Maternal effect mutations of the sponge locus affect actin cytoskeletal rearrangements in Drosophila melanogaster embryos.

In the syncytial blastoderm stage of Drosophila embryogenesis, dome-shaped actin "caps" are observed above the interphase nuclei. During mitosis, this actin rearranges to participate in the formation of pseudocleavage furrows, transient membranous invaginations between dividing nuclei. Embryos laid by homozygous sponge mothers lack these characteristic actin structures, but retain other actin associated structures and processes. Our results indicate that the sponge product is specifically required for the formation of actin caps and metaphase furrows. The specificity of the sponge phenotype permits dissection of both the process of actin cap formation and the functions of actin caps and metaphase furrows. Our data demonstrate that the distribution of actin binding protein 13D2 is unaffected in sponge embryos and suggest that 13D2 is upstream of actin in cortical cap assembly. Although actin caps and metaphase furrows have been implicated in maintaining the fidelity of nuclear division and the positions of nuclei within the cortex, our observations indicate that these structures are dispensible during the early syncytial blastoderm cell cycles. A later requirement for actin metaphase furrows in preventing the nucleation of mitotic spindles between inappropriate centrosomes is observed. Furthermore, the formation of actin caps and metaphase furrows is not a prerequisite for the formation of the hexagonal array of actin instrumental in the conversion of the syncytial embryo into a cellular blastoderm.

Expression of laminin and of a laminin-related antigen during early development of Drosophila melanogaster

This paper reports the characterization of two immunologically related proteins that may be involved in cell adhesion during Drosophila development. These proteins, laminin chain A and a 240K component, share the epitope recognized by monoclonal antibody RD3 (Mab RD3). The two antigens show different developmental expression profiles. Laminin is detected only from 6 to 8 h of development onwards; its concentration increases during embryogenesis to reach steady-state value in larvae, pupae and adult flies. By contrast, the 240K antigen, not found in oocytes, is present before blastoderm stages; its concentration increases during gastrulation, decreases at the end of organogenesis and the antigen is no longer detected in third instar larvae. Light and electron microscope immunolocalization in imaginal discs indicates that laminin is distributed apically in the lumen and basally in the basal membrane that surrounds the nonevaginated disc. During morphogenesis laminin is detected at the basal side of the evaginating part of the disc epithelium. Immunolocalization on paraffin sections of early embryos suggests that the 240K antigen is related to (1) cell formation and polarization in association with cytoskeleton components, (2) establishment of cell-extracellular substratum interactions during the blastoderm cell sheet organization and (3) basement membrane deposition during embryonic germ cell layer segregation. This 240K protein is poorly or not glycosylated, is resistant to chondroitinase ABC and collagenase and appears therefore as a new extracellular component that might be specifically involved in early processes of morphogenesis.

Blastoderm formation in the silkworm egg (Bombyx mori L.)

Embryogenesis in the egg of the silkworm, Bombyx mori, up to 24 h after oviposition was studied, by light microscopy with special reference to nuclear migration and blastoderm formation. In Bombyx eggs blastoderm cells seem to form in a mechanism different from that usually seen in many other insect species; that is, in Bombyx eggs no typical cleavage furrows were seen. Cleavage nuclei which had migrated, accompanied by the associated cytoplasm, to the egg surface pushed up the plasma membrane and protruded beyond the initial level of the periplasm. The periplasm fused with their associated cytoplasm was partitioned among and pulled around the nuclei. Then each nucleus was separated by a laterally-invading limiting membrane from the yolk-granules-occupied region to yield a blastoderm cell.

A FINE-STRUCTURE GYNANDROMORPH FATE MAP OF THE DROSOPHILA HEAD

ABSTRACT A gynandromorph fate map of the head of D. melanogaster was produced using 28 landmarks derived from one imaginal disc. An examination of the meaning of fine-structure mapping discloses that the sturt value observed between one pair of landmarks within a disc may approximate the relative physical distance of their progenitor cells at blastoderm, but for another pair of landmarks (assuming no directed cell movements), the sturt value may simply reflect their close geographic location at the time the cells are specified for their particular differentiation, a time much later in development when most cell division within the disc has come to an end. The formation of early developmental compartments has little effect on fate-map distances. Our analysis of the data suggests there are approximately ten cells present at the blastoderm stage that are head progenitors. Each blastoderm cell is likely to be the progenitor of a particular array of landmarks, but there is overlap between arrays from different blastoderm cells.