Ecdysteroids drive the uptake of yolk proteins by oocytes from Drosophila melanogaster.

David S. Richard, Noel L. Watkins, Robert B. Serafin, Jennifer M. Jones, Debra M. Hollinshead. Department of Biology, Susquehanna University, Selinsgrove, PA 17870. USA.

Juvenile hormones (JHs) are thought to drive ovarian yolk protein uptake in Drosophila melanogaster, and ecdysteroids have been considered of secondary importance to the uptake process. Recent studies have demonstrated that the levels of JH production in a mutant stock (apterous56f) are depressed yet the flies are vitellogenic. The production of ecdysteroids by these ap56f ovaries in vitro is elevated above that of the wild-type ovaries. The incubation of wild-type ovaries in the presence of 0.1mM JHB3 increased the levels of ecdysteroid biosynthesis during the first 18 h following eclosion. Female D. melanogaster undergo a pre-vitellogenic reproductive diapause when exposed to an L12:D12 photoperiod at 11oC or lower. Diapause is characterized by very low levels of JH and ecdysteroid production. Ecdysteroid production by the ovaries, but not JH production by the corpus allatum, increased sevenfold within 12 h of a temperature upshift to 25oC and vitellogenic oocytes were noted in all females. Diapause was also terminated by the injection of 1 ug of 20-hydroxyecdysone into the abdomens of diapausing females. These results are consistent with a proposed new model for the regulation of yolk protein uptake by ovaries in which ecdysteroids, and not JHs, play the prominent role. The regulation of receptor-mediated endocytosis in YP uptake was investigated by immunofluorescence using goat anti-clathrin (bovine) primary antibodies and FITC-conjugated rabbit anti-goat second antibodies. Non-fully developed oocytes (those that have not yet sequestered a full complement of YPs from the hemolymph) display more intense clathrin immuno-staining than older fully developed oocytes. Oocytes from pre-vitellogenic, diapausing females show intense clathrin immuno-staining indicating that de novo clathrin production is not required for diapause termination and that ecdysteroids may initiate YP uptake in some alternate manner, perhaps by regulating YP-receptor levels or activities. (Supported in part by NIH GM/OD54905).