ORGANIZATION OF THE ZONA PELLUCIDA ENVELOPE IN THE GILTHEAD SEABREAM (Sparus aurata)

Abstract

We have identified four eggshell proteins, or zona pellucida proteins (ZP), in the marine fish gilthead seabream (GSB), the ZPBa, ZPBb, ZPX, and ZPC. They are expressed in both the liver and the oocytes and the synthesis is 17b-estradiol regulated. ZPX is not found in mammals but show high homology to the mammalian ZPA when comparing amino acid regions believed to be important for three-dimensional structure, and assembly properties of eggshell proteins. During final oocyte maturation the oocytes from GSB are highly hydrated and therefore enlarged in size. This expansion requires that the eggshell is able to enlarge or change structure during maturation. Electron microscopy studies of developing and mature oocytes show structural differences of the eggshell at the different maturation stages. A striped appearance of the eggshell after oocyte hydration may indicate that the eggshell has elastic properties. Staining with fluorescent antibodies directed against the four known zona pellucida proteins show isoform differences in abundance and localization in the envelope. In stage III oocytes, before hydration, none of the four ZP proteins were present through the entire eggshell layer. The eggshell could be divided into three different layers with different ZP isoform composition. ZPX was the only protein found in the region proximal to the plasma membrane. The intermediate region of the eggshell was composed of ZPBa, ZPBb and ZPC, and the outer layer is made up of the ZPC. While all four isoforms are present and synthesized in the liver, isoform ZPX and ZPBa were also found to be present in the oocytes prior to stage III. The present study show that the gilthead seabream zona pellucida proteins are synthesized in different organs and thereafter unevenly distributed in the developing oocyte eggshell. (poster)