Ecdysteroid activity during oögenesis in the black blowfly, Phormia regina (Meigen)

Abstract

Haemolymph and ovarian ecdysteroid titres were measured, using a radioimmunoassay, for adult Phormia regina in relation to their first gonotrophic cycle. Immediately after adult emergence there was about 5.5 pg of ecdysteroids in each microlitre of haemolymph. This amount declined to non-detectable levels at 24 h after eclosion and remained non-detectable if flies were fed only sugar and water. Haemolymph ecdysteroid titre increased rapidly after a replete meal of liver (provided at 72 h after eclosion) and reached a maximum of 27.3 pg/μl, 32 h after the liver meal (i.e. 104 h after eclosion). In ovariectomized, liver-fed females, the haemolymph ecdysteroid titre was greatly reduced but was maintained at detectable levels indicating that the ovaries are the major, but not the only, source of dietary-induced ecdysteroids. A very small quantity of vitellogenin was also detected in the haemolymph of ovariectomized females after they fed on liver, indicating that ecdysteroids from extra-ovarian sources could initiate the synthesis of vitellogenin. If flies were treated with precocene II (20 μg/fly) at < 12 h of adult life and then allowed to feed on beef liver at 72 h of age, the liver meal failed to support increases in both the haemolymph ecdysteroid titre and the vitellogenin titre to the comparable levels attained by control flies. It is apparent that, at the dose tested, precocene II suppressed but did not stop ovarian ecdysteroid biosynthesis. The lowered ecdysteroid production may then slow down the vitellogenin biosynthesis. Ovaries of precocene II-treated flies remained undeveloped and contained ecdysteroids at barely detectable levels. A working model for the regulation of oögenesis is presented.