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
The effect of heat shock (15 min at 48 degrees C) on segmentation has been investigated in the short germ embryo of the locust (Schistocerca gregaria). Prior to formation of the germ anlage and at the disc stage heat shock considerably reduced the survival of eggs but appeared to have little effect upon segmentation. At later stages heat shock had no effect on survival but resulted in disruptions of the segmental pattern. The location of abnormal segments depended upon the stage at heat shock and the number affected depended on its severity. A constant number of normal segments developed between the last segment visible at the time of heat shock and the first abnormal segment. These results are similar to the disruptions observed in amphibian somites following heat shock. However, different parts of the segment pattern varied in their response; the head segments were very rarely affected, and disrupted regions rarely started in the middle abdomen (segments A5 and A6). The results are discussed in relation to two models (the clock and wavefront and progress zone models) that have been proposed as an explanation for the specification of the somite pattern in amphibians.
A quantitative electrophoretic analysis of glucose phosphate isomerase (GPI-1) allozymes produced by heterozygous Gpi-1sa/Gpi-1sb mouse embryos has enabled us to estimate separately the contributions of GPI-1 enzyme that were oocyte coded, encoded by the embryonic, maternally derived Gpi-1sa allele and encoded by the embryonic, paternally derived Gpi-1sb allele. The oocyte-coded GPI-1 activity is stable until 2 1/2 days and then declines and is exhausted by 5 1/2 to 6 1/2 days post coitum (p.c.). The maternally and paternally derived Gpi-1s alleles are probably usually activated synchronously but several possible exceptions were observed. This activation was first detected in 2 1/2-day embryos. Total GPI-1 activity falls to a minimum around 3 1/2 to 4 1/2 days, even though embryonic gene expression has already begun. The profile of oocyte-coded GPI-1 activity is consistent with the suggestion (Harper & Monk, 1983) that there is a mechanism for the removal of oocyte-coded gene products at around 2 1/2 days p.c. The method of analysis described is applicable to other dimeric enzymes with electrophoretic variants.
The development of ciliary band pattern in the doliolaria larva of Florometra serratissima is described based on scanning and transmission electron microscopy. The uniformly ciliated epithelium of the post-hatching larva develops four regularly spaced bands over a period of approx. 20 h generating an epithelial pattern that is, essentially, a series of stripes. The first visible events of pattern formation progress over the larval surface in a posterior-to-anterior and dorsal-to-ventral sequence, but the initial pattern is not, in fact, striped. It instead consists of a close-packed array of oval interband domains separated and surrounded by belts of band cells. Secondarily the interband domains expand laterally and coalesce to form continuous, broad stripes, while the bands remain as narrow stripes between them. Two possible explanations for this unusual sequence of events are discussed: that it can be understood in evolutionary terms with reference to band pattern in other echinoderm larvae, and that it is a morphogenetic necessity because limitations inherent in the patterning mechanism prevent the direct formation of regular stripes.
The segmentation pattern of the Drosophila wild-type embryo is characterized by a number of easily identifiable cuticular structures. They include skeletal elements of the involuted head and ventral denticle belts that define by size, pattern and orientation the anterior part of the three thoracic and eight abdominal segments. Further landmarks such as sensory organs and the posterior tracheal endings (‘Filzkorper’), in combination with the denticle belts, allow one to un-equivocally determine the polarity and quality of each segment in preparations of the larval cuticle (see Fig. 1D).
