EFFECT OF COBALT AND OTHER DIVALENT-CATIONS ON OOCYTE DEVELOPMENT OF THE BLACK BLOWFLY, PHORMIA-REGINA (MEIGEN)

Results showed that injections of the divalent cation cobalt, a calcium channel blocker, at effective haemolymph concentrations of 2.1, 3.4 or 5.0 mM, immediately prior to liver feeding, retarded or inhibited egg development by influencing vitellogenin uptake in Phormia regina. Topical application of methoprene (a juvenile hormone analogue) to cobalt-injected flies partially restored the otherwise inhibited egg development. Since vitellogenin uptake in this fly is controlled by juvenile hormone, cobalt may affect oocyte development at least partly through its effect on juvenile hormone biosynthesis. Rates of juvenile hormone biosynthesis by the corpus allatum, in vitro, in cobalt-injected flies fell into three levels of 0.48 +/- 0.01, 0.72 +/- 0.12 and 0.97 +/- 0.08 pmol/gland pair/h. These three levels of rates of juvenile hormone biosynthesis corresponded well with the three categories of ovary development observed in cobalt-injected flies: undeveloped, partially developed and fully developed. In comparison, the rates of juvenile hormone biosynthesis were 0.03 +/- 0.00, 1.16 +/- 0.08 and 1.23 +/- 0.10 pmol/gland pair/h for sugar-fed, liver-fed, and liver-fed but water-injected control flies, respectively. Effects or lack of effects of several different calcium-replacement divalent cations and a dihydropyridine calcium channel blocker (i.e. nifedipine) on oocyte development in P. regina suggested that calcium channels of the corpora allata may be involved in the regulation of juvenile hormone biosynthesis, which then affected oocyte development.