysis of 461 species, using illustrations depicting the functionally defined vertebrate phylotypic period, spanning the tailbud to the pharyngula stages, demonstrates that the module has undergone an evolutionary transformation. Its formation is homologous to the process of tubulation, the process of enclosing the body axis in nested germ layers as the embryo elongates from a sphere to a rod. Examination of zebrafish mutant literature showed that yolk extension development has a genetic basis. Experimental manipulation of zebrafish embryos showed that it forms autonomously from trunk straightening, while it is mechanically connected to dorsal tissues. The module is part of the hierarchy of tubulation. The yolk extension ontogenesis module is a nested hierarchy of three morphogenetic compartments: the innermost yolk cell, the mesendodermal mantle, and the outermost embryonic integument. Experimental evidence suggests that the contractile embryonic integument reshapes the cohesive, viscoelastic yolk mass, forming the yolk extension.
Overall, the yolk extension, as an innovative evolutionary and developmental module of the teleostean phylotypic period, will be useful as a model system in which to frame a plethora of experimental inquiries.
|Advisor:||Cooper, Mark S., Hille, Merrill B.|
|School:||University of Washington|
|School Location:||United States -- Washington|
|Source:||DAI-B 70/09, Dissertation Abstracts International|
|Subjects:||Cellular biology, Evolution and Development|
|Keywords:||Ontogenesis, Pharyhgula, Teleostean phylotypic period|
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