Manipulating the anteroposterior pattern of the Drosophila embryo.

INTRODUCTION The establishment of pattern and polarity in the insect egg has been studied intensively using a variety of experimental approaches. Drosophila, while the system of choice for genetic analysis of pattern formation has been rather neglected as an experimental organism and species with longer developmental time and larger eggs were preferred in classical studies. Among the dipteran insects, midges such as Chironomous and Smittia with their transparent chorion and synchronous development were found more rewarding. The classical methods of ligation, puncture, transplantation, destruction or removal of material, and centrifugation were applied to eggs of a variety of insect species. Although the degree of response to experimental manipulation was found to be widely different, there were similarities in the type of abnormal patterns produced by the various treatments which suggested more general conclusions:the anteroposterior pattern is probably controlled by two centres of activity, localized at the anterior and posterior egg pole respectively, with a long-range effect on the entire egg axis (reviewed by Sander, 1976). One of the earliest clear indications of an 'activation centre' localized at the posterior egg pole came from ligation experiments on the egg of the dragon fly Platycnemis (Odonata) (Seidel, 1929). Removal of the posteriormost 10 % of the egg by ligation in early cleavage stages prevents embryonic development in the larger anterior portion of the egg. If ligation is done somewhat later, partial embryos develop in the anterior portion suggesting that a time-dependent spreading of some factors is required for pattern formation in anterior egg regions. The organizing influence of material localized at the posterior pole has best been shown by Sander (1959,1960) in the leaf hopper Euscelis (Homoptera). This insect has an unusual cytoplasmic inclusion, a ball of symbiotic bacteria located at the posterior pole. Because of the fortuitously low turgor of the egg, this ball of symbionts together with adhering posterior cytoplasm can be easily pushed around in the egg without needing to penetrate the egg membranes. Sander showed that