Control of ovarian steroidogenesis in insects: A locust neurohormone is active in vitro on blowfly ovaries

Ovarian steroidogenesis controlling insect reproduction is mainly regulated by brain gonadotropins liberated from corpora cardiaca (CC). Till now, different neurohormones have been identified in two insect groups only, locusts and mosquitoes, and it is unknown whether they could be active in other insects. In order to complete previous observations on the control of ovarian steroidogenesis in the blowfly, Phormia regina, we examined whether neuropeptides isolated from locust CC have an effect in vitro on ovarian steroidogenesis in our dipteran model. Our experiments showed that crude extracts from locust CC efficiently stimulated steroidogenesis in blowfly isolated previtellogenic ovaries. However, such an activity was observed neither with authenticated neuroparsins (NPs), the putative homologs; of the ovarian ecdysteroidogenic hormone of mosquitoes, nor with ovarian maturing peptide (OMP), the putative locust steroidogenic neurohormone. Partial purifications of CC extracts were then performed using methanol and/or acidic ethanol extractions followed by reverse phase HPLC and collected fractions were assayed in vitro. A significant steroidogenic activity was found in a single group of acidic fractions, well separated from OMP and NPs, which was associated to slight but significant anti-insulin immunoreactivity. In conclusion, a locust CC neurohormone, different from NPs and OMP, is able to stimulate ecdysteroidogenesis in blowfly ovaries. Though this active factor has not been fully characterized, its behavior during extraction or HPLC and its immunoreactivity strongly suggest it could be an insulin-like peptide. This is in agreement with previous studies demonstrating the role of such peptides as steroidogenic gonadotropins in blowflies and several other insects. (C) 2009 Elsevier Inc. All rights reserved.