Maternal investment in fish oocytes and eggs: The molecular cargo and its contributions to fertility and early development

The production of fertile eggs with the capacity to develop into larvae and subsequently into marketable fish is centrally important to the aquaculture industry. This entails not only the programmed production of large numbers of eggs, but also high quality eggs with the potential to support normal development and high survival of offspring to juvenile and later stages of development and growth. Numerous studies highlight the maternal contributions to the development of embryos, including transcripts that regulate cell division and determine oocyte polarity, pattern development during early and late embryonic stages and the transition from maternal to zygotic gene expression and translation. Since most fish embryos develop independently within an enclosed egg envelope, they rely on compounds deposited within the oocytes during their various stages of development. In addition to regulatory nucleic acids (maternal DNA and RNA), these include proteins and other compounds that contribute to the structure and function of the egg envelope and the bulk molecular cargo that will be used as a source of cellular energy and structural components for formation of embryos and larvae. These latter components notably include yolk lipids and proteins deposited during oocyte growth and water acquired at the same time and during cytoplasmic maturation. In this review we highlight recent advances made in revealing the transcripts deposited within the oocyte that contribute to the structural and morphological development of the embryo, and to the regulation of gene expression and translation during oocyte development. Significant advances have been made in revealing the molecular mechanisms of lipid accumulation and metabolism within the oocyte, the intricacies of yolk protein formation via endocytosis of multiple yolk precursor proteins by multiple oocyte receptors, and the complex machinery supporting massive accumulation of water by maturing oocytes of many species. Additionally, many advances have been made in our understanding of the endocrine regulation of all of these processes during oogenesis. We provide here an overview of recent advances in our knowledge on these various aspects of oogenesis and identify several gaps in our knowledge for future studies.