Factors responsible for the initiation of a second oöcyte maturation cycle in the ovoviviparous cockroach Nauphoeta cinerea

The factors responsible for the initiation of a second oocyte maturation cycle were investigated by measuring oocyte growth, vitellogenin titre, and corpus allatum activity after injection of juvenile hormone and/or removal of the egg-case from pregnant females and by performing ovary and corpus allatum transplant experiments. Egg-case removal in late pregnancy results in immediate oocyte growth, whereas in early pregnancy oocyte growth is resumed only after a lapse of time, even after injection of juvenile hormone. This, however, induces an immediate increase in the haemolymph vitellogenin titre. A single injection of 2 or 10 μg of juvenile hormone II first stimulates some oocyte growth after this lapse of time and later activates the corpora allata, which in turn leads to completion of oocyte maturation. A repeat injection of 10 μg stimulates continuous oocyte growth without activating the corpora allata. In the presence of an egg case, activation of the corpora allata is suppressed, even after injection of 2 μg of juvenile hormone III, and the oocytes do not grow. Injection of higher doses stimulates oocyte growth and leads to expulsion of the egg case in up to 95% of the females. This, however, is not a direct consequence of the increase in size of the ovaries. Ovary transplant experiment show that in young pregnant females the second generation of oocyte is not yet competent for growth and that ovaries which are competent can mature in young pregnant females, treated with juvenile hormone, whose egg case has been removed. The results are summarized in a model demonstrating the various factors involved in regulating corpus allatum activity in oocyte maturation and pregnancy and after application of juvenile hormone. We prepose that the corpus allatum activating effect of exogenous juvenile hormone is mediated by the growing oocyte and that this activation can be suppressed by the continuous presence of exogenous juvenile hormone.