Eggshell assembly in Drosophila: Processing and localization of vitelline membrane and chorion proteins
Abstract
During oogenesis in Drosophila melanogaster, the ovarian follicle cells secrete approximately thirty proteins in a well defined sequence to construct an eggshell around the developing oocyte. The eggshell impacts on the success of several reproductive processes: it allows and facilitates fertilization, oviposition, embryogenesis, and hatching. Understanding the eggshell in molecular terms is likely to be a prerequisite for understanding its multiple functions during early development. The eggshell consists of three major proteinaceous layers. The morphologically homogeneous vitelline membrane (VM) is proximal to the oocyte surface. Overlying the VM are the progressively distal inner chorionic (ICL) and endochorion layers. The mature ICL is crystalline; the distinctive endochorion exhibits a floor-pillar-roof tripartite substructure. Compositionally aberrant eggshells produced by some female sterile mutants exhibit fatal disruptions of this three-dimensional architecture. While most biochemical, morphological, and genetic data suggest that apposition of sequentially secreted proteins contributes to layer morphogenesis, a paucity of flies mutant at eggshell structural gene loci and a lack of molecular resolution have prevented confirmation or contradiction of this assembly model. sV17 and sV23 are two proteins synthesized primarily during stages 9-10 of oogenesis when VM is laid down. s36 and s18 accumulate during early (stages 11-13) and late (stages 13-14) endochorion formation, respectively. In this study, an immunological approach has begun defining the eggshell at a molecular level by following the production, secretion, and assembly-related behavior of these four proteins. The results contradict a strict model of assembly by apposition as s18 intercalates into an endochorion floor-pillar-roof framework already defined by earlier proteins, including s36. In addition, this work shows that sV17 and sV23 are stage-specifically processed in the VM to smaller derivatives and that minor quantities of vitelline membrane and early chorion antigens are internalized by the oocyte. This work further suggests that the vitelline membrane may serve as a transient reservoir for s36 prior to its assembly into the overlying endochorion, and maintenance of chorion morphological integrity is dependent upon the presence of a vitelline membrane protein, sV23.
This paper has been withdrawn.