Drosophila salivary glands exhibit a regional reprogramming of gene expression during the third larval instar

In D. virilis salivary glands transcripts of two early gland protein genes, Egp-1 and Egp-2, which encode putative secretory proteins, accumulate in all cells from the first to mid third larval instar. Subsequently the transcripts disappear from the cytoplasm of the corpus cells, but not from their nuclei, where they accumulate at the chromosomal site of their synthesis. In the collum cells, however, Egp-transcripts continue to be detectable in the cytoplasm until the end of larval life. In the salivary glands of transgenic D. melanogaster the presence of a Egp-1/lacZ fusion protein shows the same regional shift as the cytoplasmic Egp-transcripts in D. virilis. We predict that the expression of Egp-genes is related to an early secretory function of the larval salivary glands which is executed by all cells during earlier larval stages but becomes restricted exclusively to the collum cells during the third larval instar.