Developmental and diurnal changes in ecdysteroid biosynthesis by prothoracic glands of Rhodnius prolixus (Hemiptera) in vitro during the last larval instar.

The synthesis of ecdysteroids by prothoracic glands (PGs) of male last instar larvae of Rhodnius prolixus was measured in vitro by radioimmunoassay throughout the course of larval-adult development. Large and systematic changes in relative rates of synthesis occur during development. Two bursts of elevated synthetic activity were found. The first commences as soon as development is initiated by a blood meal and lasts approximately 1 day. The second commences 4 days later and increases progressively to a peak at Days 11-13 after feeding (up to 25 ng of 20-hydroxyecdysone eq. gland-1/4 hr-1). The onset of each of these bursts of activity coincides with apparent times of PG stimulation in vivo by release of the prothoracicotropic hormone from the brain. Both bursts result in increases in hemolymph ecdysteroid titer measured in the donor animals. PGs exhibit an abrupt attenuation of synthesis on Day 14, which is followed by a rapid decline in the hemolymph ecdysteroid titer. Clearly, ecdysteroid synthesis by PGs is a major factor regulating the hemolymph titer. Ecdysteroid synthesis by PGs exhibits diurnal changes in vitro. The amount of ecdysteroid synthesized by PGs from animals during the scotophase is two to five times higher than that from animals during the photophase. A corresponding rhythm is seen in the hemolymph ecdysteroid titer. The rhythm in the titer is known to be under circadian control. It is therefore suggested that ecdysteroid synthesis in PGs of Rhodnius is regulated by a circadian system, possibly located in the PGs themselves.