Developmental expression of Drosophila melanogaster small heat-shock proteins.

We have investigated the developmental expression of the small heat-shock proteins (hsps) during embryogenesis and in adult flies by immunocytology using an antibody that specifically identifies the small hsps. Antibody staining of unstressed early embryos reveals a predominantly cytoplasmic, homogeneous distribution of the small hsps throughout the embryo. At 6h of development small hsp expression can be identified in large, neuroblast-like cells within the extended germ band and in the brain of the embryo. During germ band contraction these cells appear to migrate to the midline where they align pairwise in a segmental pattern. After germ band contraction is complete a high level of small hsp expression can be observed in the midline glia (MECs) and in a cluster of six non-neuronal cells within the midline. In contrast to several other genes that are known to be important for embryogenesis and are expressed in the central nervous system (CNS) of embryos, CNS-specific expression of the small hsps is not restricted to the embryo but is also observed in the adult fly. In adult flies strong small hsp expression is observed in the brain, the thoracic ganglion and the leg nerves. Since the small hsps seem to be expressed predominantly in the glia of the nervous system, our data suggest a protective or stabilizing function of the small hsps within the nervous system during normal fly development, which is independent of the stress response.