Post-embryonic cell lineages of the nematode, Caenorhabditis elegans

The metameric organization of the Drosophila embryo is generated in the first 5 h after fertilization. An initially rather simple pattern provides the foundation for subsequent development and diversification of the segmented part of the body. Many of the genes that control the formation of this pattern have been identified and at least twenty have been cloned. By combining the techniques of genetics, molecular biology and experimental embryology, it is becoming possible to unravel the role played by each of these genes. The repeating segment pattern is defined by the persistent expression of engrailed and of other genes of the 'segment polarity' class. The establishment of this pattern is directed by a transient molecular prepattern that is generated in the blastoderm by the activity of the 'pair-rule' genes. Maternal determinants at the poles of the egg coordinate this prepattern and define the anteroposterior sequence of pattern elements. The primary effect of these determinants is not known, but genes required for their production have been identified and the product of one of these, bicoid is known to be localized at the anterior of the egg. One early consequence of their activity is to define domains along the A-P axis within which a series of 'cardinal' genes are transcribed. The activity of the cardinal genes is required both to coordinate the process of segmentation and to define the early domains of homeotic gene expression. Further interactions between the homeotic genes and other classes of segmentation genes refine the initial establishment of segment identities.

The molecular basis for metameric pattern in the Drosophila embryo.

Analysis of the genes that control the early events of Drosophila embryogenesis is providing details of the molecular processes underlying the positional specification of cells. There are two distinct phases: the first precedes the cellularization of the blastoderm embryo and is associated with a cascade of interactions between transcriptional regulators; the second occurs after cellularization and depends on communication between cells. These processes may be conserved in a wide range of invertebrates and vertebrates.

The molecular genetics of embryonic pattern formation in Drosophila

The body plan of Drosophila is determined to a large extent by homeotic genes, which specify the identity and spatial arrangement of the body segments. Homeotic genes share a characteristic DNA segment, the homeo box, which encodes a defined domain of the homeotic proteins. The homeo domain seems to mediate the binding to specific DNA sequences, whereby the homeotic proteins exert a gene regulatory function. By isolating the normal Antennapedia gene, fusing its protein-coding sequences to an inducible promoter, and reintroducing this fusion gene into the germline of flies, it has been possible to transform head structures into thoracic structures and to alter the body plan in a predicted way. Sequence homologies suggest that similar genetic mechanisms may control development in higher organisms.

https://science.sciencemag.org/content/sci/236/4806/1245.full.pdf

Homeo boxes in the study of development

Cell fate is determined when the commitment of cells to a particular fate is autonomously maintained, irrespective of their environment. In Drosophila, fate determination is maintained through the action of the Polycomb-group and trithorax-group genes, which are required so that states of homeotic gene activity are inherited through cell division. It is shown here that the CURLY LEAF gene of Arabidopsis is necessary for stable repression of a floral homeotic gene and encodes a protein with homology to the product of the Polycomb-group gene Enhancer of zeste. We suggest that Polycomb-group genes have a similar role in fate determination in plants and animals.

A Polycomb-group gene regulates homeotic gene expression in Arabidopsis

Ovarian Development in Drosophila melanogaster By Robert C. King. Pp. x + 227. (Academic: New York and London, July 1970.) 154s.

The Drosophila Ovary

When does determination occur in Drosophila embryos

A variety of cell types develop when cells of 6 1/2-8 1/2 h Drosophila embryos are cultured in an improved medium. Nerve, muscles, fat-body, chitin-secreting, and macrophage-like cells (possibly haemocytes) appear in the first 24 h and mature over the next week. Tracheal, imaginal disc, a second stage of the macrophage-like, and a anumber of unidentified fibroblastic and epithelial cells appear in the 2nd and 3rd week, following a resumption of cell multiplication. There is some organization of some of the cell types into higher structures.

Differentiation in vitro of larval cell types from early embryonic cells of Drosophila melanogaster

Although attempts have been made over a number of decades to achieve successful tissue culture of Drosophila material, progress has been held back until recently by a lack of basic information on the chemical and physiological characteristics of the haemolymph of the organism necessary for a reasoned formulation of a culture medium. In 1963, Begg & Cruickshank published details of the mineral composition, osmotic pressure and pH of the haemolymph of third instar larvae, and this has provided the basis for a more satisfactory approach, as reflected in an increasing amount of fruitful work since reported. Much of this work has been done with embryonic material. Horikawa & Fox (1964) claimed continuous multiplication of a small type of early embryonic cell; Lesseps (1965) described re-aggregation of dissociated embryonic cells in culture, with possible development of muscle cells, nerve cells and oenocytes in the aggregates.

Characteristics of five cell types appearing during in vitro culture of embryonic material from Drosophila melanogaster.

Results are reported from the culturing in vitro of cells from individual early gastrulae of the following four groups of X-linked embryonic lethal mutants of Drosophila melanogaster . (1) Notch lethals. Five Notch mutants were studied which have been reported to give similar abnormalities in whole embryos: the nervous system displays a three-fold hypertrophy as part of a shift in the pattern of differentiation within ectodermal derivatives, and mesodermal derivatives do not differentiate. An hypertrophy of nerve was found in cell cultures prepared from embryos of all five mutants. In addition, four of the five alleles consistently gave abnormalities of muscle differentiation: when compared to controls, Notch cultures had a reduced frequency of myotubes, and displayed unusual clusters of myocytes which had either failed to fuse or had fused incompletely. Results from mixed cultures prepared from two embryos were consistent with the autonomous expression of nerve and muscle abnormalities by Notch -8 cells in the presence of wild-type cells. It is argued that the Notch locus has a direct role in the differentiation of both nerve and muscle. (2) white deficiencies. Cells carrying either of two deficiencies gave a clear-cut pattern of abnormalities: initial cellular differentiations were normal, but nerve, muscle and fat-body cells progressively deteriorated during the culture period. Mixed cultures showed that wild-type cells could not ‘rescue’ mutant muscle and fat-body cells; however, the status of the autonomy of mutant nerve abnormalities in these cultures was unclear. Both white deficiencies remove cytological band 3C1, and this permits a comparison of results with those from cultures of cells from Notch-8 embryos (also deficient for 3C1). Abnormalities displayed in cultures of the two types of mutant show no overlap. Therefore no consistent cellular abnormality can be attributed to absence of band 3C1. (3) lethal{l)myospheroid . In contrast to earlier observations on in vitro cell cultures (Donady & Seecof, 1972) muscle was seen to differentiate, though its morphology was extremely abnormal. Observations indicated that all cell types within the cultures had poor properties of adhesion to a glass substrate. It is argued that the observed abnormalities are not consistent with a mutant lesion which is restricted to the basement membrane ( contra Wright, 1960), and that all cell types carry a basic defect which may reside in the cell membrane. (4) shibirets alleles. Cultures of two temperature-sensitive lethal shibire alleles ( shits1, shits3 ) were normal at the permissive temperature of 22 °C. At the restrictive temperature (29 °C) early cell differentiation was normal but subsequent development was blocked. This blockage could be partially reversed by shifting cultures to the permissive temperature after as much as 10 days exposure to the high temperature. It is suggested that shits cells are mutant in a process which is basic to several cell types.

James H. Sang

Papers

The Drosophila Ovary

When does determination occur in Drosophila embryos

Differentiation in vitro of larval cell types from early embryonic cells of Drosophila melanogaster

Characteristics of five cell types appearing during in vitro culture of embryonic material from Drosophila melanogaster.

Cell culture of individual Drosophila embryos. I. Development of wild-type cultures.