Gamete physiology, fertilization and egg activation in teleost fish
Coward K., Bromage NR., Hibbitt O., Parrington J.
The fertilization and activation of fish oocytes are vital, but unfortunately overlooked, processes in fisheries research. This paper sets out to review our present understanding of these important events in teleost fish and, drawing comparisons with mammalian research, to highlight areas in which research effort is urgently required. Presently, the commercial culture of many important freshwater, but especially marine, teleosts is beset by problems associated with fertilization, hatching and early embryonic development. These problems have been particularly acute in certain species leading to the application of spawning induction technologies in an effort to optimize production. Increased knowledge of the processes of egg activation and fertilization in these groups of fish is likely to make significant contribution to commercial aquaculture. Studies of a wide variety of animal and plant species has demonstrated that development at fertilization is triggered by an increase in intracellular Ca2+ concentration within the egg that occurs as either a single transient or a series of distinctive oscillations depending upon the species under investigation. This increase in intracellular Ca2+ activates the egg and also appears to play an important role in later embryonic development. Teleost reproductive strategies and more importantly, teleost oocytes and spermatozoa, exhibit a remarkable variety of adaptations. Currently, studies of egg activation in teleosts are confined to laboratory species such as medaka Oryzias latipes and zebrafish Brachydanio rerio. Nevertheless, even between these two species, although an increase in intracellular Ca2+ appears to be the trigger in both cases, the mechanism of Ca2+ release may be quite different. Activation in medaka is initiated only through direct contact with conspecific sperm, suggesting the involvement of a sperm-specific factor, while zebrafish eggs appear to require only contact with the external spawning medium. In view of the highly variable fertility rates evident in many commercially cultured teleosts, it could be very rewarding to investigate the mechanism of egg activation in representative teleost groups using the findings and theories emerging from other animal groups as a starting point. In order to successfully conduct such an investigation, it will be necessary to employ a combination of physiological, molecular and recombinant approaches.