Maternal factors are essential for early animal development, including fish species. The teleost fish Danio rerio, commonly known as zebrafish, has become a valuable model system to study fish (and vertebrate) biological processes due to its potential to combine genetic, embryological and molecular methods. This chapter attempts to summarize current knowledge on the role of maternal factors in this organism, although studies on the subject carried out in other fish species, when available, are also included.

As the name implies, maternal genes are defined as those required in the adult female for the development of the offspring. This dependency on maternal genes derives from the fact that the earliest steps in embryonic development are driven by factors produced during oogenesis, which are stored in the form of mRNA, protein, or any other biomolecule. Upon egg activation and fertilization, such factors become usable for embryogenesis, sometimes after a process of activation involving translation or protein modification. Fish, as most other animals, have a substantial period of time in embryonic development which occurs prior to the activation of the zygote's own genes. The initiation of the zygotic gene program normally occurs during the blastula stages, at the so-called midblastula transition (MBT), and it appears to be precisely a new set of (zygotically derived) regulators that allow the embryo to proceed to later stages and undergo the cell-specific programs characteristic of gastrula-tion. Prior to the MBT, the entire developmental program depends solely on maternal products. The MBT constitutes a period of time in which the importance of maternal products gradually subsides and zygotic gene products become the primary factors driving development. However, a role for maternal products, often in conjunction with zygotic products derived from the same gene, has also been demonstrated in the zebrafish for a variety of developmental processes that occur after the MBT.

Much of this chapter relates information we have learned from the analysis of genetic mutants affecting early development. The function of such genes can be deduced when the effect (phenotype) in the offspring depends on the genetic constitution (genotype) of the mother. In the case of strictly maternal-effect genes, the phenotype depends solely on the genotype of the mother and is independent of the genotype of the embryo itself. In the case of maternal-zygotic genes, the function of the gene depends on gene products derived both maternally and zygotically. In this case, the phenotype of the embryo depends on the genotype of both the mother and the embryo. For simplicity, we refer to the affected embryos, which are derived from adult females that are homozygous mutant (and, in the case of maternal-zygotic genes, also homozygous mutant in the zygote), as mutant embryos.

In many cases the function of maternal factors in the developing embryo depends on a precise plan of storage and localization that is initiated during oogenesis. Therefore, we will also discuss maternally driven processes required for embryogenesis in the context of their expression during oogenesis.

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