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.

A repetitive DNA sequence has been identified in the Drosophila melanogaster genome that appears to be localized specifically within genes of the bithorax and Antennapedia complexes that are required for correct segmental development. Initially identified in cloned copies of the genes Antennapedia, Ultrabithorax and fushi tarazu, the sequence is also contained within two other DNA clones that have characteristics strongly suggesting that they derive from other homoeotic genes.

A conserved DNA sequence in homoeotic genes of the Drosophila Antennapedia and bithorax complexes

The bithorax complex in Drosophila melanogaster is a cluster of homeotic genes that specify developmental pathways for many of the body segments of the fly. The DNA of the bithorax complex has been isolated, and a region of 195,000 base pairs that covers the left half of the complex is described here. The lesions associated with many of the bithorax complex mutants have been identified, and most are due to DNA rearrangements. Most of the spontaneous mutants have insertions of a particular mobile element named "gypsy." This element affects the functions of sequences removed from the site of insertion. Mutant lesions for a given phenotypic class are distributed over large DNA distances of up to 73,000 base pairs.

https://science.sciencemag.org/content/sci/221/4605/23.full.pdf

Molecular Genetics of the Bithorax Complex in Drosophila melanogaster

The Drosophila bithorax complex is subdivided into three major genes: Ultrabithorax+, abdominal-A+ and Abdominal-B+. Each of these genes plays its principal part in a particular anatomical domain of the body, where it is specifically required to determine the correct segmental pattern.

Genetic organization of Drosophila bithorax complex

One of the first steps in segmentation of the Drosophila embryo seems to be not the formation of segments, but instead the definition of 14 domains, each of which encroaches into adjacent segments. We call these domains parasegments and discuss their developmental significance.

Parasegments and compartments in the Drosophila embryo

The bithorax genes are a group of homoeotic genes of Drosophila whose function is related to the control of segment development. They form a gene complex in which at least seven different loci have been identified1–3. Mutations at each of these loci produce a specific homoeotic transformation whereby a segment is transformed either in part or completely into another3–5. In embryos completely deficient for the bithorax genes, all the thoracic and abdominal segments resemble mesothoracic segments3. This observation, together with the segmental specificity of the mutant phenotypes, led Lewis3 to suggest a model of genetic control in which the type of development in each segment is specified by the activation of a fraction of the total number of bithorax genes. Thus, lack of activity of the bithorax genes results in mesothoracic development, activation of the bithorax (bx+) and postbithorax (pbx+) genes produces metathorax (anterior and posterior respectively), and activation of these two genes with bithoraxoid (bxd+) produces the first abdominal segment and so on. Although this model explains the genetic and developmental data, it leaves unexplained formation of the prothoracic segment. We now describe two unexpected results: deficiencies for bithorax genes can lead to segments being transformed into prothorax, and there is a temporal sequence in the function of the bithorax system.

Sequential functions of the bithorax complex of Drosophila.

Nine X-ray-induced mutations of the bithorax complex (BX-C) have been isolated and characterized. They all show the typical features of the Ultrabithorax mutations. They are homozygous lethal, produce a slight enlargement of the haltere in heterozygous condition and fail to complement the mutations at the bx, bxd and pbx loci. Some of them are associated with chromosomal aberrations in the regions 89E 1-4, where the BX-C lies, while others appear normal cytologically. The effect of six of these mutants in the adult cuticle has been studied, producing mutant marked clones in heterozygous individuals. The clones were generated by X-radiation at two points in development: the blastoderm stage and the second larval period. In all cases mutant clones showed the same phenotype: clones appearing in the dorsal structures transform metathorax and first abdominal segment towards mesothorax. That is the additive effect of bx, bxd and pbx mutations. Clones in the legs, if induced during the larval period, show an effect homologous to that seen in the dorsal structures. However, when produced at blastoderm they show in addition a transformation of the posterior second (mesothoracic) and third (metathoracic) legs into the posterior first (prothoracic) leg. This transformation, named postprothorax ( ppx ) has been recently described for the alleles Ubx 130 and Ubx 1 (Morata & Kerridge, 1981) and appears to be general for the Ubx mutations. It is concluded that the realm of action of the Ubx gene is defined by part of the rflesothoracic segment (posterior second leg compartment) and the entire metathoracic and first abdominal segments.

Developmental effects of some newly induced Ultrabithorax alleles of Drosophila

In Drosophila, genes in the centromere-proximal portion of the bithorax complex (BX-C) have been shown to control the development of the metathorax, and parts of the mesothorax and first abdominal segment. Here, we explore the roles of genes positioned more distally by examining the larval and adult phenotypes caused by a breakpoint and deletion in the middle of the complex. We find that both aberrations affect only abdominal segments, transforming the more anterior segments towards the first abdominal segment, and the remaining segments into a graded series of novel segment types which are partially transformed towards more anterior abdominal segments. Moreover, the adult transformations, which we have observed in somatic clones of mutant cells, are in close accord with the transformations observed in mutant first instar larvae, and appear to be expressed in a cell autonomous fashion. We discuss these results in the light of current views of the organization and function of the complex.

Homeotic transformations of the abdominal segments of Drosophila caused by breaking or deleting a central portion of the bithorax complex

Anatomy and Function of the Bithorax Complex of Drosophila

Of the homeotic mutants known in Drosophila, those of the bithorax system are by far the best known genetically, thanks to the extensive analyses by E.B. Lewis (1963, 1964, 1967, 1978). The interest of their study lies not only in their effect on the development of certain segments (see Morata and Lawrence, 1977) but also in the genetic structure of the system; it has been shown that the bithorax genes are organized in a gene complex which includes several adjacent loci: bithorax (bx), Contrabithorax (Cbx), Ultrabi-thorax (Ubx), bithoraxoid (bxd), postbithorax (pbx), Hyperabdominal (Hab), Ultraabdominal (Uab), and probably others. These genes seem to be under the same control elements. (See Lewis, 1978 for a recent review.) This gene complex is one of the few examples studied and characterized in eukaryotes. In general (but not always) each of these loci is associated with one particular type of homeotic transformation; a specific segment or half-segment is transformed into another.

An Analysis of the Expressivity of Some Bithorax Transformations

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