Structure and function of the bithorax complex genes of Drosophila

The bithorax complex consists of three genes, Ubx, abd-A and Abd-B, which together specify the characteristic development of parasegments 5 to 13 of Drosophila. These genes are structurally homologous; they are of similar size, are transcribed in the same orientation and they all have a homeobox near the 3' end of their transcription unit. Genetic and molecular analyses of Ubx suggest that the gene contains one transcription unit encoding the protein products and at least three cis-regulatory regions. Two of these, abx and bxd, promote the activity of the Ubx transcription unit to the levels appropriate for parasegments 5 and 6, respectively. A third regulatory element, called Cbx-like, prevents the expression of Ubx anterior to parasegment 5. The gene abd-A is not as well known, but genetic and molecular studies indicate at least one cis-regulatory region downstream of the 3' end of the transcription unit. In the gene Abd-B there are two distinct trans-acting elements, called m and r. The m element is a conventional homeotic function, which specifies the identity of parasegments 10 to 13. The r element is specific for parasegment 14 where it suppresses a number of homeotic functions (including m). Molecular analysis indicates that Abd-B contains two transcription units with a common 3' end which correspond to the m and r elements defined genetically.

Abd-B expression in Porcellio scaber Latreille, 1804 (Isopoda: Crustacea): conserved pattern versus novel roles in development and evolution

The Hox genes are intimately involved in patterning the animal body during development and are considered to have had a pivotal role in the evolution of different body plans among the metazoans. From this perspective, crustaceans, a group that has evolved an extreme diversity of body structures, represent a choice group in which to study the evolution of these genes and their expression. The expression of one of these genes, Abdominal-B (Abd-B), has only been studied in two distantly related crustaceans, Artemia and Sacculina, where it shows dissimilar patterns, highly differentiated from the one described in other arthropods. Moreover, we have no information for the Malacostraca. Thus, we cloned the gene Abd-B and followed its expression through development by in situ hybridization in the isopod Porcellio scaber. We found a highly dynamic expression pattern of PsAbd-B during embryonic development. In early stages, it is expressed in the posterior-most part of the germ band, in a domain common to several arthropods studied to date, and later it is expressed in the developing limb buds of the pleon and still later in the endopodites of the third to fifth pleopodites. This raises the interesting possibility of the involvement of this gene in the later respiratory specialization of these appendages. In association with the above expression domain, Abd-B appears to be expressed in later stages also in the ventral ectoderm, raising the further suggestion of its possible involvement in patterning the developing nervous system. Moreover, we show that the first pleopod and the endopodite of the second pleopod, whereas present as limb buds in early embryonic stages, are later reduced and actually absent in the first postembryonic stage, although they reappear again in adults. These appendages thus represent an example of Lazarus appendages. Our data show strong plasticity in the use of a key developmental gene and point out the necessity of further research that may end with a revision of the current understanding of its role in animal evolution.

Hox genes and the evolution of the arthropod body plan

In recent years researchers have analyzed the expression patterns of the Hox genes in a multitude of arthropod species, with the hope of understanding the mechanisms at work in the evolution of the arthropod body plan. Now, with Hox expression data representing all four major groups of arthropods (chelicerates, myriapods, crustaceans, and insects), it seems appropriate to summarize the results and take stock of what has been learned. In this review we summarize the expression and functional data regarding the 10 arthropod Hox genes: labial proboscipedia, Hox3/zen, Deformed, Sex combs reduced, fushi tarazu, Antennapedia, Ultrabithorax, abdominal-A, and Abdominal-B. In addition, we discuss mechanisms of developmental evolutionary change thought to be important for the emergence of novel morphological features within the arthropods.

Developmental consequences of unrestricted expression of the abd-A gene of Drosophila

The abdominal-A (abd-A) gene of Drosophila specifies abdominal segments two through eight (A2-A8). We have found that the uniform expression of the abd-A protein under heat shock control transforms embryonic segments anterior to the abd-A domain into an abdominal segment of the A2-A6 type. Posterior abdominal segments and telson undergo little or no transformation, that is, the abd-A product is phenotypically suppressed posterior to its realm of expression. The comparison of wildtype embryos with embryos carrying the heat shock-abd-A construct but no abd-A endogenous product indicate that some elements of the pattern-like shape of denticle belts or ventral pits depend on the amount of abd-A protein. The concurrent expression of abd-A and other homeotic genes suggest competition for common downstream genes. This competition has been studied by looking at the expression on the visceral mesoderm of a gene downstream of abd-A, decapentaplegic, when two homeoproteins with opposing effects on its transcription are simultaneously induced.

Multiple HOM-C gene interactions specify cell fates in the nematode central nervous system

Intricate patterns of overlapping HOM-C gene expression along the A/P axis have been observed in many organisms; however, the significance of these patterns in establishing the ultimate fates of individual cells is not well understood. We have examined the expression of the Caenorhabditis elegans Antennapedia homolog mab-5 and its role in specifying cell fates in the posterior of the ventral nerve cord. We find that the pattern of fates specified by mab-5 not only depends on mab-5 expression but also on post-translational interactions with the neighboring HOM-C gene lin-39 and a second, inferred gene activity. Where mab-5 expression overlaps with lin-39 activity, they can interact in two different ways depending on the cell type: They can either effectively neutralize one another where they are both expressed or lin-39 can predominate over mab-5. As observed for Antennapedia in Drosophila, expression of mab-5 itself is repressed by the next most posterior HOM-C gene, egl-5. Thus, a surprising diversity in HOM-C regulatory mechanisms exists within a small set of cells even in a simple organism.

Abdominal-B protein isoforms exhibit distinct cuticular transformations and regulatory activities when ectopically expressed in Drosophila embryos

The Drosophila homeotic gene Abdominal-B includes two genetically distinct elements, a morphogenetic (m) activity and a regulatory (r) activity. The proteins responsible for these activities were ectopically expressed in fly embryos. The larval cuticular transformations which result are consistent with the genetically defined role of each protein during normal embryogenesis. Both ABD-B proteins activate ectopic expression of transcripts encoding the m protein, but the levels of Antennapedia, Ultrabithorax and abdominal-A transcripts are differentially repressed. A structural and functional comparison of the ABD-B proteins and a chimeric DFD/ABD-B protein reaffirms that target specificity is largely determined by the homeodomain region and suggests protein domains outside of the homeodomain influence the activation or repression of target gene expression.

Functional analysis of the mouse homeobox gene HoxB9 in Drosophila development

Mammalian genomes contain clusters of homeobox genes (Hox-C, HOX-C) which are structurally similar to the homeotic genes of the Drosophila HOM complex. One method for assessing the functional similarity of particular Drosophila HOM and mammalian Hox genes is to test the ability of Hox genes to induce homeotic phenotypes when expressed in developing Drosophila. Here we describe such functional tests using mouse HoxB9 (formerly Hox-2.5), whose closest structural relative in Drosophila is Abdominal-B. When expressed from a heat shock promoter, HoxB9 induces transformations of head towards more posterior identities in Drosophila larvae and adults. These transformations share some similarities with the phenotypic effects produced by ectopically expressed Abdominal-B, but are also similar to the transformations induced by Antennapedia and mouse HoxB6 (Hox-2.2), suggesting that HoxB9 specifies a positional identity that is intermediate between Antennapedia and Abdominal-B.

The regulation of empty spiracles by Abdominal-B mediates an abdominal segment identity function

The empty spiracles (ems) homeo box gene is required for the development of the Drosophila larval filzkörper, which are structural specializations of the eighth abdominal segment. Filzkörper development is also dependent on the function of the homeotic selector gene Abdominal-B (Abd-B). Here, we show that ems is a downstream gene that is transcriptionally regulated by Abd-B proteins. This regulation is mediated by an Abd-B-dependent ems cis-regulatory element that in early- to mid-stage embryos is activated only in the eighth abdominal segment. Genetic epistasis tests suggest that both ems and Abd-B are required in combination for the specification of the filzkörper primordia. In a general sense, these results also provide evidence that the hierarchical level immediately downstream of the homeotic genes contains additional homeo domain transcription factors that define subsegmental domain identities.

Ectopic expression of UBX and ABD-B proteins during Drosophila embryogenesis: competition, not a functional hierarchy, explains phenotypic suppression

The Abdominal-B (Abd-B) gene, a member of the bithorax complex (BX-C), specifies the identities of parasegments (PS) 10–14 in Drosophila. Abd-B codes for two structurally related homeodomain proteins, ABD-B m and ABD-B r, that are expressed in PS10-13 and PS14-15, respectively. Although ABD-B m and r proteins have distinct developmental functions, ectopic expression of either protein during embryogenesis induces the development of filzkorper and associated spiracular hairs, structures normally located in PS13, at ectopic sites in the larval thorax and abdomen. These results suggest that other parasegmental differences contribute to the phenotype specified by ABD-B r activity in PS14. Both ABD-B m and r repress the expression of other homeotic genes, such as Ubx and abd-A, in PS10-14. However, the importance of these and other cross-regulatory interactions among homeotic genes has been questioned. Since ectopic UBX protein apparently failed to transform abdominal segments, Gonzalez-Reyes et al. (Gonzalez-Reyes, A., Urquia, N., Gehring, W.J., Struhl, G. and Morata, G. (1990). Nature 344, 78–80) proposed a functional hierarchy in which ABD-A and ABD-B activities override UBX activity. We tested this model by expressing UBX and ABD-B m proteins ectopically in wild-type and BX-C-deficient embryos. Ectopic ABD-B m does not prevent transformations induced by ectopic UBX. Instead, ectopic UBX and ABD-B m proteins compete for the specification of segmental identities in a dose-dependent fashion. Our results support a quantitative competition among the homeotic proteins rather than the existence of a strict functional hierarchy. Therefore, we suggest that cross-regulatory interactions are not irrelevant but are important for repressing the expression of competing homeotic proteins. To explain the apparent failure of ectopic UBX to transform the abdominal segments, we expressed UBX at different times during embryonic development. Our results show that ectopic UBX affects abdominal cuticular identities if expressed during early stages of embryogenesis. In later embryonic stages, abdominal segments become resistant to transformation by ectopic UBX while thoracic segments remain susceptible. Head segments also show a similar stage-dependent susceptibility to transformation by ectopic UBX in early embryogenesis but become resistant in later stages. These results suggest that abdominal and head identities are determined earlier than are thoracic identities.

Functional dissection of Drosophila abdominal-B protein

The Abdominal-B gene is unique among homeotic Drosophila genes as it encodes two proteins m and r which confer different functions. The m protein corresponds to an r protein with a large N-terminal extension; the two proteins contain the same homeodomain. We have used a transient co-transfection assay, based on HeLa cells, to analyse the intrinsic function of m and r protein in activating transcription. We find two strong transcriptional activation domains in the common part of the two proteins. The m-specific exon contains additional transcriptional activation potential. Despite this, the m protein is a weaker transcriptional activator than the r protein. Apparently, there are inhibitory sequences in the m-specific exon which, in the embryo, may have a role in masking r function in the intact m protein.

Altering the regulatory targets of the Deformed protein in Drosophila embryos by substituting the Abdominal-B homeodomain

The homeotic selector genes of Drosophila melanogaster encode transcriptional regulatory proteins that control the determination of different segmental fates. Binding of selector proteins to regulatory DNA sequences is mediated by an evolutionary conserved protein domain, the homeodomain. Although homeodomains encoded by the selector genes are very similar in their amino acid sequence and in vitro DNA-binding properties, here we provide additional evidence that the homeodomain is responsible for most of the regulatory specificity of the entire protein. A heat-shock promoter/selector gene was constructed that encodes a Deformed/Abdominal-B chimera in which the Abdominal-B homeodomain is substituted for that of Deformed. Expression of this chimeric protein throughout the embryo causes morphological transformation of anterior segments toward more posterior identities. A number of other homeotic selector genes, all normally repressed by Abdominal-B, are ectopically activated by the chimeric protein. These results support the hypothesis that the target specificity of similar homeodomain proteins is largely determined by the amino acid sequence of the homeodomain.

The molecular genetics of the bithorax complex of Drosophila: characterization of the products of the Abdominal-B domain

In Drosophila the Abdominal-B (Abd-B) domain of the bithorax complex specifies the identities of several posterior abdominal segments, comprises homeo-protein-coding regions and cis-regulatory regions, and extends from infra-abdominal-5 (iab-5) to iab-8, inclusive. Mutations that eliminate the Abd-B domain act as late embryonic lethals and result in transformations of posterior abdominal segments toward more anterior ones. The Abd-B domain gives rise to a minimum of five homeo-box-containing transcripts, 7.8, 4.7, 4.3, 3.7, and 3.3 kb in length. We examined the structure of the Abd-B domain by sequencing two Abd-B cDNA clones derived from the 4.3- and the 4.7-kb transcripts and the corresponding genomic DNA. The domain spans approximately 100 kb and contains at least eight exons. The 4.7- and 4.3-kb transcripts contain an open reading frame capable of encoding a 54-kD protein. A portion of the deduced protein-coding sequence common to all of the Abd-B transcripts was cloned into an expression vector. The resultant fusion protein then was used to derive a monoclonal antibody specific to Abd-B. By use of that antibody, we identified two embryonic Abd-B proteins, 54 and 36 kD and determined the sum of their segmental distribution by immunohistochemical analysis of whole-mounted embryos and immunofluorescent analysis of dissected embryonic nervous systems. The proteins are distributed in the fourth to the ninth abdominal segments [parasegments (PS) 10-15] inclusive. Embryos homozygous for Polycomb (Pc) show labeling over almost the entire embryo, whereas embryos deficient for the Abd-B domain show no detectable labeling.

Drosophila homoeotic genes encode transcriptional activators similar to mammalian OTF-2

Homoeotic genes in Drosophila melanogaster are active in spatially restricted metameric domains and control the morphogenesis of segment-specific features such as legs or wings within these domains. They exert their function, according to the 'selector gene' hypothesis, by regulating the expression of subordinate genes. Homoeotic genes also control their own expression and the expression of each other. The proteins encoded by these genes contain a domain, called a homoeodomain, that is strongly conserved, and that shows homologies to proteins that bind DNA and regulate transcription. Homoeoproteins have been shown to bind specific DNA sequences. We show here that the Drosophila homoeotic genes Ultrabithorax (Ubx) and Abdominal-B (Abd-B) code for proteins that are capable of activating transcription of reporter genes linked to specific cis-regulatory target sequences in transfected mammalian cells. Their activity, as well as their target specificity, is similar to that of a mammalian lymphoid-specific octamer transcription factor, OTF-2, which was recently found to contain a homoeodomain.