CONTROL OF PUPAL COMMITMENT IN THE IMAGINAL DISKS OF PRECIS-COENIA (LEPIDOPTERA, NYMPHALIDAE)

When final (5th) instar larvae of Precis coenia were treated with the juvenile hormone analog (JHA) methoprene, they underwent a supernumera ry larval molt, except for certain regions of their imaginal disks, wh ich deposited a normal pupal cuticle. Evidently those regions had alre ady become irreversibly committed to pupal development at the time JHA was applied. By applying JHA at successively later times in the insta r, the progression of pupal commitment could be studied. Pupal commitm ent in the proboscis, antenna, eye, leg and wing imaginal disks occurr ed in disk-specific patterns. In each imaginal disk there were distinc t initiation sites where pupal commitment began during the first few h ours of the final larval instar, and from which commitment spread acro ss the remainder of the disk over a 2- to 3-day period. The initiation sites were not always located in homologous regions of the various di sks. As a rule, pupal commitment also spread from imaginal disk tissue to surrounding epidermal tissue. The regions of pupal commitment in a ll disks except those of the wings, coincided with the regions of grow th of the disk. Only portions of the disk that had undergone cell divi sion and growth underwent pupal commitment. Shortening the growth peri od did not prevent pupal commitment in the wing imaginal disk, indicat ing that, in this disk at least, a normal number of cell divisions was not crucial in reprogramming of disk cells for pupal cuticle synthesi s. The apparent growth spurt of imaginal disks that occurs during the last part of the final larval instar is merely the final stage of norm al and constant exponential growth. Juvenile hormone (JH) and ecdyster oids appeared to play little role in the regulation of normal imaginal disk growth. Instead, growth of the disks may he under intrinsic cont rol. Interestingly, even though endogenous fluctuation in JH titers do not affect imaginal disk growth, exogenous JHA proved able to inhibit both pupal commitment, cell movement, and growth of the disks during the last larval instar. This function of JH could be important under c ertain adverse conditions, such as when metamorphosis is delayed in fa vor of a supernumerary larval molt. (C) 1998 Elsevier Science Ltd. All rights reserved.