Role of ecdysone, pupariation factors, and cyclic AMP in formation and tanning of the puparium of the fleshfly Sarcophaga bullata

Crosslink precursors for the dipteran puparium

During sclerotization of puparial proteins, tyrosine, lysine, and histidine were converted to highly basic aromatic metabolites. Peptides generated from the sclerotized cuticle with N-bromosuccinimide included the basic derivatives among the hydrolysis products. The absorbance maxima of the aromatic metabolites were 25 nm lower than those of the conventional tyrosyl peptides, with phenolic character poorly expressed or absent. Post-translational modification of the structural proteins preceded visual expression of tanning because aromatic conjugates also were present prior to pupariation. These results are consistent with a crosslinking mechanism favoring covalent bonding between protein chains.

Imaginal discs regulate developmental timing in Drosophila melanogaster

The regulation of body size in animals involves mechanisms that terminate growth. In holometabolous insects growth ends at the onset of metamorphosis and is contingent on their reaching a critical size in the final larval instar. Despite the importance of critical size in regulating final body size, the developmental mechanisms regulating critical size are poorly understood. Here we demonstrate that the developing adult organs, called imaginal discs, are a regulator of critical size in larval Drosophila. We show that damage to, or slow growth of, the imaginal discs is sufficient to retard metamorphosis both by increasing critical size and extending the period between attainment of critical size and metamorphosis. Nevertheless, larvae with damaged and slow growing discs metamorphose at the same size as wild-type larvae. In contrast, complete removal of all imaginal tissue has no effect on critical size. These data indicate that both attainment of critical size and the timely onset of metamorphosis are regulated by the imaginal discs in Drosophila, and suggest that the termination of growth is coordinated among growing tissues to ensure that all organs attain a characteristic final size.

Disruption of pupariation and eclosion behavior in the flesh fly, Sarcophaga bullata Parker (Diptera: Sarcophagidae), by venom from the ectoparasitic wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae)

The action of venom from the ectoparasitic wasp, Nasonia vitripennis, was monitored by examining alterations in patterned muscular movements characteristic of pupariation and eclosion behavior in the flesh fly, Sarcophaga bullata. Venom injected into larvae prior to pupariation caused a dose-dependent delay in pupariation. Eventually, such larvae did pupariate, but puparia were abnormally formed. Barographic records revealed that all elements of pupariation behavior were present in venom-injected larvae, but pupariation behavior was not well synchronized with tanning, thus implying that the venom caused disruption in the temporal organization of central motor programs. When larvae were ligated and injected with venom posterior to the ligature, no response was evident in the posterior region, suggesting that the venom does not directly stimulate muscles or neuromuscular junctions. Injection of exogenous ecdysteroid into venom-injected larvae restored some elements of pupariation behavior, consistent with ecdysone's role in stimulating the release of anterior retraction factor and puparium tanning factor, two factors that are released from the CNS to regulate pupariation. When the venom was injected into newly emerged imagoes, the duration of extrication behavior was shortened, whereas all phases of post-eclosion behavior were lengthened. These observations imply that the venom affects CNS centers that regulate the muscular systems engaged in extrication and post-eclosion behavior.

Mode of action of an insect neuropeptide leucopyrokinin (LPK) on pupariation in fleshfly (Sarcophaga bullata) larvae (Diptera: Sarcophagidae)

An insect neuropeptide leucopyrokinin (LPK) (pQTSFTPRLamide) accelerates pupariation in wandering larvae of the fleshfly Sarcophaga bullata. The period of sensitivity to the action of LPK begins approximately 4 h before pupariation. Within this period the degree of acceleration of contraction into the shape of a puparium is practically independent of the age at which the larvae are injected, while acceleration of tanning is distinctly more age dependent. From ligation experiments we conclude that intact central innervation is essential for the action of LPK on puparial contraction, whereas central neurones take no part in mediation of LPK action on tanning of the cuticle. An analysis of tensiometric recordings of muscular activity revealed that the actual time of LPK accelerated puparial contraction coincides with the beginning of the immobilisation/retraction phase. LPK accelerates the switch from wandering behaviour to immobilisation/retraction behaviour but has no effect on the onset and duration of motor patterns that normally underlie puparial contraction in controls. The morphology of an accelerated puparium is normal but its formation is temporally dissociated from normal 'contraction patterns' that are performed a long time after the puparium has contracted. It means that neuromuscular activity of larvae accelerated by LPK does not cease upon formation of the white puparium, but continues until the whole motor programme of pupariation behaviour is completed. Apparently the peptide acts on the integument by stimulating it to contract and shrink, and no specific patterns of muscular contractions are needed to properly shape the puparium. This finding sheds a new light on our understanding of the mechanism of puparium formation.