Homeotic genes and the regulation and evolution of insect wing number

Polycomb safeguards imaginal disc specification through control of the Vestigial-Scalloped complex

A fundamental goal of developmental biology is to understand how cell and tissue fates are specified. The imaginal discs of Drosophila are excellent model systems for addressing this paradigm as their fate can be redirected when discs regenerate after injury or when key selector genes are misregulated. Here, we show that when Polycomb expression is reduced, the wing selector gene vestigial is ectopically activated. This leads to the inappropriate formation of the Vestigial-Scalloped complex, which forces the eye to transform into a wing. We further demonstrate that disrupting this complex does not simply block wing formation or restore eye development. Instead, immunohistochemistry and high-throughput genomic analysis show that the eye-antennal disc unexpectedly undergoes hyperplastic growth with multiple domains being organized into other imaginal discs and tissues. These findings provide insight into the complex developmental landscape that tissues must navigate before adopting their final fate.

Polycomb safeguards imaginal disc specification through control of the Vestigial-Scalloped complex

A fundamental goal of developmental biology is to understand how cell and tissue fates are specified. The imaginal discs of Drosophila are excellent model systems for addressing this paradigm as their fate can be redirected when discs regenerate after injury or when key selector genes are mis-regulated. Here, we show that when Polycomb expression is reduced, the wing selector gene vestigial is ectopically activated. This leads to the inappropriate formation of the Vestigial-Scalloped complex which forces the eye to transform into a wing. We further demonstrate that disrupting this complex does not simply block wing formation or restore eye development. Instead, immunohistochemistry and high throughput genomic analysis show that the eye-antennal disc unexpectedly undergoes hyperplastic growth with multiple domains being organized into other imaginal discs and tissues. These findings provide insight into the complex developmental landscape that tissues must navigate before adopting their final fate.

Summary Statement

Here we describe a novel mechanism by which Pc promotes an eye fate during normal development and how the eye is reprogrammed into a wing in its absence.

The pioneering function of the hox transcription factors

Ever since the discovery that the Hox family of transcription factors establish morphological diversity in the developing embryo, major efforts have been directed towards understanding Hox-dependent patterning. This has led to important discoveries, notably on the mechanisms underlying the collinear expression of Hox genes and Hox binding specificity. More recently, several studies have provided evidence that Hox factors have the capacity to bind their targets in an inaccessible chromatin context and trigger the switch to an accessible, transcriptional permissive, chromatin state. In this review, we provide an overview of the evidences supporting that Hox factors behave as pioneer factors and discuss the potential mechanisms implicated in Hox pioneer activity as well as the significance of this functional property in Hox-dependent patterning.

Hox dosage and morphological diversification during development and evolution

Hox genes encode for evolutionary conserved transcription factors that have long fascinated biologists since the observation of the first homeotic transformations in flies. Hox genes are developmental architects that instruct the formation of various and precise morphologies along the body axes in cnidarian and bilaterian species. In contrast to these highly specific developmental functions, Hox genes encode for proteins that display poorly selective DNA-binding properties in vitro. This "Hox paradox" has been partially solved with the discovery of the TALE-class cofactors, which interact with all Hox members and form versatile Hox/TALE protein complexes on DNA. Here, we describe the role of the Hox dosage as an additional molecular strategy contributing to further resolve the Hox paradox. We present several cases where the Hox dosage is involved in the formation of different morphologies in invertebrates and vertebrates, with a particular emphasis on flight appendages in insects. We also discuss how the Hox dosage could be interpreted in different types of target enhancers within the nuclear environment in vivo. Altogether our survey underlines the Hox dosage as a key mechanism for shaping Hox molecular function during development and evolution.

Evolution of stickleback spines through independent cis-regulatory changes at HOXDB

Understanding the mechanisms leading to new traits or additional features in organisms is a fundamental goal of evolutionary biology. We show that HOXDB regulatory changes have been used repeatedly in different fish genera to alter the length and number of the prominent dorsal spines used to classify stickleback species. In Gasterosteus aculeatus (typically 'three-spine sticklebacks'), a variant HOXDB allele is genetically linked to shortening an existing spine and adding an additional spine. In Apeltes quadracus (typically 'four-spine sticklebacks'), a variant HOXDB allele is associated with lengthening a spine and adding an additional spine in natural populations. The variant alleles alter the same non-coding enhancer region in the HOXDB locus but do so by diverse mechanisms, including single-nucleotide polymorphisms, deletions and transposable element insertions. The independent regulatory changes are linked to anterior expansion or contraction of HOXDB expression. We propose that associated changes in spine lengths and numbers are partial identity transformations in a repeating skeletal series that forms major defensive structures in fish. Our findings support the long-standing hypothesis that natural Hox gene variation underlies key patterning changes in wild populations and illustrate how different mutational mechanisms affecting the same region may produce opposite gene expression changes with similar phenotypic outcomes.

Flight or protection: the genes Ultrabithorax and apterous in the determination of membranous and sclerotized wings in insects

Present-day pterygote insects have two pairs of wings, one in the mesothorax (T2), the other in the metathorax (T3), and both have diverged in structure and function in different groups. Studies in endopterygote and paraneopteran species have shown that the gene Ultrabithorax (Ubx) specifies the identity and wing structure in T3, whereas the gene apterous (ap) significantly contributes to forming modified T2 wings. We wondered whether these Ubx and ap mechanisms operate in the lineage of polyneopterans. To explore this possibility, we used the cockroach Blattella germanica (Polyneoptera and Blattodea), in which the T2 wings are sclerotized (tegmina), whereas those of the T3 are membranous. We found that Ubx determines the structure of T3 and the membranous wing, while ap significantly contributes to form the sclerotized T2 tegmina. These results along with the studies carried out on the beetle Tribolium castaneum by Tomoyasu and collaborators suggest that ap plays an important role in the sclerotization and melanization of the T2 wings in neopteran groups that have sclerotized forewings. In turn, the sclerotizing properties of ap demonstrated in beetles and cockroaches suggest that the origin of this function goes back to the emergence of Neoptera, in the mid Devonian.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

Micromanagement of Drosophila Post-Embryonic Development by Hox Genes

Hox genes function early in development to determine regional identity in animals. Consequently, the loss or gain of Hox gene expression can change this identity and cause homeotic transformations. Over 20 years ago, it was observed that the role of Hox genes in patterning animal body plans involves the fine-scale regulation of cell fate and identity during development, playing the role of ‘micromanagers’ as proposed by Michael Akam in key perspective papers. Therefore, as well as specifying where structures develop on animal bodies, Hox genes can help to precisely sculpt their morphology. Here, we review work that has provided important insights about the roles of Hox genes in influencing cell fate during post-embryonic development in Drosophila to regulate fine-scale patterning and morphology. We also explore how this is achieved through the regulation of Hox genes, specific co-factors and their complex regulation of hundreds of target genes. We argue that further investigating the regulation and roles of Hox genes in Drosophila post-embryonic development has great potential for understanding gene regulation, cell fate and phenotypic differentiation more generally.

The Hox gene Antennapedia is essential for wing development in insects

A long-standing view in the field of evo-devo is that insect forewings develop without any Hox gene input. The Hox gene Antennapedia (Antp), despite being expressed in the thoracic segments of insects, has no effect on wing development. This view has been obtained from studies in two main model species: Drosophila and Tribolium. Here, we show that partial loss of function of Antp resulted in reduced and malformed adult wings in Bombyx, Drosophila and Tribolium. Antp mediates wing growth in Bombyx by directly regulating the ecdysteriod biosynthesis enzyme gene (shade) in the wing tissue, which leads to local production of the growth hormone 20-hydroxyecdysone. Additional targets of Antp are wing cuticular protein genes CPG24, CPH28 and CPG9, which are essential for wing development. We propose, therefore, that insect wing development occurs in an Antp-dependent manner. This article has an associated 'The people behind the papers' interview.

Transcriptional Regulation and Implications for Controlling Hox Gene Expression

Hox genes play key roles in axial patterning and regulating the regional identity of cells and tissues in a wide variety of animals from invertebrates to vertebrates. Nested domains of Hox expression generate a combinatorial code that provides a molecular framework for specifying the properties of tissues along the A–P axis. Hence, it is important to understand the regulatory mechanisms that coordinately control the precise patterns of the transcription of clustered Hox genes required for their roles in development. New insights are emerging about the dynamics and molecular mechanisms governing transcriptional regulation, and there is interest in understanding how these may play a role in contributing to the regulation of the expression of the clustered Hox genes. In this review, we summarize some of the recent findings, ideas and emerging mechanisms underlying the regulation of transcription in general and consider how they may be relevant to understanding the transcriptional regulation of Hox genes.

Out from under the wing: reconceptualizing the insect wing gene regulatory network as a versatile, general module for body-wall lobes in arthropods

Body plan evolution often occurs through the differentiation of serially homologous body parts, particularly in the evolution of arthropod body plans. Recently, homeotic transformations resulting from experimental manipulation of gene expression, along with comparative data on the expression and function of genes in the wing regulatory network, have provided a new perspective on an old question in insect evolution: how did the insect wing evolve? We investigated the metamorphic roles of a suite of 10 wing- and body-wall-related genes in a hemimetabolous insect, Oncopeltus fasciatus. Our results indicate that genes involved in wing development in O. fasciatus play similar roles in the development of adult body-wall flattened cuticular evaginations. We found extensive functional similarity between the development of wings and other bilayered evaginations of the body wall. Overall, our results support the existence of a versatile development module for building bilayered cuticular epithelial structures that pre-dates the evolutionary origin of wings. We explore the consequences of reconceptualizing the canonical wing-patterning network as a bilayered body-wall patterning network, including consequences for long-standing debates about wing homology, the origin of wings and the origin of novel bilayered body-wall structures. We conclude by presenting three testable predictions that result from this reconceptualization.

Mechanisms Underlying Hox-Mediated Transcriptional Outcomes

Metazoans differentially express multiple Hox transcription factors to specify diverse cell fates along the developing anterior-posterior axis. Two challenges arise when trying to understand how the Hox transcription factors regulate the required target genes for morphogenesis: First, how does each Hox factor differ from one another to accurately activate and repress target genes required for the formation of distinct segment and regional identities? Second, how can a Hox factor that is broadly expressed in many tissues within a segment impact the development of specific organs by regulating target genes in a cell type-specific manner? In this review, we highlight how recent genomic, interactome, and cis-regulatory studies are providing new insights into answering these two questions. Collectively, these studies suggest that Hox factors may differentially modify the chromatin of gene targets as well as utilize numerous interactions with additional co-activators, co-repressors, and sequence-specific transcription factors to achieve accurate segment and cell type-specific transcriptional outcomes.

Cell-type-specific Hox regulatory strategies orchestrate tissue identity

Hox proteins are homeodomain transcription factors that diversify serially homologous segments along the animal body axis, as revealed by the classic bithorax phenotype of Drosophila melanogaster, in which mutations in Ultrabithorax (Ubx) transform the third thoracic segment into the likeness of the second thoracic segment. To specify segment identity, we show that Ubx both increases and decreases chromatin accessibility, coinciding with its dual role as both an activator and repressor of transcription. However, the choice of transcriptional activity executed by Ubx is spatially regulated and depends on the availability of cofactors, with Ubx acting as a repressor in some populations and as an activator in others. Ubx-mediated changes to chromatin accessibility positively and negatively affect the binding of Scalloped (Sd), a transcription factor that is required for appendage development in both segments. These findings illustrate how a single Hox protein can modify complex gene regulatory networks to transform the identity of an entire tissue.

Hox genes are essential for the development of eyespots in Bicyclus anynana butterflies

The eyespot patterns found on the wings of nymphalid butterflies are novel traits that originated first in hindwings and subsequently in forewings, suggesting that eyespot development might be dependent on Hox genes. Hindwings differ from forewings in the expression of Ultrabithorax (Ubx), but the function of this Hox gene in eyespot development as well as that of another Hox gene Antennapedia (Antp), expressed specifically in eyespots centers on both wings, are still unclear. We used CRISPR-Cas9 to target both genes in Bicyclus anynana butterflies. We show that Antp is essential for eyespot development on the forewings and for the differentiation of white centers and larger eyespots on hindwings, whereas Ubx is essential not only for the development of at least some hindwing eyespots but also for repressing the size of other eyespots. Additionally, Antp is essential for the development of silver scales in male wings. In summary, Antp and Ubx, in addition to their conserved roles in modifying serially homologous segments along the anterior-posterior axis of insects, have acquired a novel role in promoting the development of a new set of serial homologs, the eyespot patterns, in both forewings (Antp) and hindwings (Antp and Ubx) of B. anynana butterflies. We propose that the peculiar pattern of eyespot origins on hindwings first, followed by forewings, could be due to an initial co-option of Ubx into eyespot development followed by a later, partially redundant, co-option of Antp into the same network.

Developmental Robustness: The Haltere Case in Drosophila

Developmental processes have to be robust but also flexible enough to respond to genetic and environmental variations. Different mechanisms have been described to explain the apparent antagonistic nature of developmental robustness and plasticity. Here, we present a “self-sufficient” molecular model to explain the development of a particular flight organ that is under the control of the Hox gene Ultrabithorax (Ubx) in the fruit fly Drosophila melanogaster. Our model is based on a candidate RNAi screen and additional genetic analyses that all converge to an autonomous and cofactor-independent mode of action for Ubx. We postulate that this self-sufficient molecular mechanism is possible due to an unusually high expression level of the Hox protein. We propose that high dosage could constitute a so far poorly investigated molecular strategy for allowing Hox proteins to both innovate and stabilize new forms during evolution.

Hox dosage contributes to flight appendage morphology in Drosophila

Flying insects have invaded all the aerial space on Earth and this astonishing radiation could not have been possible without a remarkable morphological diversification of their flight appendages. Here, we show that characteristic spatial expression profiles and levels of the Hox genes Antennapedia (Antp) and Ultrabithorax (Ubx) underlie the formation of two different flight organs in the fruit fly Drosophila melanogaster. We further demonstrate that flight appendage morphology is dependent on specific Hox doses. Interestingly, we find that wing morphology from evolutionary distant four-winged insect species is also associated with a differential expression of Antp and Ubx. We propose that variation in the spatial expression profile and dosage of Hox proteins is a major determinant of flight appendage diversification in Drosophila and possibly in other insect species during evolution.

Genome-Wide Binding Analyses of HOXB1 Revealed a Novel DNA Binding Motif Associated with Gene Repression

Knowledge of the diverse DNA binding specificities of transcription factors is important for understanding their specific regulatory functions in animal development and evolution. We have examined the genome-wide binding properties of the mouse HOXB1 protein in embryonic stem cells differentiated into neural fates. Unexpectedly, only a small number of HOXB1 bound regions (7%) correlate with binding of the known HOX cofactors PBX and MEIS. In contrast, 22% of the HOXB1 binding peaks display co-occupancy with the transcriptional repressor REST. Analyses revealed that co-binding of HOXB1 with PBX correlates with active histone marks and high levels of expression, while co-occupancy with REST correlates with repressive histone marks and repression of the target genes. Analysis of HOXB1 bound regions uncovered enrichment of a novel 15 base pair HOXB1 binding motif HB1RE (HOXB1 response element). In vitro template binding assays showed that HOXB1, PBX1, and MEIS can bind to this motif. In vivo, this motif is sufficient for direct expression of a reporter gene and over-expression of HOXB1 selectively represses this activity. Our analyses suggest that HOXB1 has evolved an association with REST in gene regulation and the novel HB1RE motif contributes to HOXB1 function in part through a repressive role in gene expression.

Wing serial homologues and the diversification of insect outgrowths: insights from the pupae of scarab beetles

Modification of serially homologous structures is a common avenue towards functional innovation in developmental evolution, yet ancestral affinities among serial homologues may be obscured as structure-specific modifications accumulate over time. We sought to assess the degree of homology to wings of three types of body wall projections commonly observed in scarab beetles: (i) the dorsomedial support structures found on the second and third thoracic segments of pupae, (ii) the abdominal support structures found bilaterally in most abdominal segments of pupae, and (iii) the prothoracic horns which depending on species and sex may be restricted to pupae or also found in adults. We functionally investigated 14 genes within, as well as two genes outside, the canonical wing gene regulatory network to compare and contrast their role in the formation of each of the three presumed wing serial homologues. We found 11 of 14 wing genes to be functionally required for the proper formation of lateral and dorsal support structures, respectively, and nine for the formation of prothoracic horns. At the same time, we document multiple instances of divergence in gene function across our focal structures. Collectively, our results support the hypothesis that dorsal and lateral support structures as well as prothoracic horns share a developmental origin with insect wings. Our findings suggest that the morphological and underlying gene regulatory diversification of wing serial homologues across species, life stages and segments has contributed significantly to the extraordinary diversity of arthropod appendages and outgrowths.

Control of tissue morphogenesis by the HOX gene Ultrabithorax

Mutations in the Ultrabithorax (Ubx) gene cause homeotic transformation of the normally two-winged Drosophila into a four-winged mutant fly. Ubx encodes a HOX family transcription factor that specifies segment identity, including transformation of the second set of wings into rudimentary halteres. Ubx is known to control the expression of many genes that regulate tissue growth and patterning, but how it regulates tissue morphogenesis to reshape the wing into a haltere is still unclear. Here, we show that Ubx acts by repressing the expression of two genes in the haltere, Stubble and Notopleural, both of which encode transmembrane proteases that remodel the apical extracellular matrix to promote wing morphogenesis. In addition, Ubx induces expression of the Tissue inhibitor of metalloproteases in the haltere, which prevents the basal extracellular matrix remodelling necessary for wing morphogenesis. Our results provide a long-awaited explanation for how Ubx controls morphogenetic transformation.

Life habits and evolutionary biology of new two-winged long-proboscid scorpionflies from mid-Cretaceous Myanmar amber

Long-proboscid scorpionflies are enigmatic, mid-Mesozoic insects associated with gymnosperm pollination. One major lineage, Aneuretopsychina, consists of four families plus two haustellate clades, Diptera and Siphonaptera. One clade, Pseudopolycentropodidae, from mid-Cretaceous Myanmar amber, contains Parapolycentropus. Here, we newly establish Dualula, assigned to Dualulidae, constituting the fifth lineage. Parapolycentropus and Dualula lineages are small, two-winged, with unique siphonate mouthparts for imbibing pollination drops. A cibarial pump provides siphonal food inflow; in Dualula, the siphon base surrounds a hypopharynx housing a small, valved pump constricted to a narrow salivary duct supplying outgoing enzymes for food fluidization. Indirect evidence links long-proboscid mouthpart structure with contemporaneous tubulate ovulate organs. Direct evidence of gymnospermous Cycadopites pollen is associated with one Parapolycentropus specimen. Parapolycentropus and Dualula exhibit hind-wing reduction that would precede haltere formation, likely caused by Ultrabithorax. Distinctive, male Aneuretopsychina genitalia are evident from specimens in copulo, supplemented by mixed-sex individuals of likely male mating swarms.

Long-proboscid scorpionflies were associated with mid-Mesozoic gymnosperm pollination. Here, Lin et al. establish a new family of long-proboscid scorpionflies from Myanmar amber, elucidate evolutionary mechanisms of hind-wing reduction, and detail feeding and reproductive habits of these insects.

apterous A specifies dorsal wing patterns and sexual traits in butterflies

Butterflies have evolved different colour patterns on their dorsal and ventral wing surfaces to serve different signalling functions, yet the developmental mechanisms controlling surface-specific patterning are still unknown. Here, we mutate both copies of the transcription factor apterous in Bicyclus anynana butterflies using CRISPR/Cas9 and show that apterous A, expressed dorsally, functions both as a repressor and modifier of ventral wing colour patterns, as well as a promoter of dorsal sexual ornaments in males. We propose that the surface-specific diversification of wing patterns in butterflies proceeded via the co-option of apterous A or its downstream effectors into various gene regulatory networks involved in the differentiation of discrete wing traits. Further, interactions between apterous and sex-specific factors such as doublesex may have contributed to the origin of sexually dimorphic surface-specific patterns. Finally, we discuss the evolution of eyespot number diversity in the family Nymphalidae within the context of developmental constraints due to apterous regulation.

Specification and Patterning of Drosophila Appendages

Appendages are external projections of the body that serve the animal for locomotion, feeding, or environment exploration. The appendages of the fruit fly Drosophilamelanogaster are derived from the imaginal discs, epithelial sac-like structures specified in the embryo that grow and pattern during larva development. In the last decades, genetic and developmental studies in the fruit fly have provided extensive knowledge regarding the mechanisms that direct the formation of the appendages. Importantly, many of the signaling pathways and patterning genes identified and characterized in Drosophila have similar functions during vertebrate appendage development. In this review, we will summarize the genetic and molecular mechanisms that lead to the specification of appendage primordia in the embryo and their posterior patterning during imaginal disc development. The identification of the regulatory logic underlying appendage specification in Drosophila suggests that the evolutionary origin of the insect wing is, in part, related to the development of ventral appendages.

Origins and Specification of the Drosophila Wing

The insect wing is a key evolutionary innovation that was essential for insect diversification. Yet despite its importance, there is still debate about its evolutionary origins. Two main hypotheses have been proposed: the paranotal hypothesis, which suggests that wings evolved as an extension of the dorsal thorax, and the gill-exite hypothesis, which proposes that wings were derived from a modification of a pre-existing branch at the dorsal base (subcoxa) of the leg. Here, we address this question by studying how wing fates are initially specified during Drosophila embryogenesis, by characterizing a cis-regulatory module (CRM) from the snail (sna) gene, sna-DP (for dorsal primordia). sna-DP specifically marks the early primordia for both the wing and haltere, collectively referred to as the DP. We found that the inputs that activate sna-DP are distinct from those that activate Distalless, a marker for leg fates. Further, in genetic backgrounds in which the leg primordia are absent, the DP are still partially specified. However, lineage-tracing experiments demonstrate that cells from the early leg primordia contribute to both ventral and dorsal appendage fates. Together, these results suggest that the wings of Drosophila have a dual developmental origin: two groups of cells, one ventral and one more dorsal, give rise to the mature wing. We suggest that the dual developmental origins of the wing may be a molecular remnant of the evolutionary history of this appendage, in which cells of the subcoxa of the leg coalesced with dorsal outgrowths to evolve a dorsal appendage with motor control.

Elytra reduction may affect the evolution of beetle hind wings

Beetles are one of the largest and most diverse groups of animals in the world. Conversion of forewings into hardened shields is perceived as a key adaptation that has greatly supported the evolutionary success of this taxa. Beetle elytra play an essential role: they minimize the influence of unfavorable external factors and protect insects against predators. Therefore, it is particularly interesting why some beetles have reduced their shields. This rare phenomenon is called brachelytry and its evolution and implications remain largely unexplored. In this paper, we focused on rare group of brachelytrous beetles with exposed hind wings. We have investigated whether the elytra loss in different beetle taxa is accompanied with the hind wing shape modification, and whether these changes are similar among unrelated beetle taxa. We found that hind wings shape differ markedly between related brachelytrous and macroelytrous beetles. Moreover, we revealed that modifications of hind wings have followed similar patterns and resulted in homoplasy in this trait among some unrelated groups of wing-exposed brachelytrous beetles. Our results suggest that elytra reduction may affect the evolution of beetle hind wings.

Mimicry in butterflies: co-option and a bag of magnificent developmental genetic tricks

Butterfly wing patterns are key adaptations that are controlled by remarkable developmental and genetic mechanisms that facilitate rapid evolutionary change. With swift advancements in the fields of genomics and genetic manipulations, identifying the regulators of wing development and mimetic wing patterns has become feasible even in nonmodel organisms such as butterflies. Recent mapping and gene expression studies have identified single switch loci of major effects such as transcription factors and supergenes as the main drivers of adaptive evolution of mimetic and polymorphic butterfly wing patterns. We highlight several of these examples, with emphasis on doublesex, optix, WntA and other dynamic, yet essential, master regulators that control critical color variation and sex-specific traits. Co-option emerges as a predominant theme, where typically embryonic and other early-stage developmental genes and networks have been rewired to regulate polymorphic and sex-limited mimetic wing patterns in iconic butterfly adaptations. Drawing comparisons from our knowledge of wing development in Drosophila, we illustrate the functional space of genes that have been recruited to regulate butterfly wing patterns. We also propose a developmental pathway that potentially results in dorsoventral mismatch in butterfly wing patterns. Such dorsoventrally mismatched color patterns modulate signal components of butterfly wings that are used in intra- and inter-specific communication. Recent advances-fuelled by RNAi-mediated knockdowns and CRISPR/Cas9-based genomic edits-in the developmental genetics of butterfly wing patterns, and the underlying biological diversity and complexity of wing coloration, are pushing butterflies as an emerging model system in ecological genetics and evolutionary developmental biology. WIREs Dev Biol 2018, 7:e291. doi: 10.1002/wdev.291 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Comparative Development and Evolution > Regulation of Organ Diversity Comparative Development and Evolution > Evolutionary Novelties.

What serial homologs can tell us about the origin of insect wings

Although the insect wing is a textbook example of morphological novelty, the origin of insect wings remains a mystery and is regarded as a chief conundrum in biology. Centuries of debates have culminated into two prominent hypotheses: the tergal origin hypothesis and the pleural origin hypothesis. However, between these two hypotheses, there is little consensus in regard to the origin tissue of the wing as well as the evolutionary route from the origin tissue to the functional flight device. Recent evolutionary developmental (evo-devo) studies have shed new light on the origin of insect wings. A key concept in these studies is “serial homology”. In this review, we discuss how the wing serial homologs identified in recent evo-devo studies have provided a new angle through which this century-old conundrum can be explored. We also review what we have learned so far from wing serial homologs and discuss what we can do to go beyond simply identifying wing serial homologs and delve further into the developmental and genetic mechanisms that have facilitated the evolution of insect wings.

Ultrabithorax and the evolution of insect forewing/hindwing differentiation

Decades have passed since the stunning four-winged phenotype of the Drosophila Ultrabithorax (Ubx) mutant was reported, and accumulating knowledge obtained from studies on Ubx in Drosophila has provided a framework to investigate the role of Ubx during insect wing evolution. With several new insights emerging from recent studies in non-Drosophila insects, along with the outcomes of genomic studies focused on identifying Ubx targets, it appears to be an appropriate time to revisit the Drosophila paradigm regarding insect wing development and evolution. Here, I review the recent findings related to Ubx during wing development, and discuss the impact of these findings on the current view of how Ubx came to regulate wing differentiation in the evolution of insect flight structures.

Dinosaurs, Chameleons, Humans, and Evo-Devo Path: Linking Étienne Geoffroy's Teratology, Waddington's Homeorhesis, Alberch's Logic of "Monsters," and Goldschmidt Hopeful "Monsters"

Since the rise of evo-devo (evolutionary developmental biology) in the 1980s, few authors have attempted to combine the increasing knowledge obtained from the study of model organisms and human medicine with data from comparative anatomy and evolutionary biology in order to investigate the links between development, pathology, and macroevolution. Fortunately, this situation is slowly changing, with a renewed interest in evolutionary developmental pathology (evo-devo-path) in the past decades, as evidenced by the idea to publish this special, and very timely, issue on "Developmental Evolution in Biomedical Research." As all of us have recently been involved, independently, in works related in some way or another with evolution and developmental anomalies, we decided to join our different perspectives and backgrounds in the present contribution for this special issue. Specifically, we provide a brief historical account on the study of the links between evolution, development, and pathologies, followed by a review of the recent work done by each of us, and then by a general discussion on the broader developmental and macroevolutionary implications of our studies and works recently done by other authors. Our primary aims are to highlight the strength of studying developmental anomalies within an evolutionary framework to understand morphological diversity and disease by connecting the recent work done by us and others with the research done and broader ideas proposed by authors such as Étienne Geoffroy Saint-Hilaire, Waddington, Goldschmidt, Gould, and Per Alberch, among many others to pave the way for further and much needed work regarding abnormal development and macroevolution.

Tergal and pleural structures contribute to the formation of ectopic prothoracic wings in cockroaches

Wings were a fundamental morphological innovation for the adaptive radiation of insects, the most diversified group among all animals. Pterygote insects have two pairs of wings, the mesothoracic (T2) forewings and the metathoracic (T3) hindwings, whereas the prothorax (T1) is wingless. Using RNA interference approaches, we have found that the gene Sex combs reduced (Scr) determines the wingless identity of T1 in the cockroach Blattella germanica. Interference of Scr triggers the formation of ectopic wing structures in T1, which are formed from the expansion of the latero-posterior region of the pronotum, along with a contribution of the epimeron, a pleurite of T1. These data support the theory of a dual origin for insect wings, from pronotal (tergal origin theory) and pleural (pleural origin theory) structures and genes.

Integration of Orthogonal Signaling by the Notch and Dpp Pathways in Drosophila

The transcription factor Suppressor of Hairless and its coactivator, the Notch intracellular domain, are polyglutamine (pQ)-rich factors that target enhancer elements and interact with other locally bound pQ-rich factors. To understand the functional repertoire of such enhancers, we identify conserved regulatory belts with binding sites for the pQ-rich effectors of both Notch and BMP/Dpp signaling, and the pQ-deficient tissue selectors Apterous (Ap), Scalloped (Sd), and Vestigial (Vg). We find that the densest such binding site cluster in the genome is located in the BMP-inducible nab locus, a homolog of the vertebrate transcriptional cofactors NAB1/NAB2 We report three major findings. First, we find that this nab regulatory belt is a novel enhancer driving dorsal wing margin expression in regions of peak phosphorylated Mad in wing imaginal discs. Second, we show that Ap is developmentally required to license the nab dorsal wing margin enhancer (DWME) to read out Notch and Dpp signaling in the dorsal compartment. Third, we find that the nab DWME is embedded in a complex of intronic enhancers, including a wing quadrant enhancer, a proximal wing disc enhancer, and a larval brain enhancer. This enhancer complex coordinates global nab expression via both tissue-specific activation and interenhancer silencing. We suggest that DWME integration of BMP signaling maintains nab expression in proliferating margin descendants that have divided away from Notch-Delta boundary signaling. As such, uniform expression of genes like nab and vestigial in proliferating compartments would typically require both boundary and nonboundary lineage-specific enhancers.

Exploring the origin of insect wings from an evo-devo perspective

Although insect wings are often used as an example of morphological novelty, the origin of insect wings remains a mystery and is regarded as a major conundrum in biology. Over a century of debates and observations have culminated in two prominent hypotheses on the origin of insect wings: the tergal hypothesis and the pleural hypothesis. However, despite accumulating efforts to unveil the origin of insect wings, neither hypothesis has been able to surpass the other. Recent investigations using the evolutionary developmental biology (evo-devo) approach have started shedding new light on this century-long debate. Here, we review these evo-devo studies and discuss how their findings may support a dual origin of insect wings, which could unify the two major hypotheses.

Origin and diversification of wings: Insights from a neopteran insect

Winged insects underwent an unparalleled evolutionary radiation, but mechanisms underlying the origin and diversification of wings in basal insects are sparsely known compared with more derived holometabolous insects. In the neopteran species Oncopeltus fasciatus, we manipulated wing specification genes and used RNA-seq to obtain both functional and genomic perspectives. Combined with previous studies, our results suggest the following key steps in wing origin and diversification. First, a set of dorsally derived outgrowths evolved along a number of body segments including the first thoracic segment (T1). Homeotic genes were subsequently co-opted to suppress growth of some dorsal flaps in the thorax and abdomen. In T1 this suppression was accomplished by Sex combs reduced, that when experimentally removed, results in an ectopic T1 flap similar to prothoracic winglets present in fossil hemipteroids and other early insects. Global gene-expression differences in ectopic T1 vs. T2/T3 wings suggest that the transition from flaps to wings required ventrally originating cells, homologous with those in ancestral arthropod gill flaps/epipods, to migrate dorsally and fuse with the dorsal flap tissue thereby bringing new functional gene networks; these presumably enabled the T2/T3 wing's increased size and functionality. Third, "fused" wings became both the wing blade and surrounding regions of the dorsal thorax cuticle, providing tissue for subsequent modifications including wing folding and the fit of folded wings. Finally, Ultrabithorax was co-opted to uncouple the morphology of T2 and T3 wings and to act as a general modifier of hindwings, which in turn governed the subsequent diversification of lineage-specific wing forms.

Nymphalid eyespot serial homologues originate as a few individualized modules

Serial homologues are repeated traits that share similar development but occur in different parts of the body. Variation in number of repeats accounts for substantial diversity in animal form and considerable work has focused on identifying the factors accounting for this variation. Little is known, however, about how serial homologues originally become repeated, or about the relative timing of repeat individuation relative to repeat origin. Here, we show that the serially repeated eyespots on nymphalid butterfly wings most likely arose as a small cluster of units on the ventral hindwing that were later co-opted to the dorsal and anterior wing surfaces. Based on comparative analyses of over 400 species, we found support for a model of eyespot origin followed by redeployment, rather than by the conventional model, where eyespots arose as a complete row of undifferentiated units that later gained individuation. In addition, eyespots most likely evolved from simpler pattern elements, single-coloured spots, which were already individuated among different wing sectors. Finally, the late appearance of eyespots on the dorsal, hidden wing surface further suggests that these novel complex traits originally evolved for one function (thwarting predator attacks) and acquired a second function (sexual signalling) when moved to a different body location. This broad comparative analysis illustrates how serial homologues may initially evolve as a few units serving a particular function and subsequently become repeated in novel body locations with new functions.

Over-expression of Ultrabithorax alters embryonic body plan and wing patterns in the butterfly Bicyclus anynana

In insects, forewings and hindwings usually have different shapes, sizes, and color patterns. A variety of RNAi experiments across insect species have shown that the hox gene Ultrabithorax (Ubx) is necessary to promote hindwing identity. However, it remains unclear whether Ubx is sufficient to confer hindwing fate to forewings across insects. Here, we address this question by over-expressing Ubx in the butterfly Bicyclus anynana using a heat-shock promoter. Ubx whole-body over-expression during embryonic and larvae development led to body plan changes in larvae but to mere quantitative changes to adult morphology, respectively. Embryonic heat-shocks led to fused segments, loss of thoracic and abdominal limbs, and transformation of head limbs to larger appendages. Larval heat-shocks led to reduced eyespot size in the expected homeotic direction, but neither additional eyespots nor wing shape changes were observed in forewings as expected of a homeotic transformation. Interestingly, Ubx was found to be expressed in a novel, non-characteristic domain - in the hindwing eyespot centers. Furthermore, ectopic expression of Ubx on the pupal wing activated the eyespot-associated genes spalt and Distal-less, known to be directly repressed by Ubx in the fly׳s haltere and leg primordia, respectively, and led to the differentiation of black wing scales. These results suggest that Ubx has been co-opted into a novel eyespot gene regulatory network, and that it is capable of activating black pigmentation in butterflies.

Insights into the molecular mechanisms underlying diversified wing venation among insects

Insect wings are great resources for studying morphological diversities in nature as well as in fossil records. Among them, variation in wing venation is one of the most characteristic features of insect species. Venation is therefore, undeniably a key factor of species-specific functional traits of the wings; however, the mechanism underlying wing vein formation among insects largely remains unexplored. Our knowledge of the genetic basis of wing development is solely restricted to Drosophila melanogaster. A critical step in wing vein development in Drosophila is the activation of the decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signalling pathway during pupal stages. A key mechanism is the directional transport of Dpp from the longitudinal veins into the posterior crossvein by BMP-binding proteins, resulting in redistribution of Dpp that reflects wing vein patterns. Recent works on the sawfly Athalia rosae, of the order Hymenoptera, also suggested that the Dpp transport system is required to specify fore- and hindwing vein patterns. Given that Dpp redistribution via transport is likely to be a key mechanism for establishing wing vein patterns, this raises the interesting possibility that distinct wing vein patterns are generated, based on where Dpp is transported. Experimental evidence in Drosophila suggests that the direction of Dpp transport is regulated by prepatterned positional information. These observations lead to the postulation that Dpp generates diversified insect wing vein patterns through species-specific positional information of its directional transport. Extension of these observations in some winged insects will provide further insights into the mechanisms underlying diversified wing venation among insects.

Evidence for at least six Hox clusters in the Japanese lamprey (Lethenteron japonicum)

Cyclostomes, comprising jawless vertebrates such as lampreys and hagfishes, are the sister group of living jawed vertebrates (gnathostomes) and hence an important group for understanding the origin and diversity of vertebrates. In vertebrates and other metazoans, Hox genes determine cell fate along the anteroposterior axis of embryos and are implicated in driving morphological diversity. Invertebrates contain a single Hox cluster (either intact or fragmented), whereas elephant shark, coelacanth, and tetrapods contain four Hox clusters owing to two rounds of whole-genome duplication ("1R" and "2R") during early vertebrate evolution. By contrast, most teleost fishes contain up to eight Hox clusters because of an additional "teleost-specific" genome duplication event. By sequencing bacterial artificial chromosome (BAC) clones and the whole genome, here we provide evidence for at least six Hox clusters in the Japanese lamprey (Lethenteron japonicum). This suggests that the lamprey lineage has experienced an additional genome duplication after 1R and 2R. The relative age of lamprey and human paralogs supports this hypothesis. Compared with gnathostome Hox clusters, lamprey Hox clusters are unusually large. Several conserved noncoding elements (CNEs) were predicted in the Hox clusters of lamprey, elephant shark, and human. Transgenic zebrafish assay indicated the potential of CNEs to function as enhancers. Interestingly, CNEs in individual lamprey Hox clusters are frequently conserved in multiple Hox clusters in elephant shark and human, implying a many-to-many orthology relationship between lamprey and gnathostome Hox clusters. Such a relationship suggests that the first two rounds of genome duplication may have occurred independently in the lamprey and gnathostome lineages.

Regional and segmental differences in the embryonic expression of a putative leech Hox gene, Lox2, by central neurons immunoreactive to FMRFamide-like neuropeptides

We performed immunofluorescence experiments using a rat polyclonal antibody on formaldehyde-fixed whole-mount embryos to characterize the expression of a putative leech Hox gene, Lox2, during embryonic development. The main goal was to determine whether the differentiation of subsets of FMRFamide-like immunoreactive (FLI) neurons coincide with the expression domain of Lox2. The earliest expression of Lox2 was detected in relatively large, prominent nuclei in the posterior region at embryonic day 4, a very early stage. Lox2 expression was also detected in subsets of central neurons (neurons located in the CNS) located in midbody ganglia 6 (M6)-M21. In addition, Lox2 was expressed by a number of segment-specific and segmentally repeated central FLI neurons. Lox2-positive FLI neurons of interest included some of those previously identified: the rostral most ventral (RMV) neurons, the circular ventral (CV) neurons, and cell 261. The paired RMVs, which are located in all midbody ganglia, expressed Lox2 only in M7-M19. The CV neurons, specialized motor neurons that innervate the circular ventral muscles of the body wall, expressed Lox2 in M7-M19. The putative cell 261 expressed Lox2 in M7-M12, where Lox1 is also expressed. FMRFamide staining in putative segmental homologs of cell 261 was not detected in other segmental ganglia. Our results suggest a role for Lox2 in very early embryonic development (before the formation of the CNS), and in the differentiation of segmentally repeated and region-specific FLI neurons.

Paths less traveled: evo-devo approaches to investigating animal morphological evolution

One of the chief aims of modern biology is to understand the causes and mechanisms of morphological evolution. Multicellular animals display a stunning diversity of shapes and sizes of their bodies and individual suborganismal structures, much of it important to their survival. What is the most efficient way to study the evolution of morphological diversity? The old-new field of evolutionary developmental biology (evo-devo) can be particularly useful for understanding the origins of animal forms, as it aims to consolidate advances from disparate fields such as phylogenetics, genomics, morphometrics, cell biology, and developmental biology. We analyze the structure of some of the most successful recent evo-devo studies, which we see as having three distinct but highly interdependent components: (a) morphometrics, (b) identification of candidate mechanisms, and (c) functional experiments. Our case studies illustrate how multifarious evo-devo approaches taken within the three-winged evo-devo research program explain developmental mechanisms for morphological evolution across different phylogenetic scales.

Conservation and variation in Hox genes: how insect models pioneered the evo-devo field

Evolutionary developmental biology, or evo-devo, broadly investigates how body plan diversity and morphological novelties have arisen and persisted in nature. The discovery of Hox genes in Drosophila, and their subsequent identification in most other metazoans, led biologists to try to understand how embryonic genes crucial for proper development have changed to promote the vast morphological variation seen in nature. Insects are ideal model systems for studying this diversity and the mechanisms underlying it because phylogenetic relationships are well established, powerful genetic tools have been developed, and there are many examples of evolutionary specializations that have arisen in nature in different insect lineages, such as the jumping leg of orthopterans and the helmet structures of treehoppers. Here, we briefly introduce the field of evo-devo and Hox genes, discuss functional tools available to study early developmental genes in insects, and provide examples in which changes in Hox genes have contributed to changes in body plan or morphology.

Position matters: variability in the spatial pattern of BMP modulators generates functional diversity

Bone morphogenetic proteins (BMPs) perform a variety of functions during development. Considering a single BMP, what enables its multiple roles in tissues of varied sizes and shapes? What regulates the spatial distribution and activity patterns of the BMP in these different developmental contexts? Some BMP functions require controlling spread of the BMP morphogen, while others require formation of localized, high concentration peaks of BMP activity. Here we review work in Drosophila that describes spatial regulation of the BMP encoded by decapentaplegic (dpp) in different developmental contexts. We concentrate on extracellular modulation of BMP function and discuss the mechanisms that generate concentrated peaks of Dpp activity, subdivide territories of different activity levels or regulate spread of the Dpp morphogen from a point source. We compare these findings with data from vertebrates and non-model organisms to discuss how changes in the regulation of Dpp distribution by extracellular modulators may lead to variability in dpp function in different species.

Divergent role of the Hox gene Antennapedia in spiders is responsible for the convergent evolution of abdominal limb repression

Evolution often results in morphologically similar solutions in different organisms, a phenomenon known as convergence. However, there is little knowledge of the processes that lead to convergence at the genetic level. The genes of the Hox cluster control morphology in animals. They may also be central to the convergence of morphological traits, but whether morphological similarities also require similar changes in Hox gene function is disputed. In arthropods, body subdivision into a region with locomotory appendages ("thorax") and a region with reduced appendages ("abdomen") has evolved convergently in several groups, e.g., spiders and insects. In insects, legs develop in the expression domain of the Hox gene Antennapedia (Antp), whereas the Hox genes Ultrabithorax (Ubx) and abdominal-A mediate leg repression in the abdomen. Here, we show that, unlike Antp in insects, the Antp gene in the spider Achaearanea tepidariorum represses legs in the first segment of the abdomen (opisthosoma), and that Antp and Ubx are redundant in the following segment. The down-regulation of Antp in A. tepidariorum leads to a striking 10-legged phenotype. We present evidence from ectopic expression of the spider Antp gene in Drosophila embryos and imaginal tissue that this unique function of Antp is not due to changes in the Antp protein, but likely due to divergent evolution of cofactors, Hox collaborators or target genes in spiders and flies. Our results illustrate an interesting example of convergent evolution of abdominal leg repression in arthropods by altering the role of distinct Hox genes at different levels of their action.