Figure. 2. Germline development and asymmetries in early oogenesis and potential mitochondrial bottleneck in zebrafish.

A) Cartoons depicting the stages of germ cell development beginning with the primordial germ cells (PGCs) in larvae. Cellular asymetries have been observed in mitotic oogonia of female zebrafish. Asymmetric Bucky ball protein (light blue) is detected at zygotene stage, and is required for Balbiani body formation during prophase I (this period corresponds to stage I oocytes in zebrafish). The Balbiani body translocated to the prospective vegetal cortex and then disassembles in stage II ooctyes. B) Schematic graph depicting potential mitochondrial bottleneck and purifying selection stages in zebrafish. Grey dashed lines indicate stages/periods of oogenesis analyzed by Boke and colleagues. In their study, they observed a dramatic increase in mitochondrial DNA copy number between PGC stages and stage I oocytes, a smaller increase between stage I and stage II oocytes and mature oocytes, and a decline in embryos (black lines). Because stage I oocytes are diverse in terms of cytological stage of prophase I and phase of Balbiani body development the precise stage when the bottleneck and purifying selection, and how it relates to polarity in mitochondrial distribution in oogonia (red line), and asymmetric enrichment of Bucky ball protein (purple line) and Balbiani body formation and enrichment of mitochondria there is not known.