Leg development in flies versus grasshoppers: differences in dpp expression do not lead to differences in the expression of downstream components of the leg patterning pathway

Gene expression reveals evidence for EGFR-dependent proximal-distal limb patterning in a myriapod

Evolution of segmented limbs is one of the key innovations of Arthropoda, allowing development of functionally specific specialized head and trunk appendages, a major factor behind their unmatched evolutionary success. Proximodistal limb patterning is controlled by two regulatory networks in the vinegar fly Drosophila melanogaster, and other insects. The first is represented by the function of the morphogens Wingless (Wg) and Decapentaplegic (Dpp); the second by the EGFR-signaling cascade. While the role of Wg and Dpp has been studied in a wide range of arthropods representing all main branches, that is, Pancrustacea (= Hexapoda + Crustacea), Myriapoda and Chelicerata, investigation of the potential role of EGFR-signaling is restricted to insects (Hexapoda). Gene expression analysis of Egfr, its potential ligands, and putative downstream factors in the pill millipede Glomeris marginata (Myriapoda: Diplopoda), reveals that-in at least mandibulate arthropods-EGFR-signaling is likely a conserved regulatory mechanism in proximodistal limb patterning.

Distal-less homeobox genes of insects and spiders: genomic organization, function, regulation and evolution

The Distal-less (Dll) genes are homeodomain transcription factors that are present in most Metazoa and in representatives of all investigated arthropod groups. In Drosophila, the best studied insect, Dll plays an essential role in forming the proximodistal axis of the legs, antennae and analia, and in specifying antennal identity. The initiation of Dll expression in clusters of cells in mid-lateral regions of the Drosophila embryo represents the earliest genetic marker of limbs. Dll genes are involved in the development of the peripheral nervous system and sensitive organs, and they also function as master regulators of black pigmentation in some insect lineages. Here we analyze the complete genomes of six insects, the nematode Caenorhabditis elegans and Homo sapiens, as well as multiple Dll sequences available in databases in order to examine the structure and protein features of these genes. We also review the function, expression, regulation and evolution of arthropod Dll genes with emphasis on insects and spiders.

Formation of body appendages during caudal regeneration in Platynereis dumerilii: adaptation of conserved molecular toolsets

Background

Platynereis and other polychaete annelids with homonomous segmentation are regarded to closely resemble ancestral forms of bilateria. The head region comprises the prostomium, the peristomium, a variable number of cephalized body segments and several appendages, like cirri, antennae and palps. The trunk of such polychaetes shows numerous, nearly identical segments. Each segment bears a parapodium with species-specific morphology on either side. The posterior end of the trunk features a segment proliferation zone and a terminal pygidium with the anus and anal cirri. The removal of a substantial part of the posterior trunk is by no means lethal. Cells at the site of injury dedifferentiate and proliferate forming a blastema to regenerate both the pygidium and the proliferation zone. The pygidium forms new anal cirri, and the proliferation zone generates new segments at a rapid pace. The formation of body appendages like the cirri and the segmental parapodia can thus be studied in the caudal regenerate of Platynereis within only a few days.

Results

The development of body appendages in Platynereis is regulated by a network of genes common to polychaetes but also shared by distant taxa. We isolated DNA sequences from P. dumerilii of five genes known to be involved in appendage formation within other groups: Meis/homothorax, Pbx1/extradenticle, Dlx/Distal-less, decapentaplegic and specificprotein1/buttonhead. Analyses of expression patterns during caudal regeneration by in situ hybridization reveal striking similarities related to expression in arthropods and vertebrates. All genes exhibit transient expression during differentiation and growth of segments. As was shown previously in other phyla Pdu-Meis/hth and Pdu-Pbx1/exd are co-expressed, although the expression is not limited to the proximal part of the parapodia. Pdu-Dll is prominent in parapodia but upregulated in the anal cirri. No direct dependence concerning Pdu-Dll and Pdu-sp/btd expression is observed in Platynereis. Pdu-dpp shows an expression pattern not comparable to its expression in other taxa.

Conclusions

The expression patterns observed suggest conserved roles of these genes during appendage formation across different clades, but the underlying mechanisms utilizing this toolset might not be identical. Some genes show broad expression along the proximodistal axis indicating a possible role in proximodistal patterning of body appendages. Other genes exhibit expression patterns limited to specific parts and tissues of the growing parapodia, thus presumably being involved in formation of taxon-specific morphological differences.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0046-6) contains supplementary material, which is available to authorized users.

Embryo Development and Morphology of the Rocky Mountain Wood Tick (Acari: Ixodidae)

Dermacentor andersoni Stiles embryogenesis was observed using fluorescent and scanning electron microscopy for eggs held under laboratory conditions (25°C and at 93% relative humidity). Early embryonic cell divisions appeared to be synchronous and holoblastic, giving rise to a uniform blastoderm surrounding the yolk. The cells of the blastoderm became concentrated on one side of the embryo, forming the segmented germ band. Distinct opisthosomal and prosomal segment morphologies, which are characteristic of chelicerate embryos, were observed during germ band elongation. Mouth and leg appendages grew from the prosomal segments. As development progressed, the segments were fused into the idiosoma and capitulum of the free-living larval form. An embryo staging system was established based on embryo developmental morphology and the timing of morphogenetic events. The staging system will serve as a basis for future studies directed at understanding morphogenetic mechanisms or for observing the impact of abiotic factors, such as temperature or humidity, on tick development.

Exaggerated trait growth in insects

Animal structures occasionally attain extreme proportions, eclipsing in size the surrounding body parts. We review insect examples of exaggerated traits, such as the mandibles of stag beetles (Lucanidae), the claspers of praying mantids (Mantidae), the elongated hindlimbs of grasshoppers (Orthoptera: Caelifera), and the giant heads of soldier ants (Formicidae) and termites (Isoptera). Developmentally, disproportionate growth can arise through trait-specific modifications to the activity of at least four pathways: the sex determination pathway, the appendage patterning pathway, the insulin/IGF signaling pathway, and the juvenile hormone/ecdysteroid pathway. Although most exaggerated traits have not been studied mechanistically, it is already apparent that distinct developmental mechanisms underlie the evolution of the different types of exaggerated traits. We suggest this reflects the nature of selection in each instance, revealing an exciting link between mechanism, form, and function. We use this information to make explicit predictions for the types of regulatory pathways likely to underlie each type of exaggerated trait.

Antennapedia is involved in the development of thoracic legs and segmentation in the silkworm, Bombyx mori

Homeotic genes, which are associated closely with body patterning of various species, specify segment identity. The Wedge eye-spot (Wes) is a new homeotic mutant located on the sixth linkage group. Homozygous Wes/Wes embryos are lethal and display a pair of antenna-like appendages under the mouthparts as well as fused thoracic segments. These mutants also exhibit a narrower eye-spot at the larval stage compared with the wild type. By positional cloning, we identified the candidate gene of the Wes locus, Bombyx mori Antennapedia (BmAntp). Two BmAntp transcripts were identified in the homozygote of the Wes mutant, including a normal form and an abnormal form with a 1570-bp insertion. Our data showed that the insertion element was a long interspersed nuclear element (LINE)-like transposon that destroyed the original open reading frame of BmAntp. Quantitative RT-PCR analysis showed that the expression levels of normal BmAntp transcripts were increased markedly in the Wes heterozygous larvae compared with the wild type. Furthermore, we performed RNAi of BmAntp and observed fused thoracic segments and defective thoracic legs in the developing embryos. Our results indicated that BmAntp is responsible for the Wes mutant and has an important role in determining the proper development of the thoracic segments. Our identification of a homeotic mutation in the silkworm is an important contribution to our understanding of the regulation of Hox genes at different levels of expression.

Functional conservation and divergence of BMP ligands in limb development and lipid homeostasis of holometabolous insects

Bone morphogenetic protein (BMP) ligands play key roles in regulating morphological and physiological traits. To investigate how the functions of BMP ligands have evolved among insects, the roles of two key BMP ligands, decapentaplegic (dpp) and glass bottom boat (gbb), were studied in the flour beetle, Tribolium castaneum. RNA interference-mediated knockdown revealed that the role of dpp in establishing limb segmentation is conserved among insects. Based on the expression pattern of dpp in the presumptive leg tarsal segments, we propose that the function of dpp has evolved through heterochronic changes during the evolution of complete metamorphosis. Gbb1 was found to be necessary for sculpting the tarsal segment morphology characteristic of beetles. Knockdown of Dpp and Gbb1 expression also resulted in transparent larvae and reduced triglyceride levels, indicating their critical roles in maintaining lipid homeostasis. Both knockdown phenotypes were mediated by larval translucida. Because only Gbb regulates lipid metabolism in Drosophila, regulation of lipid homeostasis appears to have evolved by developmental systems drift. Thus, developmental systems drift may underlie evolution of both morphology and physiological processes.

Extent With Modification: Leg Patterning in the Beetle Tribolium castaneum and the Evolution of Serial Homologs

Serial homologs are similar structures that develop at different positions within a body plan. These structures share some, but not all, aspects of developmental patterning, and their evolution is thought to be constrained by shared, pleiotropic gene functions. Here we describe the functions of 17 developmental genes during metamorphic development of the legs in the red flour beetle, Tribolium castaneum. This study provides informative comparisons between appendage development in Drosophila melanogaster and T. castaneum, between embryonic and adult development in T. castaneum, and between the development of serially homologous appendages. The leg gap genes Distal-less and dachshund are conserved in function. Notch signaling, the zinc-finger transcription factors related to odd-skipped, and bric-à-brac have conserved functions in promoting joint development. homothorax knockdown alters the identity of proximal leg segments but does not reduce growth. Lim1 is required for intermediate leg development but not distal tarsus and pretarsus development as in D. melanogaster. Development of the tarsus requires decapentaplegic, rotund, spineless, abrupt, and bric-à-brac and the EGF ligand encoded by Keren. Metathoracic legs of T. castaneum have four tarsomeres, whereas other legs have five. Patterns of gene activity in the tarsus suggest that patterning in the middle of the tarsal region, not the proximal- or distal-most areas, is responsible for this difference in segment number. Through comparisons with other recent studies of T. castaneum appendage development, we test hypotheses for the modularity or interdependence of development during evolution of serial homologs.

Patterning of the adult mandibulate mouthparts in the red flour beetle, Tribolium castaneum

Specialized insect mouthparts, such as those of Drosophila, are derived from an ancestral mandibulate state, but little is known about the developmental genetics of mandibulate mouthparts. Here, we study the metamorphic patterning of mandibulate mouthparts of the beetle Tribolium castaneum, using RNA interference to deplete the expression of 13 genes involved in mouthpart patterning. These data were used to test three hypotheses related to mouthpart development and evolution. First, we tested the prediction that maxillary and labial palps are patterned using conserved components of the leg-patterning network. This hypothesis was strongly supported: depletion of Distal-less and dachshund led to distal and intermediate deletions of these structures while depletion of homothorax led to homeotic transformation of the proximal maxilla and labium, joint formation required the action of Notch signaling components and odd-skipped paralogs, and distal growth and patterning required epidermal growth factor (EGF) signaling. Additionally, depletion of abrupt or pdm/nubbin caused fusions of palp segments. Second, we tested hypotheses for how adult endites, the inner branches of the maxillary and labial appendages, are formed at metamorphosis. Our data reveal that Distal-less, Notch signaling components, and odd-skipped paralogs, but not dachshund, are required for metamorphosis of the maxillary endites. Endite development thus requires components of the limb proximal-distal axis patterning and joint segmentation networks. Finally, adult mandible development is considered in light of the gnathobasic hypothesis. Interestingly, while EGF activity is required for distal, but not proximal, patterning of other appendages, it is required for normal metamorphic growth of the mandibles.

Patterning mechanisms and morphological diversity of spider appendages and their importance for spider evolution

The prosoma of spiders bears different gnathal (labrum, chelicerae, pedipalps) and locomotory appendages (legs). In most species these appendages are also used for additional functions, e.g. sensing, mating, and courtship. The opisthosoma is equipped with four pairs of highly specialized appendages. Two pairs of spinnerets are used for silk production and manipulation. The other two pairs of appendages are internalized during development and give rise to a complex respiratory system of book lungs and tracheae. Thus spiders have a number of different appendage types with radically different adult morphologies. Furthermore, all these appendage types display significant additional species specific diversity correlating with a large spectrum of functions of the appendages. Despite this importance of appendage diversity for the evolution of the spiders we know relatively little about the genetic patterning mechanisms producing this diversity of morphology. We review recent advances concerning the developmental genetics of spider appendage diversification, mainly concentrating on open questions and future directions of research. We conclude that the deeper understanding of appendage development and diversity in spiders can contribute significantly not only to evolutionary developmental biology, but also to behavioral biology, speciation research and population genetics, and the study of sexually dimorphic traits.

Separable functions of wingless in distal and ventral patterning of the Tribolium leg

The gene wingless (wg) in Drosophila is an important factor in leg development. During embryonic development wg is involved in the allocation of the limb primordia. During imaginal disk development wg is involved in distal development and it has a separate role in ventral development. The expression pattern of wg is highly conserved in all arthropods (comprising data from insects, myriapods, crustaceans, and chelicerates), suggesting that its function in leg development is also conserved. However, recent work in other insects (e.g. the milkweed bug Oncopeltus fasciatus) argued against a role of wg in leg development. We have studied the role of wg in leg development of the flour beetle Tribolium castaneum. Using stage-specific staggered embryonic RNAi in wild-type and transgenic EGFP expressing enhancer trap lines we are able to demonstrate separable functions of Tribolium wg in distal and in ventral leg development. The distal role affects all podomeres distal to the coxa, whereas the ventral role is restricted to cells along the ventral midline of the legs. In addition, severe leg defects after injection into early embryonic stages are evidence that wg is also involved in proximal development and limb allocation in Tribolium. Our data suggest that the roles of wg in leg development are highly conserved in the holometabolous insects. Further studies will reveal the degree of conservation in other arthropod groups.

Differential recruitment of limb patterning genes during development and diversification of beetle horns

The origins of novel complex phenotypes represent one of the most fundamental, yet largely unresolved, issues in evolutionary biology. Here we explore the developmental genetic regulation of beetle horns, a class of traits that lacks obvious homology to traits in other insects. Furthermore, beetle horns are remarkably diverse in their expression, including sexual dimorphisms, male dimorphisms, and interspecific differences in location of horn expression. At the same time, beetle horns share aspects of their development with that of more traditional appendages. We used larval RNA interference-mediated gene function analysis of 3 cardinal insect appendage patterning genes, dachshund, homothorax, and Distal-less, to investigate their role in development and diversification of beetle horns within and between species. Transcript depletion of all 3 patterning genes generated phenotypic effects very similar to those documented in previous studies that focused on general insect development. In addition, we found that Distal-less and homothorax, but not dachshund, regulate horn expression in a species-, sex-, body region-, and body size-dependent manner. Our results demonstrate differential co-option of appendage patterning genes during the evolution and radiation of beetle horns. Furthermore, our results illustrate that regulatory genes whose functions are otherwise highly conserved nevertheless retain the capacity to acquire additional functions, and that little phylogenetic distance appears necessary for the evolution of sex- and species-specific differences in these functions.

Chapter 6. The origin and diversification of complex traits through micro- and macroevolution of development: insights from horned beetles

Understanding how development and ecology shape organismal evolution is a central goal of evolutionary developmental biology. This chapter highlights a class of traits and organisms that are emerging as new models in evo-devo and eco-devo research: beetle horns and horned beetles. Horned beetles are morphologically diverse, ecologically rich, and developmentally and genetically increasingly accessible. Recent studies have begun to take advantage of these attributes and are starting to link the microevolution of horned beetle development to the macroevolution of novel features, and to identify the genetic, developmental, and ecological mechanisms, and the interactions between them, that mediate organismal innovation and diversification in natural populations. Here, I review the most significant recent findings and their contributions to current frontiers in evolutionary developmental biology.

Evidence for a novel role for dachshund in patterning the proximal arthropod leg

The branchiopod crustacean Triops longicaudatus has paddlelike thoracic appendages with few joints and multiple marginal lobes. Here, we explore the degree to which the Triops limb is patterned by the same network of genes known to pattern the uniramous, multi-jointed insect appendage. Insect leg patterning proceeds through a process of subdividing the leg into proximal, intermediate, and distal regions by the activity of the transcription factors hth/exd, dac, and Dll. The immature Triops limb is subdivided into large, discrete regional domains (proximal and distal) as defined by nuclear-EXD and DLL. We show that HTH expression in Triops overlaps cell-to-cell with n-EXD expression. In addition, dac is expressed in two domains: (1) adjacent to and partially overlapping the distal Dll domain and (2) along the medial margin of the developing leg. The DAC domain adjacent to the distal Dll domain supports the early establishment of the expected intermediate domain of DAC expression. The medial expression domain resolves over time into a series of reiterated stripes located on the lower side of each medial lobe. Later, this expression pattern correlates with the sclerotized regions associated with limb flexion. We propose that these stripes of DAC expression play a role in forming reiterated medial lobes. Unlike Drosophila, where the proximal distal patterning of the leg is coincident with patterning of reiterated structures (segments), we hypothesize that the patterning in Triops may reflect an ancestral state where the patterning of reiterated medial structures was not coincident with proximodistal limb patterning.

Larval legs of mulberry silkworm Bombyx mori are prototypes for the adult legs

Morphological diversity of leg appendages is one of the hallmarks of developmental evolution. Limbs in insects may develop either from their embryonic prototypes or from imaginal discs harbored inside the larva. Bombyx mori (B. mori), a Lepidopteran insect, develops adult wings from larval wing imaginal discs. However, it has been debated whether the adult legs of B. mori arise from imaginal discs or from the larval legs. Here we addressed how the larval legs relate to their adult counterparts. We present the morphological landmarks during early leg development. We used expression of developmental genes like Distalless and extradenticle to mark leg primordia. Finally, we employed classical excision approach to develop a fate map of the adult leg. Excision and ablation of thoracic legs along proximo-distal axis at various times during larval development resulted in the loss of corresponding adult leg segments. Our data suggest that B. mori legs develop from larval appendages rather than leg imaginal discs.

Evolution of sexual dimorphism and male dimorphism in the expression of beetle horns: phylogenetic evidence for modularity, evolutionary lability, and constraint

Beetle horns are enlarged outgrowths of the head or thorax that are used as weapons in contests over access to mates. Horn development is typically confined to males (sexual dimorphism) and often only to the largest males (male dimorphism). Both types of dimorphism result from endocrine threshold mechanisms that coordinate cell proliferation near the end of the larval period. Here, we map the presence/absence of each type of dimorphism onto a recent phylogeny for the genus Onthophagus (Coleoptera: Scarabaeidae) to explore how horn development has changed over time. Our results provide empirical support for several recent predictions regarding the evolutionary lability of developmental thresholds, including uncoupled evolution of alternative phenotypes and repeated fixation of phenotypes. We also report striking evidence of a possible developmental constraint. We show that male dimorphism and sexual dimorphism map together on the phylogeny; whenever small males have horns, females also have horns (and vice versa). We raise the possibility that correlated evolution of these two phenomena results from a shared element in their endocrine regulatory mechanisms rather than a history of common selection pressures. These results illustrate the type of insight that can be gained only from the integration of developmental and evolutionary perspectives.

Sharp boundaries of Dpp signalling trigger local cell death required for Drosophila leg morphogenesis

Morphogens are secreted signalling molecules that govern many developmental processes. In the Drosophila wing disc, the transforming growth factor beta (TGFbeta) homologue Decapentaplegic (Dpp) forms a smooth gradient and specifies cell fate by conferring a defined value of morphogen activity. Thus, neighbouring cells have similar amounts of Dpp protein, and if a sharp discontinuity in Dpp activity is generated between these cells, Jun kinase (JNK)-dependent apoptosis is triggered to restore graded positional information. To date, it has been assumed that this apoptotic process is only activated when normal signalling is distorted. However, we now show that a similar process occurs during normal development: rupture in Dpp activity occurs during normal segmentation of the distal legs of Drosophila. This sharp boundary of Dpp signalling, independently of the absolute level of Dpp activity, induces a JNK-reaper-dependent apoptosis required for the morphogenesis of a particular structure of the leg, the joint. Our results show that Dpp could induce a developmental programme not only in a concentration dependent manner, but also by the creation of a sharp boundary of Dpp activity. Furthermore, the same process could be used either to restore a normal pattern in response to artificial disturbance or to direct a morphogenetic process.

Co-factors and co-repressors of Engrailed: expression in the central nervous system and cerci of the cockroach, Periplaneta americana

In the larval cockroach (Periplaneta americana), knockout of Engrailed (En) in the medial sensory neurons of the cercal sensory system changes their axonal arborization and synaptic specificity. Immunocytochemistry has been used to investigate whether the co-repressor Groucho (Gro; vertebrate homolog: TLE) and the co-factor Extradenticle (Exd; vertebrate homolog: Pbx) are expressed in the cercal system. Gro/TLE is expressed ubiquitously in cell nuclei in the embryo, except for the distal pleuropodia. Gro is expressed in all nuclei of the thoracic and abdominal central nervous system (CNS) of first instar larva, although some neurons express less Gro than others. Cercal sensory neurons express Gro protein, which might therefore act as a co-repressor with En. Exd/Pbx is expressed in the proximal portion of all segmental appendages in the embryo, with the exception of the cerci. In the first instar CNS, Exd protein is expressed in subsets of neurons (including dorsal unpaired medial neurons) in the thoracic ganglia, in the first two abdominal ganglia, and in neuromeres A8-A11 of the terminal ganglion. Exd is absent from the cerci. Because Ultrabithorax/Abdominal-A (Ubx/Abd-A) can substitute for Exd as En co-factors in Drosophila, Ubx/Abd-A immunoreactivity has also been investigated. Ubx/Abd-A immunostaining is present in abdominal segments of the embryo and first instar CNS as far caudal as A7 and faintly in the T3 segment. However, Ubx/Abd-A is absent in the cerci and their neurons. Thus, in contrast to its role in Drosophila segmentation, En does not require the co-factors Exd or Ubx/Abd-A in order to control the synaptic specificity of cockroach sensory neurons.

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.

Homology and ontogeny: pattern and process in comparative developmental biology

In this article the interface between development and homology is discussed. Development is here interpreted as a sequence of evolutionarily independent stages. Any approach stressing the importance of specific developmental stages is rejected. A homology definition is favoured which includes similarity, and complexity serves as a test for homology. Complexity is seen as the possibility of subdividing a character into evolutionarily independent corresponding substructures. Topology as a test for homology is critically discussed because corresponding positions are not necessarily indicative of homology. Complexity can be used twofold for homology assessments of development: either stages or processes of development are homologized. These two approaches must not be con-flated. This distinction leads to the conclusion that there is no ontogenetic homology "criterion".

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.

Functional analyses in the milkweed bug Oncopeltus fasciatus (Hemiptera) support a role for Wnt signaling in body segmentation but not appendage development

Specification of the proximal-distal (PD) axis of insect appendages is best understood in Drosophila melanogaster, where conserved signaling molecules encoded by the genes decapentaplegic (dpp) and wingless (wg) play key roles. However, the development of appendages from imaginal discs as in Drosophila is a derived state, while more basal insects produce appendages from embryonic limb buds. Therefore, the universality of the Drosophila limb PD axis specification mechanism has been debated since dpp expression in more basal insect species differs dramatically from Drosophila. Here, we test the function of Wnt signaling in the development of the milkweed bug Oncopeltus fasciatus, a species with the basal state of appendage development from limb buds. RNA interference of wg and pangolin (pan) produce defects in the germband and eyes, but not in the appendages. Distal-less and dachshund, two genes regulated by Wg signaling in Drosophila and expressed in specific PD domains along the limbs of both species, are expressed normally in the limbs of pan-depleted Oncopeltus embryos. Despite these apparently paradoxical results, Armadillo protein, the transducer of Wnt signaling, does not accumulate properly in the nuclei of cells in the legs of pan-depleted embryos. In contrast, engrailed RNAi in Oncopeltus produces cuticular and appendage defects similar to Drosophila. Therefore, our data suggest that Wg signaling is functionally conserved in the development of the germband, while it is not essential in the specification of the limb PD axis in Oncopeltus and perhaps basal insects.

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.

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.

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.

Gene expression in spider appendages reveals reversal of exd/hth spatial specificity, altered leg gap gene dynamics, and suggests divergent distal morphogen signaling

Leg development in Drosophila has been studied in much detail. However, Drosophila limbs form in the larva as imaginal discs and not during embryogenesis as in most other arthropods. Here, we analyze appendage genes in the spider Cupiennius salei and the beetle Tribolium castaneum. Differences in decapentaplegic (dpp) expression suggest a different mode of distal morphogen signaling suitable for the specific geometry of growing limb buds. Also, expression of the proximal genes homothorax (hth) and extradenticle (exd) is significantly altered: in the spider, exd is restricted to the proximal leg and hth expression extends distally, while in insects, exd is expressed in the entire leg and hth is restricted to proximal parts. This reversal of spatial specificity demonstrates an evolutionary shift, which is nevertheless compatible with a conserved role of this gene pair as instructor of proximal fate. Different expression dynamics of dachshund and Distal-less point to modifications in the regulation of the leg gap gene system. We comment on the significance of this finding for attempts to homologize leg segments in different arthropod classes. Comparison of the expression profiles of H15 and optomotor-blind to the Drosophila patterns suggests modifications also in the dorsal-ventral patterning system of the legs. Together, our results suggest alterations in many components of the leg developmental system, namely proximal-distal and dorsal-ventral patterning, and leg segmentation. Thus, the leg developmental system exhibits a propensity to evolutionary change, which probably forms the basis for the impressive diversity of arthropod leg morphologies.

Leg patterning driven by proximal-distal interactions and EGFR signaling

wingless and decapentaplegic signaling establishes the proximal-distal axis of Drosophila legs by activating the expression of genes such as Distalless and dachshund in broad proximal-distal domains during early leg development. However, here we show that wingless and decapentaplegic are not required throughout all of proximal-distal development. The tarsus, which has been proposed to be an ancestral structure, is instead defined by the activity of Distalless, dachshund, and a distal gradient of epidermal growth factor receptor (EGFR)-Ras signaling. Our results uncover a mechanism for appendage patterning directed by genes expressed in proximal-distal domains and possibly conserved in other arthropods and vertebrates.

Exploring myriapod segmentation: the expression patterns of even-skipped, engrailed, and wingless in a centipede

Segment formation is critical to arthropod development, yet there is still relatively little known about this process in most arthropods. Here, we present the expression patterns of the genes even-skipped (eve), engrailed, and wingless in a centipede, Lithobius atkinsoni. Despite some differences when compared with the patterns in insects and crustaceans, the expression of these genes in the centipede suggests that their basic roles are conserved across the mandibulate arthropods. For example, unlike the seven pair-rule stripes of eve expression in the Drosophila embryonic germband, the centipede eve gene is expressed strongly in the posterior of the embryo, and in only a few stripes between newly formed segments. Nonetheless, this pattern likely reflects a conserved role for eve in the process of segment formation, within the different context of a short-germband mode of embryonic development. In the centipede, the genes wingless and engrailed are expressed in stripes along the middle and posterior of each segment, respectively, similar to their expression in Drosophila. The adjacent expression of the engrailed and wingless stripes suggests that the regulatory relationship between the two genes may be conserved in the centipede, and thus this pathway may be a fundamental mechanism of segmental development in most arthropods.

Correlation of expression patterns of homothorax, dachshund, and Distal-less with the proximodistal segmentation of the cricket leg bud

We describe the expression pattern of Gryllus homothorax (Gbhth) and dachshund (Gbdac), a cricket homologue of Drosophila homothorax and dachshund, together with localization of Distal-less or Extradenticle protein during leg development. We correlated their expression patterns with the morphological segmentation of the leg bud. The boundary of Gbhth/GbDll subdivision is correlated with the segment boundary of the future trochanter/femur at early stages. Gbdac expression subdivides the leg bud into the presumptive femur and more distal region. During the leg proximodistal formation, although the early expression patterns of GbDll, Gbdac, and Gbhth significantly differ from those of Drosophila imaginal disc, their expression patterns in the fully segmented Gryllus leg were similar to those in the Drosophila late third instar disc.

Conservation and divergence in molecular mechanisms of axis formation

Genetic screens in Drosophila melanogaster have helped elucidate the process of axis formation during early embryogenesis. Axis formation in the D. melanogaster embryo involves the use of two fundamentally different mechanisms for generating morphogenetic activity: patterning the anteroposterior axis by diffusion of a transcription factor within the syncytial embryo and specification of the dorsoventral axis through a signal transduction cascade. Identification of Drosophila genes involved in axis formation provides a launch-pad for comparative studies that examine the evolution of axis specification in different insects. Additionally, there is similarity between axial patterning mechanisms elucidated genetically in Drosophila and those demonstrated for chordates such as Xenopus. In this review we examine the postfertilization mechanisms underlying axis specification in Drosophila. Comparative data are then used to ask whether aspects of axis formation might be derived or ancestral.

A complex role for distal-less in crustacean appendage development

The developing leg of Drosophila is initially patterned by subdivision of the leg into proximal and distal domains by the activity of the homeodomain proteins Extradenticle (Exd) and Distal-less (Dll). These early domains of gene expression are postulated to reflect a scenario of limb evolution in which an undifferentiated appendage outgrowth was subdivided into two functional parts, the coxapodite and telopodite. The legs of most arthropods have a more complex morphology than the simple rod-shaped leg of Drosophila. We document the expression of Dll and Exd in two crustacean species with complex branched limbs. We show that in these highly modified limbs there is a Dll domain exclusive of Exd but there is also extensive overlap in Exd and Dll expression. While arthropod limbs all appear to have distinct proximal and distal domains, those domains do not define homologous structures throughout arthropods. In addition, we find a striking correlation throughout the proximal/distal extent of the leg between setal-forming cells and Dll expression. We postulate that this may reflect a pleisiomorphic function of Dll in development of the peripheral nervous system. In addition, our results confirm previous observations that branch formation in multiramous arthropod limbs is not regulated by a simple iteration of the proximal/distal patterning module employed in Drosophila limb development.

Developmental modularity and the evolutionary diversification of arthropod limbs

Segmentation is one of the most salient characteristics of arthropods, and differentiation of segments along the body axis is the basis of arthropod diversification. This article evaluates whether the evolution of segmentation involves the differentiation of already independent units, i.e., do segments evolve as modules? Because arthropod segmental differentiation is commonly equated with differential character of appendages, we analyze appendages by comparing similarities and differences in their development. The comparison of arthropod limbs, even between species, is a comparison of serially repeated structures. Arthropod limbs are not only reiterated along the body axis, but limbs themselves can be viewed as being composed of reiterated parts. The interpretation of such reiterated structures from an evolutionary viewpoint is far from obvious. One common view is that serial repetition is evidence of a modular organization, i.e., repeated structures with a common fundamental identity that develop semi-autonomously and are free to diversify independently. In this article, we evaluate arthropod limbs from a developmental perspective and ask: are all arthropod limbs patterned using a similar set of mechanisms which would reflect that they all share a generic coordinate patterning system? Using Drosophila as a basis for comparison, we find that appendage primordia, positioned along the body using segmental patterning coordinates, do indeed have elements of common identity. However, we do not find evidence of a single coordinate system shared either between limbs or among limb branches. Data concerning the other diagnostic of developmental modularity--semi-autonomy of development--are not currently available for sufficient taxa. Nonetheless, some data comparing patterns of morphogenesis provide evidence that limbs cannot always be temporally or spatially decoupled from the development of their neighbors, suggesting that segment modularity is a derived character.

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

Insects bear a stereotyped set of limbs, or ventral body appendages. In the highly derived dipteran Drosophila melanogaster, the homeodomain transcription factor encoded by the Distal-less (Dll) gene plays a major role in establishing distal limb structures. We have isolated the Dll orthologue (TcDll) from the beetle Tribolium castaneum, which, unlike Drosophila, develops well-formed limbs during embryogenesis. TcDll is initially expressed at the sites of limb primordia formation in the young embryo and subsequently in the distal region of developing legs, antennae and mouthparts except the mandibles. Mutations in the Short antennae (Sa) gene of Tribolium delete distal limb structures, closely resembling the Dll phenotype in Drosophila. TcDll expression is severely reduced or absent in strong Sa alleles. Genetic mapping and molecular analysis of Sa alleles also support the conclusion that TcDll corresponds to the Sa gene. Our data indicate functional conservation of the Dll gene in evolutionarily distant insect species. Implications for evolutionary changes in limb development are discussed.

Correlation of diversity of leg morphology in Gryllus bimaculatus (cricket) with divergence in dpp expression pattern during leg development

Insects can be grouped into mainly two categories, holometabolous and hemimetabolous, according to the extent of their morphological change during metamorphosis. The three thoracic legs, for example, are known to develop through two overtly different pathways: holometabolous insects make legs through their imaginal discs, while hemimetabolous legs develop from their leg buds. Thus, how the molecular mechanisms of leg development differ from each other is an intriguing question. In the holometabolous long-germ insect, these mechanisms have been extensively studied using Drosophila melanogaster. However, little is known about the mechanism in the hemimetabolous insect. Thus, we studied leg development of the hemimetabolous short-germ insect, Gryllus bimaculatus (cricket), focusing on expression patterns of the three key signaling molecules, hedgehog (hh), wingless (wg) and decapentaplegic (dpp), which are essential during leg development in Drosophila. In Gryllus embryos, expression of hh is restricted in the posterior half of each leg bud, while dpp and wg are expressed in the dorsal and ventral sides of its anteroposterior (A/P) boundary, respectively. Their expression patterns are essentially comparable with those of the three genes in Drosophila leg imaginal discs, suggesting the existence of the common mechanism for leg pattern formation. However, we found that expression pattern of dpp was significantly divergent among Gryllus, Schistocerca (grasshopper) and Drosophila embryos, while expression patterns of hh and wg are conserved. Furthermore, the divergence was found between the pro/mesothoracic and metathoracic Gryllus leg buds. These observations imply that the divergence in the dpp expression pattern may correlate with diversity of leg morphology.

Deletions of 9p and the quest for a conserved mechanism of sex determination

Distal chromosome 9p contains a locus that, when deleted, is a cause of 46,XY gonadal dysgenesis in the absence of extragenital anomalies. This locus might account for the frequently observed cases of 46,XY pure gonadal dysgenesis who do not harbor mutations in SRY, the sex master regulator gene found in mammalian species. The genomic organization of 9p positional candidate genes is currently being studied and mutational screens are ongoing. Among other positional candidates, including two additional doublesex-related genes, the evidence to support a role for the gene DMRT1 in vertebrate male sexual development is accumulating. Although formal proof of the requirement of DMRT1 in gonadal sex fate choice has not been obtained so far, the particular interest in this gene and perhaps other doublesex-related genes identified in vertebrates lies in that they may provide an entry point to a conserved mechanism of sex determination across animal phyla. We discuss recent results and emerging views on the genetics of sex determination, while stressing that the majority of cases of 46,XY gonadal dysgenesis remain unexplained. The latter is likely to be efficiently addressed by positional cloning efforts, particularly by considering the wealth of sequence data provided by the Human Genome Project.