Juvenile hormone controls the onset of pupal commitment in the imaginal disks and epidermis of Precis coenia (Lepidoptera: Nymphalidae)

Abstract

The commitment of tissues to pupal development occurs early during the last larval instar of many holometabolous insects, usually at least several days before pupal differentiation begins in response to the moulting hormone, 20-hydroxyecdysone. Pupal commitment of the imaginal disks and epidermis of the butterfly, Precis coenia, was studied by two approaches: treatment with juvenile hormone or a juvenile hormone mimic to prevent or delay metamorphosis, and treatment with 20-hydroxyecdysone to induce premature metamorphosis. With both treatments, tissues already committed to pupal development at the time of treatment responded by producing pupal cuticle at the next moult, while tissues which were larval in commitment produced larval cuticle. The same temporal and spatial sequence of pupal commitment was observed by both methods, and pre-treatment with juvenile hormone did not alter the results of 20-hydroxyecdysone treatment. Dosage of the juvenile hormone mimic, as long as it was above threshold, did not affect the degree of pupal commitment, which instead was dependent on the age of the animal alone. Paradoxically, the sensitivity of larvae to the juvenile hormone mimic increased with age, even though a greater proportion of the tissues of older animals were already pupally committed, and hence juvenile hormone-insensitive. We suggest that the onset of pupal commitment is determined by the decline of the juvenile hormone titre: once underway, pupal commitment spreads from tissue to tissue in an intrinsic, programmed manner.