Maternal Factors Regulate Embryonic Development in Zebrafish

During fertilization in animals, a sperm launches its pronucleus into the egg. The zygotic diploid nucleus is formed by fusion of male and female pronuclei, however, the zygotic genome is transcriptionally inactive during the first few mitotic divisions. A logic speculation is that early embryonic development relies on maternal factors such as RNA and proteins that are supplied by the egg. Forward genetic studies in invertebrates, mainly performed in the 1980s, uncover essential roles of maternal factors in determining the oocyte polarities, the embryonic axes and pattern formation. In the past two decades, analyses of maternal mutants in zebrafish and mice also reveal important functions of maternal factors in embryonic development. This review mainly focuses on identified functions of maternal factors in developmental processes of the zebrafish embryo, including oocyte maturation, egg activation, blastomere cleavage, clearance of maternal mRNAs, zygotic genome activation as well as germ layer induction and patterning and body axis formation. The findings in model animals will provide a base for studying mechanisms of human infertility, miscarriage and birth defects.