Abstract
The development of last instar Heliothis virescens larvae was experimentally arrested by ligation between the head and thorax or by injection with calyx fluid from the endoparasitoid Campoletis sonorensis. Prothoracic glands were removed from these larvae at various times after treatment and the gland cells were either measured or prepared for electron microscopy. At 4 days postligation, gland cell ultrastructure was similar to that of cells from unligated larvae and cell size was not significantly reduced until 7 days after ligation. In contrast to ligated controls, cells from calyx fluid-injected larvae were significantly smaller 24 hr after injection. Significant degeneration in gland cell ultrastructure was observed 2 days after injection and by 7 days postinjection, prothoracic glands showed gross degeneration. Similarly, injection of purified polydnavirus caused gross gland degeneration by 7 days after injection. Irradiating calyx fluid abolished its ability to cause gland degeneration. Our results indicate that C. sonorensis polydnavirus stimulates prothoracic gland degeneration. This virally induced degeneration of the prothoracic glands may account for our previously reported observation that injection of calyx fluid causes a drastic reduction in hemolymph ecdysteroid titer and consequent developmental arrest.
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