Introduction

The silk moth eggshell (chorion) has been studied intensively since the first biochemical report on its composition and synthesis was published twenty years ago (Kawasaki, Sato, and Suzuki, 1971). To date, the work has made significant contributions to two distinct but interrelated fields: molecular evolution and developmental biology. This chapter will serve as a general introduction to the biology of the chorion, as well as a review of our current understanding of the mechanisms for developmentally regulated, chorion gene expression. Other aspects of the system are treated in greater detail in chapters by Eickbush and Izzo, Regier et al., and Goldsmith.

Two features have made eggshell formation (choriogenesis) an excellent model system for the study of development: a high degree of tissue, spatial and temporal regulation of the structural genes encoding chorion proteins, and informative similarities and differences in choriogenesis among different insects. Indeed, the comparative approach has been an important hallmark of chorion research from the outset. Thus, although our primary interest here is choriogenesis in Lepidoptera, it is important to place the information in the context of what is known about eggshell formation in the other well–studied group of insects, the Diptera. It should be noted that these two orders had their last common ancestor approximately 250 million years ago, and thus have been separated in evolution approximately as long as mammals from birds.In both Lepidoptera and Diptera, the eggs are formed in follicles (often called egg chambers in Drosophila), which consist of three cell types. The first is a single oocyte.