Cell death in the salivary glands of metamorphosing Calliphora vomitoria.

The salivary gland cells of Calliphora vomitoria larvae initiate and complete their own destruction in a programmed manner at the onset of metamorphosis. On entering the post-feeding period the larvae come to rest and the polytene salivary gland cells show a significant increase in DNA synthesis followed closely by a surge of mRNA synthesis accompanied by increasing protein production. During this prelude to cell death the new mRNA gives rise to at least 10 new proteins. The first new proteins having a MWt between 30 and 100kD appear by day 8 of the life-cycle and a number persist until the advent of cell death on day 9. Other new proteins appear in a cascade of production during day 8 and in vitro translation of mRNA produced at this time shows a new 55kD protein appearing before cell destruction. Significantly no evidence of DNA degeneration or laddering associated with classical apoptosis was observed, on the contrary considerable DNA synthesis in the form of chromosomal endoduplication or "genomic amplification" was seen; selective gene expression being apparently controlled at translational level. Overall the results strongly suggest a synthetically mediated programmed cell death in the metamorphosing salivary glands of the blow-fly which is distinct from apoptosis.