Book Description

In zebrafish (Danio rerio) and many other organisms, the differentiation of primordial germ cells (PGCs) requires the reliable transmission of maternally-derived germline determinants, collectively termed germ plasm. PGCs give rise to the germline; thus, the fidelity of PGC specification and development is intimately tied to the survival and fitness of sexually-reproducing species. In this dissertation, I systematically investigate the formation, structure, and composition of germ plasm aggregates in zebrafish and related fish species in the Danio and Devario genera. I provide a comprehensive overview of preformation (germ plasm-dependent) and induction (non-germ plasm) as mechanisms of PGC specification, including major findings in several model vertebrate species. PGC specification is also discussed in a phylogenetic context, with particular attention to conserved gene families and the ongoing efforts to characterize germline determination in non-model vertebrate species. In Chapters 3 and 4, I use advanced methods of fluorescence in situ hybridization (detailed in Appendix I), microscopy, and image analysis to characterize the formation and structure of zebrafish germ plasm aggregates at a macromolecular-level, from the identification of homotypic germline-determinant RNA particles (Chapter 3) to the description of and proposed mechanism for organized supramolecular assembly of germ plasm in cleavage-stage embryos (Chapter 4). Appendix II presents the creation and initial characterization of mutations in bucky ball 2-like, the only zebrafish paralog of the germ plasm organizing protein Bucky ball. In Chapter 5, I identify germ plasm in three non-zebrafish species within the Danionin clade: Danio kyathit, Danio albolineatus, and Devario aequipinnatus, and find that eleven RNAs, including several not previously reported as germ plasm components in any species beyond zebrafish, exhibit conserved localization patterns within the embryos of all three examined non-zebrafish species. One component of zebrafish germ plasm, ca15b, was observed in the germ plasm of the two non-zebrafish Danio species but lacked specific localization in Devario aequipinnatus embryos. Together, these works present an improved understanding of the molecular components and behavior of maternally-derived germ plasm in zebrafish and related species.