VITELLOGENESIS IN THE TERRESTRIAL ISOPOD, ONISCUS-ASELLUS (L) - CHARACTERIZATION OF VITELLINS AND VITELLOGENINS AND CHANGES IN THEIR SYNTHESIS THROUGHOUT THE INTERMOLT CYCLE

Egg vitellin of the terrestrial isopod Oniscus asselus is a 290 kD, fe male-specific lipoglycoprotein (V-1) comprising two subunits, A (98 kD ) and B (82 kD). An additional related protein (V-2, 180 kD) was found in developing oocytes but not in mature eggs. Vitellogenin (VG) is el ectrophoretically and immunologically indistinguishable to V-1 and was detected in oocytes, fat body and haemolymph of vitellogenic females. In vitro incorporation studies revealed that VG is synthesized by bot h fat body and ovaries, but by no other tissues of either females or m ales. Oniscus VG, therefore, has both intra- and extra-ovarian origins . Following long-term incorporation, the VG synthesized by either fat body or ovaries comprises only subunits A and B of V-1. However, follo wing short-term incorporation, larger polypeptides are present in thes e tissues, which are also recognized by an antiserum raised against V- 1. It is speculated that VG in Oniscus is synthesized as a large polyp eptide which is subsequently processed to smaller fragments. Synthesis of VG by explants of fat body was studied throughout the intermoult c ycle. A single large peak of VG synthesis was seen in the second half of premoult; during this time, VG comprises 40% of the total protein s ynthesized by fat body and oocytes increase in volume 100-fold. The ch ronology of VG synthesis and oocyte growth are related to known times of release of hormones in Oniscus. It is suggested that vitellogenesis is regulated by both ecdysteroids and neuropeptide(s) from the sinus gland.