Vitellogenesis in the Northern octopus, Eledone cirrhosa
The Northern octopus, Eledone cirrhosa, has a lifespan of approximately two years, breeding once in its second year. The ovary of E. cirrhosa ranges from around 0.15% of the body weight of immature animals to around 20% at full maturity. This hundredfold increase in ovary weight is due mostly to the accumulation of yolk in the oocytes, i.e. to vitellogenesis. Gradient PAGE techniques have identified a total of five yolk lipophosphoglycoproteins (designated YP 1-5) at various stages of egg maturation. YP-4 is the first to appear at the onset of vitellogenesis and this remains quantitatively the major egg protein throughout maturity. Fully mature eggs contain YPs 1-4, which have increased slightly in molecular weight during maturation, but YP-5 disappears at this stage. The yolk proteins of this cephalopod species are of unusually high molecular weight (625,000-1,230,000) and are also highly soluble relative to most other types of animal yolk previously studied. In the later stages of vitellogenesis glycoproteins of similar molecular weight to yolk proteins 1,2 and 4 have been identified in the digestive gland. However, no putative vitellogenins could be identified in the haemolymph using the same electrophoretic technique, possibly due to masking by the highly abundant, 'band 10' protein identified by various methods as haemocyanin. A tissue culture system was developed in which rates of protein synthesis in the oocyte and surrounding follicular cell layer could be measured. No stimulatory effect of optic gland extracts on these rates could be detected in vitellogenic eggs and thus no direct endocrine control of vitellogenesis by this tissue was demonstrated. Stimulation in vitro of protein synthesis by optic glands was, however, detected in previtellogenic eggs. The implications of these results, contradictory to those of previous authors, are discussed. Various vertebrate sex steroids, corticosteroids and insect hormones had no effect on protein synthetic rate, except dexamethasone (10nMolar), which depressed synthesis by 38%.