Molecular Identity of Maternal Regulators of the Egg to Embryo Transition

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In all animals, large-scale zygotic transcription typically initiates several cell cycles following fertilization at the maternal-zygotic transition (MZT). Prior to wholesale zygotic transcription initiation, the embryo depends on maternal factors in the egg to drive its early development. Hence, eggs are endowed with a multitude of factors critical for fertilization, egg activation, early cell cycles, and other processes acting prior to zygotic genome activation. Although essential to reproduction, these pre-MZT processes have been little studied. A unique collection of maternal-effect mutants in the zebrafish with pre-MZT defects will be investigated here and are expected to reveal new molecular players in these processes. Because the egg supplies all the factors needed by the embryo for its early development, eggs are very large. Little is known about how the enormous cytoplasm of these large embryonic cells, full of components needed for several cell divisions, is restricted in its assembly of the stored components to each ensuing cell division. Two maternal-effect mutant genes that restrict cytoskeletal assembly in the egg will be studied and the molecular nature of the genes determined. These are expected to be novel repressors of cytoskeletal assembly that function specifically in the egg to regulate the maternal supply of these factors. Several additional maternal mutant genes that affect specific aspects of egg activation and the initiation of cell cycles will also be studied. Based on the chromosomal locations of the mutant genes, novel genes or previously known genes with new, unexpected functions are expected to be identified. The molecular identity of the genes will be determined through an RNA-Seq approach, followed by CRISPR-Cas9 genome editing to verify the identities of the genes. Future studies will then integrate these new factors into the known molecular framework for each particular maternally controlled process, which is expected to fill gaps and/or generate new molecular entry points for future investigation.