The Short antennae gene of Tribolium is required for limb development and encodes the orthologue of the Drosophila Distal-less protein

The genome of the model beetle and pest Tribolium castaneum

Tribolium castaneum is a member of the most species-rich eukaryotic order, a powerful model organism for the study of generalized insect development, and an important pest of stored agricultural products. We describe its genome sequence here. This omnivorous beetle has evolved the ability to interact with a diverse chemical environment, as shown by large expansions in odorant and gustatory receptors, as well as P450 and other detoxification enzymes. Development in Tribolium is more representative of other insects than is Drosophila, a fact reflected in gene content and function. For example, Tribolium has retained more ancestral genes involved in cell-cell communication than Drosophila, some being expressed in the growth zone crucial for axial elongation in short-germ development. Systemic RNA interference in T. castaneum functions differently from that in Caenorhabditis elegans, but nevertheless offers similar power for the elucidation of gene function and identification of targets for selective insect control.

From development to biodiversity--Tribolium castaneum, an insect model organism for short germband development

Insect embryogenesis is best understood in the fruit fly Drosophila. However, Drosophila embryogenesis shows evolutionary-derived features: anterior patterning is controlled by a highly derived Hox gene bicoid, the body segments form almost simultaneously and appendages develop from imaginal discs. In contrast, embryogenesis of the red flour beetle Tribolium castaneum displays typical features in anterior patterning, axis and limb formation shared with most insects, other arthropods as well as with vertebrates. Anterior patterning depends on the conserved homeobox gene orthodenticle, the main body axis elongates sequentially and limbs grow continuously starting from an appendage bud. Thus, by analysing developmental processes in the beetle at the molecular and cellular level, inferences can be made for similar processes in other arthropods. With the completion of sequencing the Tribolium genome, the door is now open for post-genomic studies such as RNA expression profiling, proteomics and functional genomics to identify beetle-specific gene circuits.

Complex genetic interactions govern the temporal effects of Antennapedia on antenna-to-leg transformations in Drosophila melanogaster

The putative regulatory relationships between Antennapedia (Antp), spalt major (salm) and homothorax (hth) are tested with regard to the sensitive period of antenna-to-leg transformations. Although Antp expression repressed hth as predicted, contrary to expectations, hth did not show increased repression at higher Antp doses, whereas salm, a gene downstream of hth, did show such a dose response. Loss of hth allowed antenna-to-leg transformations but the relative timing of proximal-distal transformations was reversed, relative to transformations induced by ectopic Antp. Finally, overexpression of Hth was only partially able to rescue transformations induced by ectopic Antp. These results indicate that there may be additional molecules involved in antenna/leg identity and that spatial, temporal and dosage relationships are more subtle than suspected and must be part of a robust understanding of molecular network behaviour involved in determining appendage identity in Drosophila melanogaster.

Emerging model systems in evo-devo: horned beetles and the origins of diversity

Horned beetles and beetle horns are emerging as a model system suited to address fundamental questions in evolutionary developmental biology. Here we briefly review the biology of horned beetles and highlight the unusual opportunities they provide for evo-devo research. We then summarize recent advances in the development of new approaches and techniques that are now available to scientists interested in working with these organisms. We end by discussing ways to implement and combine these new approaches to explore new frontiers in evo-devo research previously unavailable to reseachers working outside traditional model organisms.

Molecular patterning mechanism underlying metamorphosis of the thoracic leg in Manduca sexta

The tobacco hornworm Manduca sexta, like many holometabolous insects, makes two versions of its thoracic legs. The simple legs of the larva are formed during embryogenesis, but then are transformed into the more complex adult legs at metamorphosis. To elucidate the molecular patterning mechanism underlying this biphasic development, we examined the expression patterns of five genes known to be involved in patterning the proximal-distal axis in insect legs. In the developing larval leg of Manduca, the early patterning genes Distal-less and Extradenticle are already expressed in patterns comparable to the adult legs of other insects. In contrast, Bric-a-brac and dachshund are expressed in patterns similar to transient patterns observed during early stages of leg development in Drosophila. During metamorphosis of the leg, the two genes finally develop mature expression patterns. Our results are consistent with the hypothesis that the larval leg morphology is produced by a transient arrest in the conserved adult leg patterning process in insects. In addition, we find that, during the adult leg development, some cells in the leg express the patterning genes de novo suggesting that the remodeling of the leg involves changes in the patterning gene regulation.

Gene silencing in the spider mite Tetranychus urticae: dsRNA and siRNA parental silencing of the Distal-less gene

A major prerequisite to understanding the evolution of developmental programs includes an appreciation of gene function in a comparative context. RNA interference (RNAi) represents a powerful method for reverse genetics analysis of gene function. However, RNAi protocols exist for only a handful of arthropod species. To extend functional analysis in basal arthropods, we developed a RNAi protocol for the two-spotted spider mite Tetranychus urticae focusing on Distal-less (Dll), a conserved gene involved in appendage specification in metazoans. First, we describe limb morphogenesis in T. urticae using confocal and scanning electron microscopy. Second, we examine T. urticae Dll (Tu-Dll) mRNA expression patterns and correlate its expression with appendage development. We then show that fluorescently labeled double-stranded RNA (dsRNA) and short interfering RNA (siRNA) molecules injected into the abdomen of adult females are incorporated into the oviposited eggs, suggesting that dsRNA reagents can be systemically distributed in spider mites. Injection of longer dsRNA as well as siRNA induced canonical limb truncation phenotypes as well as the fusion of leg segments. Our data suggest that Dll plays a conserved role in appendage formation in arthropods and that such conserved genes can serve as reliable starting points for the development of functional protocols in nonmodel organisms.

Conservation, innovation, and the evolution of horned beetle diversity

Beetle horns represent an evolutionary novelty exhibiting remarkable diversity above and below the species level. Here, we show that four typical appendage patterning genes, extradenticle (exd), homothorax (hth), dachshund (dac), and Distal-less (Dll) are expressed in the context of the development of sexually dimorphic thoracic horns in three Onthophagus species. At least two of these genes, Dll and hth, exhibited expression patterns consistent with a conservation of patterning function during horn development relative to their known roles in the development of insect legs. exd, hth, and dac expression patterns during horn development were largely invariable across species or sexes within species. In contrast, Dll expression was far more discrete and exhibited consistent differences between sexes and species. Most importantly, differences in location and domain size of Dll expression tightly correlated with the degree to which prepupal horn primordia were retained or resorbed before the final adult molt. Our results lend further support to the hypothesis that the origin of beetle horns relied, at least in part, on the redeployment of already existing developmental mechanisms, such as appendage patterning processes and that changes in the exact location and domain size of Dll expression may represent important modifier mechanisms that modulate horn expression in different species or sexes. If correct, this would imply that certain components of genetic basis of horn development may be able to diversify rapidly within lineages and largely independent of phylogenetic distance. We present a first model that integrates presently available data on the genetic regulation of horn development and diversity.

Insulin signaling and limb-patterning: candidate pathways for the origin and evolutionary diversification of beetle ‘horns’

Beetle 'horns' are rigid outgrowths of the insect cuticle used as weapons in contests for access to mates. Relative to their body size, beetle horns can be enormous. They protrude from any of five different regions of the head or thorax; they are curved, straight, branched or bladed; and their development is often coupled with the nutrient environment (male dimorphism) or with sex (sexual dimorphism). Here, we show that this extraordinary diversity of horns can be distilled down to four trajectories of morphological change--horn location, shape, allometry and dimorphism--and we illustrate how the developmental mechanisms regulating horn growth could generate each of these types of horn evolution. Specifically, we review two developmental pathways known to regulate growth of morphological structures in Drosophila and other insects: a limb-patterning pathway that specifies the location and shape of a structure, and the insulin pathway, which modulates trait growth in response to larval nutrition. We summarize preliminary evidence indicating that these pathways are associated with the development of beetle horns, and we show how subtle changes in the relative activities of these two pathways would be sufficient to generate most of the extant diversity of horn forms. Our objective is to intuitively connect genotype with phenotype, and to advocate an informed 'candidate gene' approach to studies of the developmental basis of evolution. We end by using this insight from development to offer a solution to the long-standing mystery of the scarabs: the observation by Darwin, Lameere, Arrow and others that this one family of beetles appeared to have a 'special tendency' towards the evolution of horns.

The roles of wingless and decapentaplegic in axis and appendage development in the red flour beetle, Tribolium castaneum

Axis patterning and appendage development have been well studied in Drosophila melanogaster, a species in which both limb and segment morphogenesis are derived. In Drosophila, positional information from genes important in anteroposterior and dorsoventral axis formation, including wingless (wg) and decapentaplegic (dpp), is required for allocating and patterning the appendage primordia. We used RNA interference to characterize the functions of wg and dpp in the red flour beetle, Tribolium castaneum, which retains more ancestral modes of limb and segment morphogenesis. We also characterized the expression of potential targets of the WG and DPP signaling pathways in these embryos. Tribolium embryos in which dpp had been downregulated had defects in the dorsalmost body wall, but did not appear to have been globally repatterned and had normal appendages. Downregulation of wg led to the loss of segment boundaries, gnathal and thoracic appendages, and lateral head lobes, and to changes in the expression of dpp, Distal-less, and Engrailed. The functions of wg varied along both the anteroposterior and dorsoventral axes of the embryo. Phylogenetic comparisons indicate that the role of WNT signaling in segment boundary formation is evolutionarily old, but that its role in appendage allocation originated in the common ancestor of holometabolous insects.

Maintenance of segment and appendage primordia by the Tribolium gene knödel

For homeotic and segment-polarity genes in Drosophila, a switch in gene regulation has been described that distinguishes patterning and maintenance phases. Maintenance of segment and organ primordia involves secondary patterning and differentiation steps, as well as survival factors regulating proliferation and organ size. In a screen for embryonic lethal mutations in the flour beetle Tribolium castaneum, we have recovered two alleles of the knödel gene, which result in short, bag-like embryos. These embryos have severely reduced appendages and differentiate a cuticle that lacks most overt signs of segmentation. In addition, they lack bristles and display defects in the nervous system. Early patterning in knödel mutant embryos is normal up to the extended germ band stage, as indicated by the formation of regular even-skipped (Tc'eve) and wingless (Tc'wg) stripes. Afterwards, however, these patterns degenerate. Similarly, proximo-distal growth and patterning of limbs are nearly normal initially, but limb primordia shrink, and proximo-distal patterns degenerate, during subsequent stages. knödel could be a segment polarity gene required for segment border maintenance in both trunk and appendages. Alternatively, it may have a more general role in tissue or organ maintenance.

Insect appendages and comparative ontogenetics

It is arguable that the evolutionary and ecological success of insects is due in large part to the versatility of their articulated appendages. Recent advances in our understanding of appendage development in Drosophila melanogaster, as well as functional and expression studies in other insect species have begun to frame the general themes of appendage development in the insects. Here, we review current studies that provide for a comparison of limb developmental mechanisms acting at five levels: (1) the specification of ventral appendage primordia; (2) specification of the limb axes; (3) regulation and interactions of genes expressed in specific domains of the proximal-distal axis, such as Distal-less; (4) the specification of appendage identity; and (5) genetic regulation of appendage allometry.

Diverse developmental mechanisms contribute to different levels of diversity in horned beetles

An ongoing challenge to evolutionary developmental biology is to understand how developmental evolution on the level of populations and closely related species relates to macroevolutionary transformations and the origin of morphological novelties. Here we explore the developmental basis of beetle horns, a morphological novelty that exhibits remarkable diversity on a variety of levels. In this study, we examined two congeneric Onthophagus species in which males develop into alternative horned and hornless morphs and different sexes express marked sexual dimorphism. In addition, both species differ in the body region (head vs. thorax) that develops the horn. Using a comparative morphological approach we show that prepupal growth of horn primordia during late larval development, as well as reabsorption of horn primordia during the pupal stage, contribute to horn expression in adults. We also show that variable combinations of both mechanisms are employed during development to modify horn expression of different horns in the same individual, the same horn in different sexes, and different horns in different species. We then examine expression patterns of two transcription factors, Distal-less (Dll) and aristaless (al), in the context of prepupal horn growth in alternative male morphs and sexual dimorphisms in the same two species. Expression patterns are qualitatively consistent with the hypothesis that both transcription factors function in the context of horn development similar to their known roles in patterning a wide variety of arthropod appendages. Our results suggest that the origin of morphological novelties, such as beetle horns, rests, at least in part, on the redeployment of already existing developmental mechanisms, such as appendage patterning processes. Our results also suggest, however, that little to no phylogenetic distance is needed for the evolution of very different modifier mechanisms that allow for substantial modulation of trait expression at different time points during development in different species, sexes, or tissue regions of the same individual. We discuss the implications of our results for our understanding of the evolution of horned beetle diversity and the origin and diversification of morphological novelties.

Isolation of white gene orthologue of the sawfly, Athalia rosae (Hymenoptera) and its functional analysis using RNA interference

We isolated and characterized the white gene orthologue of the sawfly, Athalia rosae (Hymenoptera). The A. rosae white (Ar white) cDNA cloned was 2058-bp long encoding 685 amino acids in a single open reading frame (ORF). Comparison of the cDNA sequence with the genomic DNA sequence revealed that the ORF was derived from 11 exons. Ar white was a single copy gene as evidenced by genomic Southern blotting and its cytological localization on the metaphase chromosomes. The deduced amino acid sequence aligned well with known insect white orthologous gene products sharing conserved regions such as the ATP-binding motif and the six transmembrane-spanning segments. Expression of Ar white was detected at embryonic and pupal stages by Northern blotting. In situ hybridization detected the embryonic expression in a pair of the lateral tips of protocephalic placodes from where optic organs are formed. Ar white function was examined using double-stranded RNA (dsRNA)-mediated interference. The synthesized dsRNA targeting Ar white transcripts caused a decrease in the level of the original mRNAs, and resulted in the white phenocopy in the embryonic eye pigmentation when microinjected into eggs from wild-type females. The effects occurred in a dose-dependent manner.

Patterning of the branched head appendages in Schistocerca americana and Tribolium castaneum

Much of our understanding of arthropod limb development comes from studies on the leg imaginal disc of Drosophila melanogaster. The fly limb is a relatively simple unbranched (uniramous) structure extending out from the body wall. The molecular basis for this outgrowth involves the overlap of two signaling molecules, Decapentaplegic (Dpp) and Wingless (Wg), to create a single domain of distal outgrowth, clearly depicted by the expression of the Distal-less gene (Dll). The expression of wg and dpp during the development of other arthropod thoracic limbs indicates that these pathways might be conserved across arthropods for uniramous limb development. The appendages of crustaceans and the gnathal appendages of insects, however, exhibit a diverse array of morphologies, ranging from those with no distal elements, such as the mandible, to appendages with multiple distal elements. Examples of the latter group include branched appendages or those that possess multiple lobes; such complex morphologies are seen for many crustacean limbs as well as the maxillary and labial appendages of many insects. It is unclear how, if at all, the known patterning genes for making a uniramous limb might be deployed to generate these diverse appendage forms. Experiments in Drosophila have shown that by forcing ectopic overlaps of Wg and Dpp signaling it is possible to generate artificially branched legs. To test whether naturally branched appendages form in a similar manner, we detailed the expression patterns of wg, dpp, and Dll in the development of the branched gnathal appendages of the grasshopper, Schistocerca americana, and the flour beetle, Tribolium castaneum. We find that the branches of the gnathal appendages are not specified through the redeployment of the Wg-Dpp system for distal outgrowth, but our comparative studies do suggest a role for Dpp in forming furrows between tissues.

Hypothesis Testing in Evolutionary Developmental Biology: A Case Study from Insect Wings

Developmental data have the potential to give novel insights into morphological evolution. Because developmental data are time-consuming to obtain, support for hypotheses often rests on data from only a few distantly related species. Similarities between these distantly related species are parsimoniously inferred to represent ancestral aspects of development. However, with limited taxon sampling, ancestral similarities in developmental patterning can be difficult to distinguish from similarities that result from convergent co-option of developmental networks, which appears to be common in developmental evolution. Using a case study from insect wings, we discuss how these competing explanations for similarity can be evaluated. Two kinds of developmental data have recently been used to support the hypothesis that insect wings evolved by modification of limb branches that were present in ancestral arthropods. This support rests on the assumption that aspects of wing development in Drosophila, including similarities to crustacean epipod patterning, are ancestral for winged insects. Testing this assumption requires comparisons of wing development in Drosophila and other winged insects. Here we review data that bear on this assumption, including new data on the functions of wingless and decapentaplegic during appendage allocation in the red flour beetle Tribolium castaneum.

The evolution of patterning of serially homologous appendages in insects

Arthropod bodies are formed by a series of appendage-bearing segments, and appendages have diversified both along the body axis within species and between species. Understanding the developmental basis of this variation is essential for addressing questions about the evolutionary diversification of limbs. We examined the development of serially homologous appendages of two insect species, the beetle Tribolium castaneum and the grasshopper Schistocerca americana. Both species retain aspects of ancestral appendage morphology and development that have been lost in Drosophila, including branched mouthparts and direct development of appendages during embryogenesis. We characterized the expression of four genes important in proximodistal axis development of Drosophila appendages: the secreted signaling factors wingless and decapentaplegic, and the homeodomain transcription factors extradenticle and Distal-less. Our comparisons focus on two aspects of appendage morphology: differentiation of the main axis of serial homologues and the appearance of proximal branches (endites) in the mouthparts. Although Distal-less expression is similar in endites and palps of the mouthparts, the expression of other genes in the endites does not conform to their known roles in axial patterning, leading us to reject the hypothesis that branched insect mouthparts develop by reiteration of the limb patterning network. With the exception of decapentaplegic, patterning of the main appendage axis is generally more similar in direct homologues than in serial homologues. Interestingly, however, phylogenetic comparisons suggest that patterning of serial homologues was more similar in ancestral insects, and thus that the observed developmental differences did not cause the evolutionary divergence in morphology among serial homologues.

Functional analyses in the hemipteran Oncopeltus fasciatus reveal conserved and derived aspects of appendage patterning in insects

The conservation of expression of appendage patterning genes, particularly Distal-less, has been shown in a wide taxonomic sampling of animals. However, the functional significance of this expression has been tested in only a few organisms. Here we report functional analyses of orthologues of the genes Distal-less, dachshund, and homothorax in the appendages of the milkweed bug Oncopeltus fasciatus (Hemiptera). This hemimetabolous insect has typical legs but highly derived mouthparts. Distal-less, dachshund, and homothorax are conserved in their individual expression patterns and functions in the legs of Oncopeltus, but their functions in other appendages are in some cases divergent. We find that specification of antennal identity does not require wild-type Distal-less activity in Oncopeltus as it does in Drosophila. Additionally, the mouthparts of Oncopeltus show novel patterns of gene expression and function, relative to other insects. Expression of Distal-less in the maxillary stylets of Oncopeltus does not seem necessary for proper development of this appendage, while dachshund and homothorax are crucial for formation of the mandibular and maxillary stylets. These data are used to evaluate hypotheses for the evolution of hemipteran mouthparts and the evolution of developmental mechanisms in insect appendages in general.

Expression patterns of leg genes in the mouthparts of the spider Cupiennius salei (Chelicerata: Arachnida)

The leg genes extradenticle, homothorax, dachshund, and Distal-less define three antagonistic developmental domains in the legs, but not in the antenna, of Drosophila. Here we report the expression patterns of these leg genes in the prosomal appendages of the spider Cupiennius salei. The prosoma of the spider bears six pairs of appendages: a pair of cheliceres, a pair of pedipalps, and four pairs of walking legs. Three types of appendages thus can be distinguished in the spider. We show here that in the pedipalp, the leg-like second prosomal appendage, the patterns are very similar to those in the legs themselves, indicating the presence of three antagonistic developmental domains in both appendage types. In contrast, in the chelicera, the fang-like first prosomal appendage, the patterns are different and there is no evidence for antagonistic domains. Together with data from Drosophila this suggests that leg-shaped morphology of arthropod appendages requires an underlying set of antagonistic developmental domains, whereas other morphologies (e.g. antenna, chelicera) may result from the loss of such antagonistic domains.

The mechanism of Drosophila leg development along the proximodistal axis

During development of higher organisms, most patterning events occur in growing tissues. Thus, unraveling the mechanism of how growing tissues are patterned into final morphologies has been an essential subject of developmental biology. Limb or appendage development in both vertebrates and invertebrates has attracted great attention from many researchers for a long time, because they involve almost all developmental processes required for tissue patterning, such as generation of the positional information by morphogen, subdivision of the tissue into distinct parts according to the positional information, localized cell growth and proliferation, and control of adhesivity, movement and shape changes of cells. The Drosophila leg development is a good model system, upon which a substantial amount of knowledge has been accumulated. In this review, the current understanding of the mechanism of Drosophila leg development is described.

The expression of the proximodistal axis patterning genes Distal-less and dachshund in the appendages of Glomeris marginata (Myriapoda: Diplopoda) suggests a special role of these genes in patterning the head appendages

The genes Distal-less, dachshund, extradenticle, and homothorax have been shown in Drosophila to be among the earliest genes that define positional values along the proximal-distal (PD) axis of the developing legs. In order to study PD axis formation in the appendages of the pill millipede Glomeris marginata, we have isolated homologues of these four genes and have studied their expression patterns. In the trunk legs, there are several differences to Drosophila, but the patterns are nevertheless compatible with a conserved role in defining positional values along the PD axis. However, their role in the head appendages is apparently more complex. Distal-less in the mandible and maxilla is expressed in the forming sensory organs and, thus, does not seem to be involved in PD axis patterning. We could not identify in the mouthparts components that are homologous to the distal parts of the trunk legs and antennnae. Interestingly, there is also a transient premorphogenetic expression of Distal-less in the second antennal and second maxillary segment, although no appendages are eventually formed in these segments. The dachshund gene is apparently involved both in PD patterning as well as in sensory organ development in the antenna, maxilla, and mandible. Strong dachshund expression is specifically correlated with the tooth-like part of the mandible, a feature that is shared with other mandibulate arthropods. homothorax is expressed in the proximal and medial parts of the legs, while extradenticle RNA is only seen in the proximal region. This overlap of expression corresponds to the functional overlap between extradenticle and homothorax in Drosophila.

Early steps in building the insect brain: neuroblast formation and segmental patterning in the developing brain of different insect species

In insects, morphological, molecular and genetic studies have provided a detailed insight into the ontogenetic processes that shape the ventral nerve cord. On the other hand, owing to its complexity and less obvious segmental composition, the knowledge about the development of the brain is still fragmentary. A promising approach towards gaining insight into fundamental processes underlying brain development is the comparison of embryonic brain development among different insect species. However, so far such comparative analyses are scarce. In this review, we summarize and compare data on the early steps in brain formation in different hemi- and holometabolous insects. We show that basic aspects of the spatial and temporal development of the embryonic brain neuroblast pattern are conserved among insects. Furthermore, we compare the number and proliferation patterns of neuroblasts related to major neuropil structures such as mushroom bodies, central complex, and antennal lobe. Finally, comparing the expression patterns of engrailed in different species, and considering new data from Drosophila melanogaster, we discuss the segmental organization of the insect brain.

The Caenorhabditis elegans distal-less ortholog ceh-43 is required for development of the anterior hypodermis

Homeobox genes of the Distal-less (Dll) class are expressed in developing appendages as well as in the central nervous system in invertebrates and vertebrates. Mutant analyses in mice and Drosophila have implicated these genes in outgrowth of structures, cell adhesion, cell migration, and cell fate decisions. We have investigated the expression and function of ceh-43, the Dll ortholog from the nematode Caenorhabditis elegans, by using gfp reporter constructs and double-stranded RNA-mediated interference (RNAi). Our results show that, as in the fly, the C. elegans Dll ortholog seems to play a role in cell adhesion. An antibody against the butterfly Distal-less homeodomain stains the nervous system of C. elegans embryos (Panganiban et al. [1997] Proc Natl Acad Sci USA. 94:5162-5166). GFP expression under the control of the ceh-43 promoter looks similar, although strong expression is primarily confined to the head hypodermis and to neuronal support cells. ceh-43(RNAi) results in 100% lethality at embryonic or early larval stages. At the beginning of morphogenesis, ceh-43(RNAi) embryos start to lose cells through a hole in the head hypodermis. They either rupture anteriorly as elongation proceeds, or they elongate normally to threefold egg length with the pharynx not connected to the mouth. Elongated ceh-43(RNAi) animals die before or soon after hatching with a fluid-filled pseudocoel and large vacuoles. These phenotypes suggest a role for ceh-43 in development of adhesive properties in the head hypodermis that connects the epithelia of the skin and the digestive tract. Furthermore, possible defects in the excretory system may result at least in part from a requirement for ceh-43 in the CAN neurons where ceh-43:gfp is also expressed.