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vg w is a homozygous lethal mutation killing embryos prior to formation of the syncitial blastoderm. In heterozygous condition it causes duplications of the posterior wing, ranging from very small duplications of the axillary cord and alar lobe to large duplications including much of the wing blade and the posterior row of bristles. No anterior margin structures are ever observed. The thorax is sometimes slightly abnormal, but rarely shows large duplications. The size of the wing is related to the number of pattern elements deleted or duplicated. Heterozygous vgw flies also show homoeosis of the haltere to wing. This occurs in the capitellum, where wing blade is observed, but no wing margin structures are found. As with the bithorax (bx) mutation which transforms anterior haltere to anterior wing this aspect of the phenotype is repressed by the Contrabithorax (Cbx) mutation. The transformed haltere discs show more growth than wild-type haltere discs. Flies heterozygous for vgw also show a high frequency of pupal lethality, those forming pharate adults generally show the most extreme vgw phenotype. No cell death has been observed in the imaginai discs of third instar larvae, suggesting that if the wing defects result from cell death this must occur early in development. The homoeosis in the haltere discs and duplications of the wing disc are reflected by the altered morphology and growth of these discs. There are some minor differences in the expressivity of the phenotype when flies are reared at different temperatures. Chromosome substitutions suggested that all aspects of the phenotype related to the vgw mutation and that other mutations had not occurred in the stock. Cytological analysis indicated that vgw is a deletion or inversion on the right arm of chromosome 2 from 47F/48A to 49C. Complementation studies with various mutants thought to be located within the deletion, or inversion and which affect wing morphology have been undertaken. Cbx causes transformations of wing to haltere; this occurs in the posterior compartment far more frequently than in the anterior compartment. Cbx; vgw flies have wings where one of the duplicates is no longer present, presumably transformed to haltere, though this is difficult to identify. One copy of the axillary cord, alar lobe etc, the structures commonly duplicated in vgw, are present, but they are the anterior duplicate rather than the original posterior copy of these structures. Thus Cbx acts upon genuine posterior structures but not those posterior structures in vgw which form in anterior wing locations, suggesting that although these structures differentiate into posterior wing, to the Cbx gene product the cells are still ‘anterior’.

The wing discs from larvae undergoing the moult from 1st to 2nd instar are able to differentiate some parts of the adult wing when forced to undergo a premature metamorphosis. The first structures which differentiate are parts of the wing hinge and the wing blade. As development proceeds and older discs are forced through metamorphosis, the capacity to differentiate moves out both proximally and distally until gradually all of the derivatives of a mature wing disc are formed. Individual structures often differentiate from young discs in an incomplete form and pattern elements, such as bristles or sensilla, are added as older discs are tested.

The changes which can be seen occurring during the development of a living embryo of Drosophila melanogaster are described in detail, and represented photographically as a series of developmental stages. This provides an easy, but accurate technique for selecting eggs at precise developmental stages for experiments.

When complementary fragments of the imaginal wing disc of Drosophila are cultured for several days prior to inducing metamorphosis, usually one fragment will regenerate while the second duplicates. It has been proposed that wound healing plays an important part in disc regulation by initiating cell proliferation and determining the mode of regulation (regeneration/ duplication). To test the latter proposal 15 types of wing disc fragments were examined for variability both in the mode of wound healing and the mode of pattern regulation. Two modes of wound healing were observed, regular - the two wound edges heal with each other, and irregular - each wound edge heals with itself. When cultured separately fragments that healed regularly regenerated, while fragments that healed irregularly duplicated. This suggests that the mode of wound healing determines the mode of pattern regulation.

A sequence of 51 visible changes is described during the course of metamorphosis in Drosophila melanogaster, and a series of 24 convenient stages is defined for use in the experimental analysis and exploitation of this part of the insect life cycle. The duration of each stage is estimated and times are suggested for batch collections of symphasic animals.

Cells were removed from three regions (anterior, mid-lateral and posterior) of Drosophila eggs at blastoderm formation, by pricking. Embryonic defects were generally correlated with the site of damage, as is also found with microcautery and with u.v. irradiation. But many structures can develop autonomously, independently of surrounding damage. Most hatching adults were normal, suggesting that there may be mechanisms which permit compensation for cell loss. Malformed or missing adult structures are those expected from the fate map obtained from studies of genetic mosaics.

Drosophila embryos were damaged by microcautery at the cellular blastoderm stage at the sites of presumptive imaginal cells, identified from fate maps. The resulting adults were analyzed for abnormal structures. Cautery of any of the presumptive imaginal regions can lead to defects in the adult cuticle, though the majority of adults which hatch are morphologically normal. The abnormal adults had one or more discs either deleted, incompletely formed, or misarranged. Several of the structures which were incomplete had duplicated regions. The results suggest that, from the time of their initial formation in the cellular blastoderm, a group of cells determined to be an ‘adult’ structure possesses a gradient of developmental capacity which is expressed by certain regions duplicating and other cells regenerating. The types of duplications found were similar to those resulting from other experimental treatments of imaginal discs at later stages in development, indicating that the presumptive imaginal disc cells, when they are first established in the cellular blastoderm, have a similar organization to mature imaginal discs.

The extent and type of adult transformations in the tumorous head (tuh) mutation of Drosophila melanogaster were studied. The observations indicate homeotic transformations, duplications and deficiencies of eye antennal disc derivatives. Contrary to previous observations there is no transformation of eye to abdomen and the only homeotic transformations identifiable are antenna to leg and rostralhaut to genitalia. Embryonic and post-embryonic lethality was also examined. Specific anterior abnormalities were found in embryos leading to early death. The amount of embryonic lethality was not affected by increased temperature during oogenesis. When analysing the adult phenotype, however, the whole period of oogenesis was sensitive to pulses of increased temperature; the pulses however did not have to be restricted to any particular stage of oogenesis to be effective in increasing tuh penetrance. The increase in the penetrance of the adult transformations is also exhibited when eggs are moved from 25 °C to 29 °C from the 8th to the 12th hour of embryonic development. Experiments indicate that the tuh-3 gene is active before the 8th hour of embryogenesis.

The three yolk-protein genes of Drosophila are normally expressed only in adult female fat bodies and ovaries. 20-hydroxyecdysone can affect the transcription of these genes in males and females, as can mutations in the sex-determining genes tra, tra-2, ix and dsx. We have asked a number of basic questions about how these genes are regulated, using an inhibitor of protein synthesis (cycloheximide), labelling RNA in vivo, a temperature-sensitive sex-determination mutant (tra-2tsl), and 20-hydroxyecdysone. We have found that the yolk-protein genes are continuously transcribed in the fat bodies of adult females and that maintenance of this transcription requires protein synthesis. Hormone induction in males is also inhibited by cycloheximide, suggesting that the products of other genes are essential both for 20-hydroxyecdysone to be able to switch on the genes, and for their continuous transcription in the female fat body. The products of the tra-2 gene are also required for continuous transcription of the yolk-protein genes, suggesting that the pathway inhibited by the cycloheximide is that of the sex-determination hierarchy. 20-hydroxyecdysone can override the sex-determination system and induce yolk protein synthesis in normal males and tra-2ts reared and maintained at the restrictive temperature.

Drosophila eggs at nuclear multiplication or blastoderm stages were regionally irradiated with monochromatic u.v. Twenty-four hours after irradiation, the results were classified as normal larvae, undifferentiated eggs, or defective embryos; the latter were subdivided into embryos with anterior or posterior defects, or embryos without anterior and posterior specificity. The irradiation of anterior (posterior) egg regions and the occurrence of anterior (posterior) defects were strongly correlated. These correlations were found using eggs of both stages. The size of the irradiated area did not obviously influence the types of defect. Dose-response curves were established irradiating anterior quarters of eggs at 245, 265, 285, or 305 nm wavelength. The frequency of embryonic defects increased with increasing u.v. doses, whereas the dosages applied did not increase the frequency of undifferentiated eggs over the control level. In eggs during nuclear multiplication 285 nm radiation was most effective in producing embryonic defects. After blastoderm formation, the efficiency of irradiation was generally increased but similar at all wavelengths employed. The induction of embryonic defects was photoreversible after u.v.-irradiation at the blastoderm stage. The data reported also include transmittance spectra of chorion and egg membrane preparations from Droso-phila eggs.

A number of female sterile mutations on the first and third chromosomes of Drosophila melanogaster have been screened for defects in the yolk proteins using polyacrylamide gel electrophoresis. Two new mutants were identified. 6m45 accumulates all three yolk proteins (YP1, YP2 and YP3) in the haemolymph but they are all absent from the ovaries suggesting it is a yolk-protein-uptake mutant. In contrast, 1163 is a temperature-sensitive mutation with a large reduction in the quantity of YP1 in the haemolymph and ovaries at 29 °C. Both mutants are autonomous in ovary transplant experiments.

Localized regions of Drosophila eggs were damaged at both nuclear multiplication and blastoderm stages, using the technique of microcautery. The resulting defects in embryos and adults were recorded in order to study the effects of microcautery on determination of larval and adult structures. Using 70 °C microcautery, few specific defects were found in either embryos or adults. Sensitivity maps relating embryonic mortality to sites of damage were prepared for both egg stages and compared. Microcautery at 75 °C was used to check experimentally the putative localization in the egg of adult disc cells, as indicated by genetic mosaic studies. Although some of the expected adult defects were found, there was also evidence for regulation. Localized embryonic defects were found using this higher temperature, such that anterior damage produced anterior defects and posterior damage led to posterior defects. Mid-region microcautery resulted in both anterior and mid-region defects, the latter being most common after damage to the blastoderm. Comparisons are made with results obtained using other experimental manipulations and with the development of some embryonic lethal mutants.

The three yolk proteins of Drosophila melanogaster begin to be synthesized at eclosion. Transcription of the genes is regulated by the genes tra, tra-2 and dsx and also by the insect hormones, juvenile hormone and 20-hydroxyecdysone. We show that there is yet another level of control which is dependent upon feeding. Females that are starved from eclosion show a basal level of yolk protein gene transcription, which is rapidly increased when a complete diet is supplied. We show that the effect is not due to incorrect development of the fat body and is unlikely to be solely due to a general effect on protein synthesis. Later in development, cessation of feeding leads to selective inhibition of yolk protein synthesis and hence egg production. The effects of starvation can be partially overcome by 20-hydroxyecdysone, juvenile hormone, casein, amino acid mix or sucrose, but only a complete medium or live yeast brings about total recovery. Using ypl-Adh fusions (fusions of the promoter region of ypl to the structural gene for Adh), the DNA sequence required for this diet-enhanced transcription has been located within an 890 bp fragment upstream of the ypl gene. The insect hormones do not operate on this same DNA fragment.

Nuclei isolated from a permanent cell line derived from Drosophila melanogaster embryos have been injected, along with a radioactive DNA precursor [3H]TTP, into Xenopus laevis eggs. In culture, less than 7 % of the cells were in S phase. After a 90 min incubation, following injection into eggs, 99% of the nuclei were shown by autoradiography to have synthesized DNA. In a similar experiment, a density label BrdUTP was injected into eggs along with the nuclei. Subsequent analysis on caesium chloride gradients showed that this DNA synthesis was semi-conservative replication. Therefore we conclude that signals present in Xenopus egg cytoplasm can initiate and sustain true semi-conservative DNA replication in nuclei from an invertebrate organism.