Mass determination of the unit cell of the innermost chorionic layer in Drosophilidae by scanning transmission electron microscopy

The innermost chorionic layer (ICL) in eggshells of Drosophila melanogaster is a naturally occurring patchwork of thin three-dimensional crystalline p lates located between the inner endochorion and the vitelline envelope. The mass-per-unit area of the ICL has been measured from scanning transmission electron microscope images of isolated unstained material and it was possi ble to distinguish up to four layers with the majority of the crystalline s heets being one to three layers thick. Taking into account the unit cell ar eas for the different crystals, we have estimated the mean ICL subunit size s to be 36 kDa for Drosophila melanogaster, 35 kDa for Drosophila auraria, and 33 kDa for Drosophila teissieri. The results suggest that the three dif ferent Drosophilidae species have very similar average subunit masses. (C) 1999 Academic Press.