Louis Safranek

EFFECTS OF JUVENILE HORMONE ON ECDYSONE-DEPENDENT DEVELOPMENT IN THE TOBACCO HORNWORM, MANDUCA SEXTA

1. Juvenile hormone delayed or prevented the onset of metamorphosis by neck-ligated fourth-instar tobacco hornworm larvae and by fifth instars neck-ligated prior to the cessation of feeding.2. After the onset of the wandering period J H had precisely the opposite effects in that it accelerated the onset of metamorphosis. This was the case both for wandering larvae and for pupae, irrespective of whether they were intact or subjected to brain removal or neck ligation.3. These findings point to a previously unsuspected shifting role of JH in the control of metamorphosis.4. Fourth-instar larvae underwent no further development after brain removal unless they were also effectively allatectomized by neck ligation. Evidently the secretion and inhibitory action of JH persists in brainless fourth-instar larvae.5. Brainless fifths, by contrast, were often able to initiate metamorphosis despite the continued presence of their CA. Development could be prevented by daily application of JH. The disappearance of effecti...

Hormonal control of cuticle coloration in the tobacco hornworm, Manduca sexta: Basis of an ultrasensitive bioassay for juvenile hormone

Juvenile hormone inhibits cuticular melanization in tobacco hornworm larvae. This action of juvenile hormone was used as the basis of a new bioassay which is sensitive to less than 0·01 ng (3 × 10−14 moles) of topically applied hormone and requires only 2 days.

The biology of the black larval mutant of the tobacco hornworm, Manduca sexta

Abstract A mutant of the tobacco hornworm, Manduca sexta, was found to form black melanized cuticle in the last larval instar. This black phenotype is due to a single sex-linked gene whose expression can be changed by one or more modifier genes. The expression of the mutant phenotype is prevented by juvenile hormone (JH) application at the time of head cap apolysis during the moulting cycle to the last larval instar. The bl mutant is equally as sensitive to JH at this time as is a neck-ligated wild type larva, ruling out an absence of hormone receptors or a difference in JH metabolism. The bl corpora allata were found to be less active at this time than were those of the wild type larva, suggesting that the defect resides in the control of the corpora allata. Since selection for complete expression of the bl phenotype is easy, this mutant provides the basis for an ultrasensitive JH bioassay to be described in a forthcoming paper.

STUDIES OF THE PROTHORACICOTROPIC HORMONE IN THE TOBACCO HORNWORM, MANDUCA SEXTA

1. Brain removal at appropriate times in the life cycle uniformly delayed or prevented the onset of all known ecdysone-mediated transitions. This effect was especially pronounced at larval-larval molts and at the pupal-adult transformation.2. Loose brains reduced but did not eliminate the delay induced by brain removal.3. Isolated larval abdomens never developed nor were they induced to do so by implantation of an active brain.4. Loose pupal brains retained their original commitment to diapause or not to diapause. This was true even when the loose brain was implanted into a brainless host pupa with an opposite diapause commitment.5. Larval brains effectively elicited the development of pupae, and conversely, pupal brains elicited the development of larvae.6. Prothoracicotropic activity was found in the brain but in no other ganglia.7. Extracts of Bombyx mori PTTH were inactive in Manduca in the concentrations tested.8. Consideration is given to several mechanisms which might mediate the development of bra...

CRITICAL WEIGHTS FOR METAMORPHOSIS IN THE TOBACCO HORNWORM, MANDUCA SEXTA

Fifth-instar larvae of the tobacco hornworm underwent a supernumerary larval molt under several feeding regimens. Supernumerary molting occurred only when the molt was initiated at a live weight less than about 3 g; at higher weights pupation took place. The supernumerary sixth instars were normal in appearance and behavior and went on to form normal pupae only when the fifth instar had begun at weights less than 1 g. Otherwise the supernumerary sixth instars exhibited precocious metamorphosis of their crochets and imaginal discs and failed to undergo normal pupation. Thus, the commitment to pupal development appears to occur in stages. A first stage, initiated at a weight of about 1 g, commits the crochets and imaginal discs to metamorphosis at the next molt. A second stage, entered at a weight of about 3 g, permits the full and complete pupation of the remaining tissues; this transition probably reflects the elimination of juvenile hormone (JH). Starvation apparently alters the normal patterns of ecdysone production, breakdown, or sensitivity so as to elicit ecdysone-dependent development at lower than normal weights. When starvation results in fifth-instar larvae initiating a developmental response at weights below 3 g, a supernumerary larval molt occurs. But in this case the critical effect of starvation is on the timing of the molt rather than on the JH titer, since the latter even in normally fed early fifth instars is high at weights below 3 g.

DETERMINANTS OF LARVAL MOLT INITIATION IN THE TOBACCO HORNWORM, MANDUCA SEXTA

Hornworm larvae adhere to Dyars rule under normal growth conditions, increasing their live weight by an average of 5- to 6-fold from the outset of one instar to the next. This adherence to Dyars rule is largely maintained even in instars subsequent to those in which larvae have been severely malnourished. The importance of relative weight gain for the normal onset of larval molting contrasts with the requirement for attainment of an absolute body weight prior to pupation. But body size is by no means the sole cue for the initiation of a larval molt. Thus, under conditions of malnutrition larvae can initiate a molt at any weight and can do so even in the absence of any weight gain in an instar: in these circumstances the duration of an instar is inversely related to the weight at the outset of malnutrition. In larvae fed ad lib as well as in malnourished larvae the initiation of a molt appears to be limited to a discrete phase of the photocycle. The failure of larvae to molt on schedule at body weights lower than those predicted by Dyars rule cannot be attributed to an inability of the brain to stimulate ecdysone-dependent development as is the case in diapausing pupae. Though body size, instar duration, and photocycle all interact to determine the onset of the molt, the role of the brain remains ill-defined.

Precocious termination of diapause in neck- and abdomen-ligated pupal preparations of the tobacco hornworm, Manduca sexta

When ligatures were placed between the head and thorax of freshly pupated hornworms, the resulting brainless preparations initiated adult development weeks earlier than intact diapausing controls and months earlier than similar preparations from which the brain had been surgically extirpated. This phenomenon could not be reproduced by removal of any single recognized cephalic neural or endocrine organ or any combination of organs. A similar accelerated development took place in many isolated pupal abdomens prepared by ligature or surgical section between the thorax and abdomen.Extirpation of the prothoracic glands of either diapause or non-diapause pupae resulted in only a very slight delay in the onset of adult development. Nevertheless, this development remained highly dependent on the brain, since preparations lacking brains as well as prothoracic glands underwent a prolonged developmental arrest. Preparations lacking prothoracic glands demonstrated elevated levels of ecdysone at the outset of adult de...

Inactivation of the Corpora Allata in the Final Instar of the Tobacco Hornworm, Manduca sexta, Requires Integrity of Certain Neural Pathways from the Brain

Neither the implantation of active CA nor treatment with O-ethyl,S-phenylphosphoramidothiolate (EPPAT), a potent inhibitor of the juvenile hormone esterase (JHE), prevented metamorphosis of final instar tobacco hornworms. However, a combination of the two treatments often blocked metamorphosis and caused the formation of supernumerary larvae or larval-pupal intermediates. So also, in conjunction with EPPAT treatment, unilateral severance of the medial nerve from the brain to the corpus cardiacum-corpus allatum complex often resulted in abnormal supernumerary or intermediate larval forms. Thus, clearance of JH from mature hornworm larvae prior to metamorphosis appears normally to depend on two mechanisms: (1) cessation of JH production by inhibition of the CA via the innervation of these glands, and (2) destruction of previously secreted existing JH via production of JHE. In the present experiments, each of these mechanisms appeared fully able to clear JH sufficiently to permit normal metamorphosis, becaus...