Vitellogenesis in the terrestrial isopod, Oniscus asellus (L.): Characterization of vitellins and vitellogenins and changes in their synthesis throughout the intermoult cycle

摘要

Egg vitellin of the terrestrial isopod Oniscus asselus is a 290 kD, female-specific lipoglyco-protein (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 matureeggs. Vitellogenin (VG) is electrophoretically 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 both fat body andovaries, but by no other tissues of either females or males. Oniscus VG, therefore, has both ultra- 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, following short-term incorporation, larger polypeptides are present in these tissues, which are also recognized by an antiserum raised against V_1. It is speculated that VG in Oniscus is synthesized as a large polypeptide which is subsequently processed to smaller fragments. Synthesis of VG by explants of fat body was studied throughout the intermoult cycle. A single large peak of VG synthesis was seen in the second half of premou during this time, VG comprises 40% of the total protein synthesized by fat body and oocytes increase in volume 100-fold. The chronology 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 neuro-peptide(s) from the sinus gland.