Developmental changes in the fine structure of the salivary gland of Trichosia pubescens (Morgante) (Diptera : Sciaridae) in relation to puffing activity

Abstract

Two differentiated sections (S 1 and S 2) of the salivary gland of Trichosia pubescens (Morgante) (Diptera : Sciaridae) have been examined by electron microscopy for fine structural alterations that occur in the cell cytoplasm during larval development. Such changes have been correlated with the puffing patterns of the polytene chromsomes. During stage 1 (end of the 3rd instar to mid 4th instar), the puffing pattern and the ultrastructure of S 1 and S 2 cells are rather constant. Nevertheless, marked differences are noted when the puffs and the fine structure of the 2 sections are compared. In S 1, secretory material is concentrated and eliminated as membrane-bound granules, while in S 2, secretory granules are not detected and the elimination of secretion seems to occur continuously. At stage 2 (end of the 4th instar), the puffing pattern undergoes considerable alterations simultaneously with the appearance of many ultrastructural modifications. In S 1, the morphological aspect of the secretory granules is altered, while in S 2 a decline in the secretory activity is detected. At stage 3 (older 4th-instar larvae), most of DNA puffs are active, there being no striking differences in the puffing pattern between S 1 and S 2. This stage is marked by the onset of gland histolysis, with the appearance of an intense autophagic activity of lysosomes in S 1 and S 2. As histolysis progresses during stage 4 (prepupae and early pupae) the activity of the polytene chromosomes decreases; most of the cells present a large number of autophagic vacuoles and an increasing disorganization of the cytoplasm, leading to the final lysis of the gland.