The role of the corpora allata and associated nerves in the regulation of ovarian cycles in the oviparous cockroach Periplaneta americana

Abstract

Sexually mature females of the oviparous cockroach P. americana take an average of 4.17 days to complete each gonotrophic cycle. Neither sham operations on the corpora allata nor surgical sectioning of the paired nervi corporis allati I (NCA-1) or II (NCA-2) had any obvious effect on this mean cycle length. Total denervation of the corpora allata did bring about a slight lengthening of the mean cycle duration (to 5.04±0.30 days) but each of these operated insects was able nevertheless to produce at least five successive oöthecae. Surgical removal of a single corpus allatum (unilateral allatectomy) was likewise seen to slow down the rate of oötheca production. The mean cycle duration in these insects being 5.21±0.10 days. Total allatectomy abolished oötheca production completely. Measurements of juvenile hormone biosynthesis by freshly isolated corpora allata indicate that a single corpus allatum can compensate for the absence of its missing partner. Parallel experiments reveal that totally denervated corpora allata retain the ability to synthesise juvenile hormone for a considerable time after surgery. A single topical dose (100 μg) of the non-terpenoid juvenile hormone analogue fenoxycarb, in conjunction with a diet impregnated with the same compound at 10 or 100 ppm, was easily sufficient to restore oötheca production in long-term allatectomized females. The mean cycle duration in these replacement-therapy insects was 3.25±0.19 days which was significantly shorter than the interval between successive oöthecae in the control group. Topical application and feeding of fenoxycarb to otherwise intact females greatly enhanced the speed at which gonotrophic cycles were completed. The mean cycle length in this latter group was 2.69±0.05 days. It appears that the corpora allata of P. americana have a considerable capacity to function in the absence of nervous connections. Surgical operations which may reduce endogenous juvenile hormone titre have the effect of merely slowing down the maturation of oöcytes. They do not abolish the process altogether. In contrast to this an artificial elevation of juvenoid titre greatly enhances the rate of oötheca production.. Multiple gonotrophic cycles can even be induced in the total absence of fluctuating titres of endogenous juvenile hormone. It seems conceivable, therefore, that neither the build-up phase nor fall-off phase of corpus allatum biosynthetic activity are the important regulatory signals in the induction and modulation of vitellogenesis in successive batches of oöcytes in P. americana. It is rather as though the corpora allata are acting instead like a variable throttle which serves either to speed up or to slow down an endogenous “clock-like” system. This they do by merely altering the total amount of juvenile hormone which is produced over a somewhat longer period. The more hormone there is available the faster will be the process of oöcyte maturation.