Transcriptional expression of variant histone genes during development of xenopus: [formula omitted], [formula omitted], [formula omitted] and [formula omitted]. Dept. of Anatomy and Embryology, Univ. of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands

Abstract

Two differentiated sections (S1 and S2) 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 S1 and S2 cells are rather constant. Nevertheless, marked differences are noted when the puffs and the fine structure of the 2 sections are compared. In S1, secretory material is concentrated and eliminated as membrane-bound granules, while in S2, 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 S1, the morphological aspect of the secretory granules is altered, while in S2 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 S1 and S2. This stage is marked by the onset of gland histolysis, with the appearance of an intense autophagic activity of lysosomes in S1 and S2. 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.