Three-dimensional reconstruction of innermost chorion layer of Drosophila grimshawi and Drosophila melanogaster eggshell mutant fs(1)384.

A low-resolution three-dimensional structure of the crystalline innermost chorionic layer (ICL) of the Hawaiian species Drosophila grimshawi and the Drosophila melanogaster eggshell mutant fs(1)384 has been calculated from electron microscope images of tilted negatively stained specimens. The isolated ICL of Drosophila grimshawi is a three-layer structure, about 36 nm thick, whereas the ICL of Drosophila melanogaster eggshell mutant fs(1)384 is a single layer, about 12 nm thick. Each unit in both crystalline structures includes octamers made up of four heterodimers. Crosslinks between the structural elements, both within and between unit cells form an interconnecting network, apparently important in maintaining the integrity of the layer. A model which may account for the ICL self-assembly formation in vivo and the ICL observed lattice polymorphism is proposed, combining data from the three-dimensional reconstruction work and secondary structure features of the ICL component proteins s36 and s38.