An examination by time lapse movie of the process of pupation in Drosophila melanogaster shows a series of muscular contractions to effect breakage of the tracheae and subsequent head invagination.
In order to gain an understanding of the biochemical changes during pre-pupal development, salivary glands from animals in the late larval to the pupal stages were pulse-labeled with methionine or cysteine and the patterns of the proteins were analyzed on a sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Many of the proteins detectable by Coomassie are generally present in all stages from late larvae to pupae. However, autoradiographs of the same gel show rapid rates of synthesis in the prepupal stages especially for a set of low molecular weight proteins (3K-12K). These small polypeptides show a dramatic reduction in their rates of synthesis at the time of pupation.
Analyses of proteins from the salivary glands of animals injected with labeled methionine and cysteine show a general maintenance of most, if not all, of the proteins except for the low molecular weight components which diminish drastically at pupation. Results also suggest a transport of these small polypeptides from the salivary glands to the fluid between the pupal case and the prepupal cuticle (pupation fluid). These low molecular weight proteins are very basic and each component actually consists of several sub-components.
Another aspect of development that was investigated concerns the regulation of bristle and hair formation. Heat treatment (40.2°C for 40 min) of pupae at different stages resulted in the production of four separate phenocopies designated as angle bristle, smooth bristle, multiple hairs and spear bristle. Each phenocopy is induced at a specific time when the cell is in a susceptible state and the sensitive period lasts for less than two hours.
When animals are heat treated under conditions which do not turn off the protein synthesis in progress but which induce the heat shock proteins, this results in increased survival and protection against phenocopy production. Comparison of the resumption of protein synthesis to that of RNA synthesis suggests the storage of mRNA as a factor in the protection against phenocopy induction.
