Regulation of proboscipedia in Drosophila by homeotic selector genes

Food hardness preference reveals multisensory contributions of fly larval gustatory organs in behaviour and physiology

Food presents a multisensory experience, with visual, taste, and olfactory cues being important in allowing an animal to determine the safety and nutritional value of a given substance. Texture, however, remains a surprisingly unexplored aspect, despite providing key information about the state of the food through properties such as hardness, liquidity, and granularity. Food perception is achieved by specialised sensory neurons, which themselves are defined by the receptor genes they express. While it was assumed that sensory neurons respond to one or few closely related stimuli, more recent findings challenge this notion and support evidence that certain sensory neurons are more broadly tuned. In the Drosophila taste system, gustatory neurons respond to cues of opposing hedonic valence or to olfactory cues. Here, we identified that larvae ingest and navigate towards specific food substrate hardnesses and probed the role of gustatory organs in this behaviour. By developing a genetic tool targeting specifically gustatory organs, we show that these organs are major contributors for evaluation of food hardness and ingestion decision-making. We find that ablation of gustatory organs not only results in loss of chemosensation, but also navigation and ingestion preference to varied substrate hardnesses. Furthermore, we show that certain neurons in the primary taste organ exhibit varied and concurrent physiological responses to mechanical and multimodal stimulation. We show that individual neurons house independent mechanisms for multiple sensory modalities, challenging assumptions about capabilities of sensory neurons. We propose that further investigations, across the animal kingdom, may reveal higher sensory complexity than currently anticipated.

Anterior Hox Genes and the Process of Cephalization

During evolution, bilateral animals have experienced a progressive process of cephalization with the anterior concentration of nervous tissue, sensory organs and the appearance of dedicated feeding structures surrounding the mouth. Cephalization has been achieved by the specialization of the unsegmented anterior end of the body (the acron) and the sequential recruitment to the head of adjacent anterior segments. Here we review the key developmental contribution of Hox1-5 genes to the formation of cephalic structures in vertebrates and arthropods and discuss how this evolved. The appearance of Hox cephalic genes preceded the evolution of a highly specialized head in both groups, indicating that Hox gene involvement in the control of cephalic structures was acquired independently during the evolution of vertebrates and invertebrates to regulate the genes required for head innovation.

Insulator foci distance correlates with cellular and nuclear morphology in early Drosophila embryos

The three-dimensional (3D) organization of the genome is highly dynamic, changing during development and varying across different tissues and cell types. Recent studies indicate that these changes alter regulatory interactions, leading to changes in gene expression. Despite its importance, the mechanisms that influence genomic organization remain poorly understood. We have previously identified a network of chromatin boundary elements, or insulators, in the Drosophila Antennapedia homeotic complex (ANT-C). These genomic elements interact with one another to tether chromatin loops that could block or promote enhancer-promoter interactions. To understand the function of these insulators, we assessed their interactions by measuring their 3D nuclear distance in developing animal tissues. Our data suggest that the ANT-C Hox complex might be in a folded or looped configuration rather than in a random or extended form. The architecture of the ANT-C complex, as read out by the pair-wise distance between insulators, undergoes a strong compression during late embryogenesis, coinciding with the reduction of cell and nuclear diameters due to continued cell divisions in post-cleavage cells. Our results suggest that genomic architecture and gene regulation may be influenced by cellular morphology and movement during development.

Disparate expression specificities coded by a shared Hox-C enhancer

Can a single regulatory sequence be shared by two genes undergoing functional divergence? Here we describe a single promiscuous enhancer within the Drosophila Antennapedia Complex, EO053, that directs aspects of the expression of two adjacent genes, pb (a Hox2 ortholog) and zen2 (a divergent Hox3 paralog), with disparate spatial and temporal expression patterns. We were unable to separate the pb-like and zen2-like specificities within EO053, and we identify sequences affecting both expression patterns. Importantly, genomic deletion experiments demonstrate that EO053 cooperates with additional pb- and zen2-specific enhancers to regulate the mRNA expression of both genes. We examine sequence conservation of EO053 within the Schizophora, and show that patterns of synteny between the Hox2 and Hox3 orthologs in Arthropods are consistent with a shared regulatory relationship extending prior to the Hox3/zen divergence. Thus, EO053 represents an example of two genes having evolved disparate outputs while utilizing this shared regulatory region.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Cap'n'collar differentiates the mandible from the maxilla in the beetle Tribolium castaneum

BACKGROUND

The biting mandible of the arthropods is thought to have evolved in the ancestor of the insects, crustaceans and myriapods: the Mandibulata. A unique origin suggests a common set of developmental genes will be required to pattern the mandible in different arthropods. To date we have functional studies on patterning of the mandibular segment of Drosophila melanogaster showing in particular the effects of the gene cap'n'collar (cnc), however, the dipteran head is far from representative of insects or of more distantly related mandibulates; Drosophila does not even possess a mandibular appendage. To study the development of a more representative insect mandible, we chose the red flour beetle Tribolium castaneum and investigated the function of the Tribolium orthologs of cap'n'collar (Tc-cnc) and the Hox gene Deformed (Tc-Dfd). In order to determine the function of Tc-cnc and Tc-Dfd, transcripts were knocked down by maternal RNA interference (RNAi). The effects of gene knockdown were examined in the developing embryos and larvae. The effect of Tc-cnc and Tc-Dfd knockdown on the expression of other genes was determined by using in situ hybridization on Tribolium embryos.

RESULTS

Our analyses show that Tc-cnc is required for specification of the identity of the mandibular segment of Tribolium and differentiates the mandible from maxillary identity. Loss of Tc-cnc function results in a transformation of the mandible to maxillary identity as well as deletion of the labrum. Tc-Dfd and the Tribolium homolog of proboscipedia (Tc-mxp = maxillopedia), Hox genes that are required to pattern the maxillary appendage, are expressed in a maxilla-like manner in the transformed mandible. Tribolium homologs of paired (Tc-prd) and Distal-less (Tc-Dll) that are expressed in the endites and telopodites of embryonic appendages are also expressed in a maxilla-like manner in the transformed mandible.We also show that Tc-Dfd is required to activate the collar of Tc-cnc expression in the mandibular segment but not the cap expression in the labrum. Tc-Dfd is also required for the activation of Tc-prd in the endites of the mandible and maxillary appendages.

CONCLUSIONS

Tc-cnc is necessary for patterning the mandibular segment of Tribolium. Together, Tc-cnc and Tc-Dfd cooperate to specify mandibular identity, as in Drosophila. Expression patterns of the homologs of cnc and Dfd are conserved in mandibulate arthropods suggesting that the mandible specifying function of cnc is likely to be conserved across the mandibulate arthropods.

Global posterior prevalence is unique to vertebrates: a dance to the music of time?

We reach the conclusion that posterior prevalence, a collinear property considered important for Hox complex function, is so far unique, in a global form, to vertebrates. Why is this? We suspect this is because posterior prevalence is explicitly connected to the vertebrate form of Hox temporal collinearity, which is central to axial patterning.

Zebrafish Tshz3b negatively regulates Hox function in the developing hindbrain

In flies, the zinc-finger protein Teashirt promotes trunk segmental identities, in part, by repressing the expression and function of anterior hox paralog group (PG) 1-4 genes that specify head fates. Anterior-posterior patterning of the vertebrate hindbrain also requires Hox PG 1-4 function, but the role of vertebrate teashirt-related genes in this process has not been investigated. In this work, we use overexpression and structure-function analyses to show that zebrafish tshz3b antagonizes Hox-dependent hindbrain segmentation. Ectopic Tshz3b perturbs the specification of rhombomere identities and leads to the caudal expansion of r1, the only rhombomere whose identity is specified independently of Hox function. This overexpression phenotype does not require the homeodomain and C-terminal zinc fingers that are unique to vertebrate Teashirt-related proteins, but does require that Tshz3b function as a repressor. Together, these results argue that the negative regulation of Hox PG 1-4 function is a conserved characteristic of Teashirt-related proteins.

The Drosophila gap gene giant has an anterior segment identity function mediated through disconnected and teashirt

The C2H2 zinc-finger-containing transcription factors encoded by the disconnected (disco) and teashirt (tsh) genes contribute to the regionalization of the Drosophila embryo by establishing fields in which specific Homeotic complex (Hom-C) proteins can function. In Drosophila embryos, disco and the paralogous disco-related (disco-r) are expressed throughout most of the epidermis of the head segments, but only in small patches in the trunk segments. Conversely, tsh is expressed extensively in the trunk segments, with little or no accumulation in the head segments. Little is known about the regulation of these genes; for example, what limits their expression to these domains? Here, we report the regulatory effects of gap genes on the spatial expression of disco, disco-r, and tsh during Drosophila embryogenesis. The data shed new light on how mutations in giant (gt) affect patterning within the anterior gt domain, demonstrating homeotic function in this domain. However, the homeosis does not occur through altered expression of the Hom-C genes but through changes in the regulation of disco and tsh.

Comparative analysis of Hox downstream genes in Drosophila

Functional diversification of body parts is dependent on the formation of specialized structures along the various body axes. In animals, region-specific morphogenesis along the anteroposterior axis is controlled by a group of conserved transcription factors encoded by the Hox genes. Although it has long been assumed that Hox proteins carry out their function by regulating distinct sets of downstream genes, only a small number of such genes have been found, with very few having direct roles in controlling cellular behavior. We have quantitatively identified hundreds of Hox downstream genes in Drosophila by microarray analysis, and validated many of them by in situ hybridizations on loss- and gain-of-function mutants. One important finding is that Hox proteins, despite their similar DNA-binding properties in vitro, have highly specific effects on the transcriptome in vivo, because expression of many downstream genes respond primarily to a single Hox protein. In addition, a large fraction of downstream genes encodes realizator functions, which directly affect morphogenetic processes, such as orientation and rate of cell divisions, cell-cell adhesion and communication, cell shape and migration, or cell death. Focusing on these realizators, we provide a framework for the morphogenesis of the maxillary segment. As the genomic organization of Hox genes and the interaction of Hox proteins with specific co-factors are conserved in vertebrates and invertebrates, and similar classes of downstream genes are regulated by Hox proteins across the metazoan phylogeny, our findings represent a first step toward a mechanistic understanding of morphological diversification within a species as well as between species.

Chromosomal distribution of PcG proteins during Drosophila development

Polycomb group (PcG) proteins are able to maintain the memory of silent transcriptional states of homeotic genes throughout development. In Drosophila, they form multimeric complexes that bind to specific DNA regulatory elements named PcG response elements (PREs). To date, few PREs have been identified and the chromosomal distribution of PcG proteins during development is unknown. We used chromatin immunoprecipitation (ChIP) with genomic tiling path microarrays to analyze the binding profile of the PcG proteins Polycomb (PC) and Polyhomeotic (PH) across 10 Mb of euchromatin. We also analyzed the distribution of GAGA factor (GAF), a sequence-specific DNA binding protein that is found at most previously identified PREs. Our data show that PC and PH often bind to clustered regions within large loci that encode transcription factors which play multiple roles in developmental patterning and in the regulation of cell proliferation. GAF co-localizes with PC and PH to a limited extent, suggesting that GAF is not a necessary component of chromatin at PREs. Finally, the chromosome-association profile of PC and PH changes during development, suggesting that the function of these proteins in the regulation of some of their target genes might be more dynamic than previously anticipated.

Requirement of Abdominal-A and Abdominal-B in the developing genitalia of Drosophila breaks the posterior downregulation rule

The genitalia of Drosophila derive from the genital disc and require the activity of the Abdominal-B (Abd-B) Hox gene. This gene encodes two different proteins, Abd-B M and Abd-B R. We show here that the embryonic genital disc, like the larval genital disc, is formed by cells from the eighth (A8), ninth (A9) and tenth (A10) abdominal segments, which most likely express the Abd-B M, Abd-B R and Caudal products, respectively. Abd-B m is needed for the development of A8 derivatives such as the external and internal female genitalia, the latter also requiring abdominal-A (abd-A), whereas Abd-B r shapes male genitalia (A9 in males). Although Abd-B r represses Abd-B m in the embryo, in at least part of the male A9 such regulation does not occur. In the male A9, some Abd-B m(-)r(-) or Abd-B r(-) clones activate Distal-less and transform part of the genitalia into leg or antenna. In the female A8, many Abd-B m(-)r(-) mutant clones produce similar effects, and also downregulate or eliminate abdominal-A expression. By contrast, although Abd-B m is the main or only Abd-B transcript present in the female A8, Abd-B m(-) clones induced in this primordium do not alter Distal-less or abd-A expression, and transform the A8 segment into the A4. The relationship between Abd-B and abd-A in the female genital disc is opposite to that of the embryonic epidermis, and contravenes the rule that posteriorly expressed Hox genes downregulate more anterior ones.

Hox gene function and interaction in the milkweed bug Oncopeltus fasciatus (Hemiptera)

Studies in genetic model organisms such as Drosophila have demonstrated that the homeotic complex (Hox) genes impart segmental identity during embryogenesis. Comparative studies in a wide range of other insect taxa have shown that the Hox genes are expressed in largely conserved domains along the anterior-posterior body axis, but whether they are performing the same functions in different insects is an open question. Most of the Hox genes have been studied functionally in only a few holometabolous insects that undergo metamorphosis. Thus, it is unclear how the Hox genes are functioning in the majority of direct-developing insects and other arthropods. To address this question, we used a combination of RNAi and in situ hybridization to reveal the expression, functions, and regulatory interactions of the Hox genes in the milkweed bug Oncopeltus fasciatus. Our results reveal many similarities and some interesting differences compared to Drosophila. We find that the gene Antennapedia is required for the identity of all three thoracic segments, while Ultrabithorax, abdominal-A and Abdominal-B cooperate to pattern the abdomen. The three abdominal genes exhibit posterior prevalence like in Drosophila, but apparently via some post-transcriptional mechanism. The functions of the head genes proboscipedia, Deformed, and Sex combs reduced were shown previously, and here we find that the complex temporal expression of pb in the labium is like that of other insects, but its regulatory relationship with Scr is unique. Overall, our data reveal that the evolution of insect Hox genes has included many small changes within general conservation of expression and function, and that the milkweed bug provides a useful model for understanding the roles of Hox genes in a direct-developing insect.

The HOX Gene Cluster in the Bivalve Mollusc Mytilus galloprovincialis

The clustered Hox genes play a central role in the regulation of development in bilaterian animals. In this study, we analyzed the homeobox-containing genes in a bivalve mollusc, the mussel Mytilus galloprovincialis, an unsegmented spiralian lophotrochozoan. We isolated and characterized four Hox cluster genes using the polymerase chain reaction with specific primers. Molecular alignments and phylogenetic analysis indicate that these mussel genes are homologs of the anterior group (pb ortholog), paralog group 3, and central group (PG4/Dfd and PG5/Scr) genes. The putative homeodomain sequences were designated Mgox1, Mgox2, Mgox3, and Mgox4.

An interactive network of zinc-finger proteins contributes to regionalization of the Drosophila embryo and establishes the domains of HOM-C protein function

During animal development, the HOM-C/HOX proteins direct axial patterning by regulating region-specific expression of downstream target genes. Though much is known about these pathways, significant questions remain regarding the mechanisms of specific target gene recognition and regulation, and the role of co-factors. From our studies of the gnathal and trunk-specification proteins Disconnected (DISCO) and Teashirt (TSH), respectively, we present evidence for a network of zinc-finger transcription factors that regionalize the Drosophila embryo. Not only do these proteins establish specific regions within the embryo, but their distribution also establishes where specific HOM-C proteins can function. In this manner, these factors function in parallel to the HOM-C proteins during axial specification. We also show that in tsh mutants, disco is expressed in the trunk segments, probably explaining the partial trunk to head transformation reported in these mutants, but more importantly demonstrating interactions between members of this regionalization network. We conclude that a combination of regionalizing factors, in concert with the HOM-C proteins,promotes the specification of individual segment identity.

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.

The Anopheles genome and comparative insect genomics

The Anopheles gambiae genome sequence, coupled with the Drosophila melanogaster genome sequence, provides a better understanding of the insects, a group that contains our friends, foes, and competitors.

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

The development and evolution of insect mouthparts as revealed by the expression patterns of gnathocephalic genes

To understand better both the development and evolution of insect mouthparts, we have compared the expression pattern of several developmentally important genes in insects with either mandibulate or stylate-haustellate mouthparts. Specifically, we examined the expression of the proboscipedia (pb) and Distal-less (Dll) gene products as well as three regulators of pb, Sex combs reduced (Scr), Deformed (Dfd), and cap 'n' collar (cnc). These genes are known to control the identity of cells in the gnathal segments of Drosophila melanogaster and would appear to have similar conserved functions in other insects. Together we have made an atlas of gene expression in the heads of three insects: Thermobia domestica and Acheta domestica, which likely exemplify the mandibulate mouthparts present in the common insect ancestor, and Oncopeltus fasciatus, which has piercing-sucking mouth parts that are typical of the Hemiptera. At the earliest stages of embryogenesis, only the expression of pb was found to differ dramatically between Oncopeltus and the other insects examined, although significant differences were observed later in development. This difference in pb expression reflects an apparent divergence in the specification of gnathal identity between mandibulate and stylate-haustellate mouthparts, which may result from a "phylogenetic homeosis" that occurred during the evolution of the Hemiptera.

Interactions of the Tribolium Sex combs reduced and proboscipedia orthologs in embryonic labial development

The role of Hox genes in the development of insect gnathal appendages has been examined in three insects: the fruitfly, Drosophila melanogaster; the milkweed bug, Oncopeltus fasciatus; and the red flour beetle, Tribolium castaneum. In each of these organisms, the identity of the labium depends on the homeotic genes Sex combs reduced (Scr) and proboscipedia (pb). Loss of pb function in each of the three insects results in homeotic transformation of the labial appendages to legs. In contrast, loss of Scr function yields a different transformation in each species. Interestingly, mutations in Cephalothorax (Cx), the Tribolium ortholog of Scr, transform the labial appendages to antennae, a result seen in the other insects only when both pb and Scr are removed. We show here that the Tribolium labial appendages also develop as antennae in double mutants. Further, we demonstrate that expression of the Tribolium proboscipedia ortholog maxillopedia (mxp) is greatly reduced or absent in the labium of Cx mutant larvae. Thus, in the wild-type labial segment, Cx function is required (directly or indirectly) for mxp transcription. A similar interaction between Scr and pb during Drosophila embryogenesis has been described recently. Thus, this regulatory paradigm appears to be conserved at least within the Holometabola.

TheDrosophilaproboscis is specified by two Hox genes,proboscipediaandSex combs reduced, via repression of leg and antennal appendage genes

The proboscis is one of the most highly modified appendages in Drosophila melanogaster. However, the phenotypes of proboscipedia (pb) mutants, which transform the proboscis into leg or antenna, indicate a basic homology among these limbs. Recent genetic studies have revealed a developmental system for patterning appendages and identified several genes required for limb development. Among these are: extradenticle (exd), homothorax (hth), dachshund (dac), Distal-less (Dll) and spalt (sal). These limb genes have not been well studied in wild-type mouthparts and their role if any in this appendage is not well understood. Here we demonstrate that the homeotic gene products Proboscipedia (Pb) and Sex combs reduced (Scr) regulate the limb genes in the labial disc to give rise to a unique type of appendage, the proboscis. Pb inhibits exd, dac and sal expression and downregulates Dll. This observation explains the ability of Pb to inhibit the effects of ectopically expressed trunk Hox genes in the proboscis, to suppress leg identity in the trunk and to transform antenna to maxillary palp. Scr suppresses sal expression and also downregulates Dll in the labial discs; discs mutant for both pb and Scr give rise to complete antennae, further demonstrating appendage homology. In the labial disc, Pb positively regulates transcription of Scr, whereas in the embryo, Scr positively regulates pb. Additionally, our results suggests a revised fate map of the labial disc. We conclude that the proboscis constitutes a genetically distinct type of appendage whose morphogenesis does not require several important components of leg and/or antennal patterning systems, but retains distal segmental homology with these appendages.

Comparative insect developmental genetics: phenotypes without mutants

The last decade has seen a dramatic increase in interest in the extent to which morphological evolution depends on changes in regulatory pathways. Insects provide a fertile ground for study because of their diversity and our high level of understanding of the genetic regulation of development in Drosophila melanogaster. However, comparable genetic approaches are presently possible in only a small number of non-Drosophilid insects. In a recent paper, Hughes and Kaufman have used a new methodology, RNA interference, in the milkweed bug, Oncopeltus fasciatus, to phenocopy the effects of mutations in Hox genes. RNA interference involves the injection of double-stranded RNA of the same sequence as the relevant mRNA resulting in a depletion of that transcript. Hughes and Kaufman focused on the gnathal segments, which elaborate specialized appendages important to feeding. Their results indicate that gnathal adaptations in this bug are correlated with changes in Hox gene functions and interactions.