Molecular components related to egg viability in the gilthead sea bream, Sparus aurata

In pelagic egg spawners, the production of large numbers of sinking eggs, u nable to develop into embryos, represents one of the major limiting factors in controlled reproduction. The aim of this study is to elucidate the mole cular differences between floating and nonfloating eggs at cytoplasmic and nuclear level. Comparison of analyses between floating and nonfloating sea bream Sparus aurata eggs evidenced differences in vitelline envelope protei n components, such differences being probably related with the hydration pr ocess but not with fertilization as supported by the assessment of DNA that doubled after in vitro insemination. These data clearly indicated that the absence of embryo development in nonfloating eggs is not due to lack of fe rtilization. The cytoplasmic composition was also different, the number of protein components being higher in floating eggs, and these extra component s may generate the appropriate osmotic pressure at the base of the hydratio n process. Some lysosomal enzymes, such as cathepsin D and L both involved in yolk proteolysis, in virgin nonfloating eggs were significantly higher w ith respect to floating ones; the levels of these two enzymes significantly increased in the latter after fertilization. On the contrary, in nonfloati ng eggs cathepsin L significantly decreased after fertilization. These chan ges may be related with a series of metabolic processes vital for the produ ction of viable offspring. The capacity of egg transcription and the protei n synthesis in these two types of eggs, indicated by the RNA/DNA and RNA/pr otein ratios, evidenced that the status of cell transcription rate and prot ein synthesis capacity is significantly higher in floating eggs. This, in t urn, suggested that the lack of embryo development may be due to low levels of proteins involved in cell cycle regulation. (C) 2001 Wiley-Liss, Inc.