Chromosomal continuity in the abdominal region of the bithorax complex of Drosophila is not essential for its contribution to metameric identity

We have examined the developmental consequences for larval and imaginal segmental cuticular structure of a chromosomal translocation involving a breakpoint in the abdominal region of the bithorax complex (BX-C). This complex makes an essential contribution to the development of metameric differences in part of the thorax and in all abdominal segments. The breakpoint is proximal to the most distal (iab-7) homeobox, and results in the translocation to the Y chromosome of the Ultrabithorax (Ubx) and abdominal-A (abd-A) domains. The genotype deficient for the distal part of the complex shows normal function for Ubx and abd-A but has a phenotype typical for severe Abd-B mutations. Conversely, the distal fragment retains a segment identity function which must represent a contribution from Abd-B in parasegments 13 and 14; the latter metamere is wild type, indicating that it does not require the contribution of Ubx or abd-A. We also constructed a genotype comprising the proximal fragment of this translocation together with an overlapping distal fragment of the BX-C derived from Df(3R)Ubx109. It therefore contained all sequences of the BX-C though in the abdominal region the abd-A and Abd-B domains were not adjacent to each other in the chromosome. This genotype was phenotypically normal and demonstrates that DNA sequences in the abd-A and Abd-B regions do not require cis-arrangement for their activity.     

Influence of RNAi knockdown for E-complex genes on the silkworm proleg development

Larvae of many holometabolous insects possess abdominal appendages called prolegs. Lepidoptera larvae have prolegs in the segments A3-A6. Functions of Lepidoptera hox genes on these abdominal appendages development is still a controversial issue. In this article, we report the use of double strand RNA (dsRNA)-mediated interference (RNAi) to dissect the function of some hox genes, specifically E-complex genes Ubx, abd-A, and Abd-B, in the ventral appendage development of the Lepidoptera silkworm, Bombyx mori. We found that Ubx RNAi caused leg identity in A1 segment, abd-A RNAi caused severe defect of abdominal prolegs and Abd-B RNAi allowed proleg identity in more posterior abdominal segments. These results confirm that Lepidoptera hox genes Ubx and Abd-B have evolved the repressing function to ventral appendage development, which is similar to those of Drosophila. However, Lepidoptera abd-A might have been modified distinctively during evolution, and has important roles in directing the development of prolegs.     

Transvection in the Iab-5,6,7 Region of the Bithorax Complex of Drosophila: Homology Independent Interactions in Trans

The Abdominal-B (Abd-B) gene of the bithorax complex (BX-C) of Drosophila controls the identities of the fifth through seventh abdominal segments and segments in the genitalia (more precisely, parasegments 10-14). Here we focus on iab-5, iab-6 and iab-7, regulatory regions of Abd-B that control expression in the fifth, sixth and seventh abdominal segments (parasegments 10-12). By analysis of partial BX-C deficiencies, we show that these regions are able to promote fifth and sixth abdominal segment identities in the absence of an Abd-B gene in cis. We establish that this ability does not result from cis-regulation of the adjacent abd-A or Ubx genes of the BX-C but rather occurs because the iab-5,6,7 region is able to interact with Abd-B in trans. We demonstrate that this interaction is proximity dependent and is, therefore, a case of what E. B. LEWIS has called transvection. Interactions of this type are presumably facilitated by the synapsis of homologues that occurs in somatic cells of Dipterans. Although transvection has been detected in a number of Drosophila genes, transvection of the iab-5,6,7 region is exceptional in two ways. First, interaction in trans with Abd-B does not require that homologues share homologous sequences within, or for some distance to either side of, the BX-C. This is the first case of transvection shown to be independent of local synapsis. A second unusual feature of iab-5,6,7 transvection is that it is remarkably difficult to disrupt by heterozygosity for chromosome rearrangements. The lack of requirement for local synapsis and the tenacity of trans-interaction argue that the iab-5,6,7 region can locate and interact with Abd-B over considerable distance. This is consistent with the normal role of iab-5,6,7, which must act over some 20-60 kb to influence its regulatory target in cis at the Abd-B promoter. Evidence is presented that trans-action of iab-5,6,7 requires, and may be mediated by, the region between distal iab-7 and Abd-B. Also, we show that iab-5,6,7 transvection is independent of the allelic state of zeste, a gene that influences several other cases of transvection. The long-range nature of interactions in trans between iab-5,6,7 and Abd-B suggests that similar interactions could operate effectively in organisms lacking extensive somatic pairing. Transvection may, therefore, be of more general significance than previously suspected.     

Interacting insulators from the Drosophila melanogaster bithorax complex can form independent expression domains

Regulatory region of three bithorax complex genes, Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) can be divided into nine iab domains, capable of directing expression of one of the genes in certain abdominal parasegment of Drosophila. In the Abd-B regulatory region, three insulators were identified, including Fab-7 and Fab-8, which flanked the iab-7domain, and Mcp, which separated the Abd-B and abd-A regulatory regions. It was suggested that boundary insulators formed a barrier between active and repressed chromatin. In the present study, using the yellow and white reporter genes and different combinations of known insulators, Mcp, Fab-7, and Fab-8, it was demonstrated that only specific interaction of two insulators was capable of isolation of active and repressed chromatin, i.e., the formation of independent expression domains.     

Modeling the regulation of the bithorax complex in Drosophila melanogaster: the phenotypic effects of Ubx, abd-A and Abd-B heterozygotic larvae, and a homozygous Ubx- abd A hybrid gene

As an intermediate step in the development of a defined quantitative model of pattern formation during Drosophila segmentation, we present here a model capable of predicting the experimentally determined levels of gene activity and their phenotypic consequences. In its present form, the model includes only four genes: the three genes of the bithorax complex (Ubx, abd-A and Abd-B) and Antennapedia. It is shown that the model is quite robust, predicting many properties in the behavior of these genes. A previously undescribed property is that all of these genes should phenotypically exhibit some kind of haploinsufficiency when present in only a single dose in the genetic background of the animal. This is shown both by the model and by a new method of quantitatively analyzing the differences in the more obvious cuticular features of the larvae, i.e., the patterns in the ventral denticle belts. The model is also capable of dealing with a complicated genetic situation, a hybrid gene of Ubx and abd-A produced by the C1 deletion.     

Dissecting the regulatory landscape of the Abd-B gene of the bithorax complex

The three homeotic genes of the bithorax complex (BX-C), Ubx, abd-A and Abd-B control the identity of the posterior thorax and all abdominal segments. Large segment-specific cis-regulatory regions control the expression of Ubx, abd-A or Abd-B in each of the segments. These segment-specific cis-regulatory regions span the whole 300 kb of the BX-C and are arranged on the chromosome in the same order as the segments they specify. Experiments with lacZ reporter constructs revealed the existence of several types of regulatory elements in each of the cis-regulatory regions. These include initiation elements, maintenance elements, cell type- or tissue-specific enhancers, chromatin insulators and the promoter targeting sequence. In this paper, we extend the analysis of regulatory elements within the BX-C by describing a series of internal deficiencies that affect the Abd-B regulatory region. Many of the elements uncovered by these deficiencies are further verified in transgenic reporter assays. Our results highlight four key features of the iab-5, iab-6 and iab-7 cis-regulatory region of Abd-B. First, the whole Abd-B region is modular by nature and can be divided into discrete functional domains. Second, each domain seems to control specifically the level of Abd-B expression in only one parasegment. Third, each domain is itself modular and made up of a similar set of definable regulatory elements. And finally, the activity of each domain is absolutely dependent on the presence of an initiator element.     

Ectopic expression of homeotic genes caused by the elimination of the Polycomb gene in Drosophila imaginal epidermis

The morphological patterns in the adult cuticle of Drosophila are determined principally by the homeotic genes of the bithorax and Antennapedia complexes. We find that many of these genes become indiscriminately active in the adult epidermis when the Pc gene is eliminated. By using the Pc3 mutation and various BX-C mutant combinations, we have generated clones of imaginal cells possessing different combinations of active homeotic genes. We find that, in the absence of BX-C genes, Pc- clones develop prothoracic patterns; this is probably due to the activity of Sex combs reduced which overrules Antennapedia. Adding contributions of Ultrabithorax, abdominal-A and Abdominal-B results in thoracic or abdominal patterns. We have established a hierarchical order among these genes: Antp less than Scr less than Ubx less than abd-A less than Abd-B. In addition, we show that the engrailed gene is ectopically active in Pc- imaginal cells.     

Double and triple mutant combinations of bithorax complex of Drosophila

We have constructed double and triple mutant combinations for the Ubx, abd-A and Abd-B genes of the bithorax complex and have examined the homeotic transformations they produce in the larval and adult patterns. Embryos hemizygous for the triple combination exhibit a metameric pattern consisting of parasegments 5-12 being transformed into parasegment 4. In addition, parasegment 13 develops like a mixture of parasegment 3 and 4, and parasegment 14 is abnormal. The same phenotype is displayed by embryos homozygous for DfP9, lacking all the BX-C DNA, >300 kb. This result strongly supports the notion that the BX-C contains only three genes which account for all the developmental functions of the complex. The phenotypes of the different double combinations also support the same view; the Ubx abd-a comthoracic and several abdominal functions. The abd-A Abd-B combination exhibits the same phenotype of DpP10 DfP9, lacking all the abdominal functions except those specific for A1. Our results also indicate that each BX-C gene becomes active autonomously regardless of the presence or functional state of the other BX-C genes.     

Control of the expression of the bithorax complex genes abdominal-A and abdominal-B by cis-regulatory regions in Drosophila embryos.

The abdominal-A (abd-A) and Abdominal-B (Abd-B) genes of the bithorax complex (BX-C) specify the identity of most of the Drosophila abdomen. Six different classes of infraabdominal (iab) mutations within the BX-C transform a subset of the parasegments affected by the lack of these two genes. It is thought that these mutations define parasegmental cis-regulatory regions that control the expression of abd-A and Abd-B. By staining embryos mutant for different iab mutations with anti-abd-A and anti-Abd-B antibodies I show here that the expression of Abd-B (and probably also abd-A) exhibit a parasegmental regulation. I have also studied the significance of the chromosomal order of parasegmental iab regulatory sequences, and the possible presence of chromosomal 'boundaries' between them, by looking at the expression of abd-A and Abd-B in embryos carrying the Uab and Mcp mutations. These data are discussed in the light of models of parasegmental-specific regulatory regions within the BX-C.     

Quantitative analysis of ventral denticular patterns of Drosophila melanogaster larvae and the regulation of the bithorax complex

A quantitative model of the effect of the bithorax complex on segmentation is presented which could explain the known data of the spatiotemporal regulation of key gene complex during early Drosophila development, in relation to their effects on some of the segmentation landmarks. The model tries to put together the two different genetic levels, the genotypic and the phenotypic. At the genotypic level, a minimal cross-regulatory network of the different genes involved, Antp, Ubx, abd-A and Abd-B which explains the reported levels of expressions of these genes. At the phenotypic level, the pattern of the ventral denticle belts across the larva which are characteristics of the different segments have been compared by calculating a value of the degree of similarity in the case of the wild-type and several mutant combinations. Finally the two parts of the model are combined, showing that a satisfactory agreement between the two can be achieved. Therefore, this work is a first attempt to develop a method which will provide an explanatory solution of the old question in morphogenesis of how the phenotype is directed by the genotype of a cell or organism.