Differential control of transcription appears to be a particularly important level in the pathway of information flow (from gene to supramolecular assembly) for interpreting developmental information in a time-dependent and cell-specific manner. The chorion-producing epithelium surrounding polyphemus silkmoth oocytes has proved to be a valuable model system. Chorion proteins number more than 100 and are encoded by a small number of evolutionarily related gene families. Chorion genes are expressed in a time-dependent manner when measured at the levels of specific RNA accumulation and protein synthesis. During the very late period of choriogenesis, a group of genes are expressed predominantly by only one of the two major populations of follicle cells. Very late period proteins assemble into topographically restricted and morphologically distinct surface structures called aeropyle crowns. It has been proposed that chorion genes expressed at similar times in development are clustered separately from other chorion genes. Analysis of very late period gene organization will test this hypothesis. Possible clustering of cell-specific chorion genes will also be tested. Cloned cDNAs of two very late period sequences (called E1 and E2) have been used to identify cloned chromosomal fragments containing E1 and E2 genes. Different types of E1 and E2 gene linkage relationships are possible--clustering of E1 and E2 genes separately or together, linkage to other very late period genes and/or to non-regionally expressed chorion genes. The primary structures of E1 and E2 genes (and possibly other very late period genes) will be determined to characterize these novel gene families and to search for noncoding sequences essential for developmental regulation. These studies should be helpful in constructing models for the evolution of chorion multigene families, and possibly their developmental regulation. In the cecropia silkmoth, aeropyle crowns are absent. The mechanism by which very late period gene expression in these two silkmoths became altered during evolution is unknown, but may be related to species-specific differences in gene structure, gene organization or transcriptional efficiency. The first two hypotheses will be tested by characterizing cloned E1 and E2 genes from cecropia in terms of their structure and organization; the third, by Northern analysis using cloned E genes as probes. Multiple E genes from cecropia will be sequenced for comparison with each other and with polyphemus. This study may shed light on the evolution of developmental pathways.
