Maternal regulation of the vertebrate oocyte-to-embryo transition

Maternally-loaded factors in the egg accumulate during oogenesis and are essential for the acquisition of oocyte and egg developmental competence to ensure the production of viable embryos. However, their molecular nature and functional importance remain poorly understood. Here, we present a collection of 9 recessive maternal-effect mutants identified in a zebrafish forward genetic screen that reveal unique molecular insights into the mechanisms controlling the vertebrate oocyte-to-embryo transition. Three genes, over easy, p33bjta, poached and black caviar, were found to control initial steps in yolk globule sizing and protein cleavage during oocyte maturation that act independently of nuclear maturation. The krang, kazukuram, p28tabj, and spotty genes play distinct roles in egg activation, including cortical granule biology, cytoplasmic segregation, the regulation of microtubule organizing center assembly and microtubule nucleation, and establishing the basic body plan. Furthermore, we cloned two of the mutant genes, identifying the over easy gene as a subunit of the Adaptor Protein complex 5, Ap5m1, which implicates it in regulating intracellular trafficking and yolk vesicle formation. The novel maternal protein Krang/Kiaa0513, highly conserved in metazoans, was discovered and linked to the function of cortical granules during egg activation. These mutant genes represent novel genetic entry points to decipher the molecular mechanisms functioning in the oocyte-to-embryo transition, fertility, and human disease. Additionally, our genetic adult screen not only contributes to the existing knowledge in the field but also sets the basis for future investigations. Thus, the identified maternal genes represent key players in the coordination and execution of events prior to fertilization. The oocyte-to-embryo transition consists of the coordinated regulation of multiple molecular processes acting in the late oocyte, egg and early zygote. This transcriptionally silent period requires the precisely timed function of maternally-supplied gene products during oogenesis. However, knowledge of their molecular nature and in vivo function remains incomplete. Importantly, defects associated with the oocyte-to-embryo transition in humans cause reduced female reproductive success, in addition to a possible increased incidence of miscarriage. The mutants reported here provide access to maternal factors regulating the processes that prepare an oocyte for reproductive competence and embryogenesis. We have identified essential regulators of yolk granule formation, cortical granule biology, and microtubule organizing assembly control. Specifically, we found that the highly conserved maternal Ap5m1 protein regulates yolk granule maturation, which generate essential nutrients and immunity for growth and development in oviparous animals. We characterized the Krang factor, which regulates aspects of egg activation likely by modulating the secretory pathway. The mutants presented here represent attractive genetic models to investigate the molecular and cell biological mechanisms that control the oocyte-to-embryo transition, as well as reveal a collection of genetic factors indispensable for reproduction and survival. Importantly, knowledge of their genetic underpinnings and biological importance in reproduction will also pave the way to investigate the evolution of maternal genes during vertebrate development.