Activation of butterfly eyespots by Distal-less is consistent with a reaction-diffusion process

Socket Array Irregularities and Wing Membrane Distortions at the Eyespot Foci of Butterfly Wings Suggest Mechanical Signals for Color Pattern Determination

Eyespot foci on butterfly wings function as organizers of eyespot color patterns during development. Despite their importance, focal structures have not been examined in detail. Here, we microscopically examined scales, sockets, and the wing membrane in the butterfly eyespot foci of both expanded and unexpanded wings using the Blue Pansy butterfly Junonia orithya. Images from a high-resolution light microscope revealed that, although not always, eyespot foci had scales with disordered planar polarity. Scanning electron microscopy (SEM) images after scale removal revealed that the sockets were irregularly positioned and that the wing membrane was physically distorted as if the focal site were mechanically squeezed from the surroundings. Focal areas without eyespots also had socket array irregularities, but less frequently and less severely. Physical damage in the background area induced ectopic patterns with socket array irregularities and wing membrane distortions, similar to natural eyespot foci. These results suggest that either the process of determining an eyespot focus or the function of an eyespot organizer may be associated with wing-wide mechanics that physically disrupt socket cells, scale cells, and the wing membrane, supporting the physical distortion hypothesis of the induction model for color pattern determination in butterfly wings.

Cis-regulatory modes of Ultrabithorax inactivation in butterfly forewings

Hox gene clusters encode transcription factors that drive regional specialization during animal development: for example the Hox factor Ubx is expressed in the insect metathoracic (T3) wing appendages and differentiates them from T2 mesothoracic identities. Hox transcriptional regulation requires silencing activities that prevent spurious activation and regulatory crosstalks in the wrong tissues, but this has seldom been studied in insects other than Drosophila, which shows a derived Hox dislocation into two genomic clusters that disjoined Antennapedia (Antp) and Ultrabithorax (Ubx). Here, we investigated how Ubx is restricted to the hindwing in butterflies, amidst a contiguous Hox cluster. By analysing Hi-C and ATAC-seq data in the butterfly Junonia coenia, we show that a Topologically Associated Domain (TAD) maintains a hindwing-enriched profile of chromatin opening around Ubx. This TAD is bordered by a Boundary Element (BE) that separates it from a region of joined wing activity around the Antp locus. CRISPR mutational perturbation of this BE releases ectopic Ubx expression in forewings, inducing homeotic clones with hindwing identities. Further mutational interrogation of two non-coding RNA encoding regions and one putative cis-regulatory module within the Ubx TAD cause rare homeotic transformations in both directions, indicating the presence of both activating and repressing chromatin features. We also describe a series of spontaneous forewing homeotic phenotypes obtained in Heliconius butterflies, and discuss their possible mutational basis. By leveraging the extensive wing specialization found in butterflies, our initial exploration of Ubx regulation demonstrates the existence of silencing and insulating sequences that prevent its spurious expression in forewings.

Butterfly eyespots exhibit unique patterns of open chromatin

Background: How the precise spatial regulation of genes is correlated with spatial variation in chromatin accessibilities is not yet clear. Previous studies that analysed chromatin from homogenates of whole-body parts of insects found little variation in chromatin accessibility across those parts, but single-cell studies of Drosophila brains showed extensive spatial variation in chromatin accessibility across that organ. In this work we studied the chromatin accessibility of butterfly wing tissue fated to differentiate distinct colors and patterns in pupal wings of Bicyclus anynana. Methods: We dissected small eyespot and adjacent control tissues from 3h pupae and performed ATAC-Seq to identify the chromatin accessibility differences between different sections of the wings. Results: We observed that three dissected wing regions showed unique chromatin accessibilities. Open chromatin regions specific to eyespot color patterns were highly enriched for binding motifs recognized by Suppressor of Hairless (Su(H)), Krüppel (Kr), Buttonhead (Btd) and Nubbin (Nub) transcription factors. Genes in the vicinity of the eyespot-specific open chromatin regions included those involved in wound healing and SMAD signal transduction pathways, previously proposed to be involved in eyespot development. Conclusions: We conclude that eyespot and non-eyespot tissue samples taken from the same wing have distinct patterns of chromatin accessibility, possibly driven by the eyespot-restricted expression of potential pioneer factors, such as Kr.

Lepidopteran prolegs are novel traits, not leg homologs

Lepidopteran larvae have both thoracic legs and abdominal prolegs, yet it is unclear whether these are serial homologs. A RNA-seq analysis with various appendages of Bicyclus anynana butterfly larvae indicated that the proleg transcriptome resembles the head-horn transcriptome, a novel trait in the lepidoptera, but not a thoracic leg. Under a partial segment abdominal-A (abd-A) knockout, both thoracic leg homologs (pleuropodia) and prolegs developed in the same segment, arguing that both traits are not serial homologs. Further, three of the four coxal marker genes, Sp5, Sp6-9, and araucan, were absent from prolegs, but two endite marker genes, gooseberry and Distal-less, were expressed in prolegs, suggesting that prolegs may be using a modular endite gene-regulatory network (GRN) for their development. We propose that larval prolegs are novel traits derived from the activation of a pre-existing modular endite GRN in the abdomen using abd-A, the same Hox gene that still represses legs in more lateral positions.

Frizzled2 receives WntA signaling during butterfly wing pattern formation

Butterfly color patterns provide visible and biodiverse phenotypic readouts of the patterning processes. Although the secreted ligand WntA has been shown to instruct the color pattern formation in butterflies, its mode of reception remains elusive. Butterfly genomes encode four homologs of the Frizzled-family of Wnt receptors. Here, we show that CRISPR mosaic knockouts of frizzled2 (fz2) phenocopy the color pattern effects of WntA loss of function in multiple nymphalids. Whereas WntA mosaic clones result in intermediate patterns of reduced size, fz2 clones are cell-autonomous, consistent with a morphogen function. Shifts in expression of WntA and fz2 in WntA crispant pupae show that they are under positive and negative feedback, respectively. Fz1 is required for Wnt-independent planar cell polarity in the wing epithelium. Fz3 and Fz4 show phenotypes consistent with Wnt competitive-antagonist functions in vein formation (Fz3 and Fz4), wing margin specification (Fz3), and color patterning in the Discalis and Marginal Band Systems (Fz4). Overall, these data show that the WntA/Frizzled2 morphogen-receptor pair forms a signaling axis that instructs butterfly color patterning and shed light on the functional diversity of insect Frizzled receptors.

Spatial and temporal regulation of Wnt signaling pathway members in the development of butterfly wing patterns

Wnt signaling members are involved in the differentiation of cells associated with eyespot and band color patterns on the wings of butterflies, but the identity and spatio-temporal regulation of specific Wnt pathway members remains unclear. Here, we explore the localization and function of Armadillo/β-catenin dependent (canonical) and Armadillo/β-catenin independent (noncanonical) Wnt signaling in eyespot and band development in Bicyclus anynana by localizing Armadillo (Arm), the expression of all eight Wnt ligand and four frizzled receptor transcripts present in the genome of this species and testing the function of some of the ligands and receptors using CRISPR-Cas9. We show that distinct Wnt signaling pathways are essential for eyespot and band patterning in butterflies and are likely interacting to control their active domains.

The genetic basis of wing spots in Pieris canidia butterflies

Spots in pierid butterflies and eyespots in nymphalid butterflies are likely non-homologous wing colour pattern elements, yet they share a few features in common. Both develop black scales that depend on the function of the gene spalt, and both might have central signalling cells. This suggests that both pattern elements may be sharing common genetic circuitry. Hundreds of genes have already been associated with the development of nymphalid butterfly eyespot patterns, but the genetic basis of the simpler spot patterns on the wings of pierid butterflies has not been investigated. To facilitate studies of pierid wing patterns, we report a high-quality draft genome assembly for Pieris canidia, the Indian cabbage white. We then conducted transcriptomic analyses of pupal wing tissues sampled from the spot and non-spot regions of P. canidia at 3-6 h post-pupation. A total of 1352 genes were differentially regulated between wing tissues with and without the black spot, including spalt, Krüppel-like factor 10, genes from the Toll, Notch, TGF-β, and FGFR signalling pathways, and several genes involved in the melanin biosynthetic pathway. We identified 14 genes that are up-regulated in both pierid spots and nymphalid eyespots and propose that spots and eyespots share regulatory modules despite their likely independent origins.

Colour patterns: Predicting patterns without knowing the details

A bewildering array of colour patterns occurs across animals. New work studying five lizard species reveals that reaction-diffusion models can be remarkably predictive of future adult skin patterning, even though molecular details are unknown. This has implications for understanding how complex patterns evolve.

Ultrabithorax modifies a regulatory network of genes essential for butterfly eyespot development in a wing sector-specific manner

Nymphalid butterfly species often have a different number of eyespots in forewings and hindwings, but how the hindwing identity gene Ultrabithorax (Ubx) drives this asymmetry is not fully understood. We examined a three-gene regulatory network for eyespot development in the hindwings of Bicyclus anynana butterflies and compared it with the same network previously described for forewings. We also examined how Ubx interacts with each of these three eyespot-essential genes. We found similar genetic interactions between the three genes in fore- and hindwings, but we discovered three regulatory differences: Antennapedia (Antp) merely enhances spalt (sal) expression in the eyespot foci in hindwings, but is not essential for sal activation, as in forewings; Ubx upregulates Antp in all hindwing eyespot foci but represses Antp outside these wing regions; and Ubx regulates sal in a wing sector-specific manner, i.e. it activates sal expression only in the sectors that have hindwing-specific eyespots. We propose a model for how the regulatory connections between these four genes evolved to produce wing- and sector-specific variation in eyespot number.

Butterfly Wing Color Pattern Modification Inducers May Act on Chitin in the Apical Extracellular Site: Implications in Morphogenic Signals for Color Pattern Determination

Butterfly wing color patterns are modified by various treatments, such as temperature shock, injection of chemical inducers, and covering materials on pupal wing tissue. Their mechanisms of action have been enigmatic. Here, we investigated the mechanisms of color pattern modifications usingthe blue pansy butterfly Junoniaorithya. We hypothesized that these modification-inducing treatments act on the pupal cuticle or extracellular matrix (ECM). Mechanical load tests revealed that pupae treated with cold shock or chemical inducers were significantly less rigid, suggesting that these treatments made cuticle formation less efficient. A known chitin inhibitor, FB28 (fluorescent brightener 28), was discovered to efficiently induce modifications. Taking advantage of its fluorescent character, fluorescent signals from FB28 were observed in live pupae in vivo from the apical extracellular side and were concentrated at the pupal cuticle focal spots immediately above the eyespot organizing centers. It was shown that chemical modification inducers and covering materials worked additively. Taken together, various modification-inducing treatments likely act extracellularly on chitin or other polysaccharides to inhibit pupal cuticle formation or ECM function, which probably causes retardation of morphogenic signals. It is likely that an interactive ECM is required for morphogenic signals for color pattern determination to travel long distances.

A transcriptomic atlas underlying developmental plasticity of seasonal forms of Bicyclus anynana butterflies

Organisms residing in regions with alternating seasons often develop different phenotypes, or forms, in each season. These forms are often adaptations to each season and result from an altered developmental response to specific environmental cues such as temperature. While multiple studies have examined form-specific gene expression profiles in a diversity of species, little is known about how environments and developmental transitions, cued by hormone pulses, alter post-transcriptional patterns. In this study, we examine how gene expression, alternative splicing, and miRNA-mediated gene silencing in Bicyclus anynana butterfly hindwing tissue, varies across two rearing temperatures at four developmental timepoints. These timepoints flank two temperature-sensitive periods that coincide with two pulses of the insect hormone 20E. Our results suggest that developmental transitions, coincident with 20E pulses, elicit a greater impact on all these transcriptomic patterns than rearing temperatures per se. More similar transcriptomic patterns are observed pre-20E pulses than those observed post-20E pulses. We also found functionally distinct sets of differentially expressed and differentially spliced genes in the seasonal forms. Furthermore, around 10% of differentially expressed genes are predicted to be direct targets of, and regulated by, differentially expressed miRNAs between the seasonal forms. Many differentially expressed genes, miRNAs, or differentially spliced genes potentially regulate eyespot size plasticity, and we validated the differential splicing pattern of one such gene, daughterless. We present a comprehensive and interactive transcriptomic atlas of the hindwing tissue of both seasonal forms of B. anynana throughout development, a model organism of seasonal plasticity.

Distal-less and spalt are distal organisers of pierid wing patterns

Two genes, Distal-less (Dll) and spalt (sal), are known to be involved in establishing nymphalid butterfly wing patterns. They function in several ways: in the differentiation of the eyespot’s central signalling cells, or foci; in the differentiation of the surrounding black disc; in overall scale melanisation (Dll); and in elaborating marginal patterns, such as parafocal elements. However, little is known about the functions of these genes in the development of wing patterns in other butterfly families. Here, we study the expression and function of Dll and sal in the development of spots and other melanic wing patterns of the Indian cabbage white, Pieris canidia, a pierid butterfly. In P. canidia, both Dll and Sal proteins are expressed in the scale-building cells at the wing tips, in chevron patterns along the pupal wing margins, and in areas of future scale melanisation. Additionally, Sal alone is expressed in the future black spots. CRISPR knockouts of Dll and sal showed that each gene is required for the development of melanic wing pattern elements, and repressing pteridine granule formation, in the areas where they are expressed. We conclude that both genes likely play ancestral roles in organising distal butterfly wing patterns, across pierid and nymphalid butterflies, but are unlikely to be differentiating signalling centres in pierids black spots. The genetic and developmental mechanisms that set up the location of spots and eyespots are likely distinct in each lineage.

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.

Stage- and sex-specific transcriptome analyses reveal distinctive sensory gene expression patterns in a butterfly

Background

Animal behavior is largely driven by the information that animals are able to extract and process from their environment. However, the function and organization of sensory systems often change throughout ontogeny, particularly in animals that undergo indirect development. As an initial step toward investigating these ontogenetic changes at the molecular level, we characterized the sensory gene repertoire and examined the expression profiles of genes linked to vision and chemosensation in two life stages of an insect that goes through metamorphosis, the butterfly Bicyclus anynana.

Results

Using RNA-seq, we compared gene expression in the heads of late fifth instar larvae and newly eclosed adults that were reared under identical conditions. Over 50 % of all expressed genes were differentially expressed between the two developmental stages, with 4,036 genes upregulated in larval heads and 4,348 genes upregulated in adult heads. In larvae, upregulated vision-related genes were biased toward those involved with eye development, while phototransduction genes dominated the vision genes that were upregulated in adults. Moreover, the majority of the chemosensory genes we identified in the B. anynana genome were differentially expressed between larvae and adults, several of which share homology with genes linked to pheromone detection, host plant recognition, and foraging in other species of Lepidoptera.

Conclusions

These results revealed promising candidates for furthering our understanding of sensory processing and behavior in the disparate developmental stages of butterflies and other animals that undergo metamorphosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07819-4.

Porcupine/Wntless-dependent trafficking of the conserved WntA ligand in butterflies

Wnt ligands are key signaling molecules in animals, but little is known about the evolutionary dynamics and mode of action of the WntA orthologs, which are not present in the vertebrates or in Drosophila. Here we show that the WntA subfamily evolved at the base of the Bilateria + Cnidaria clade, and conserved the thumb region and Ser209 acylation site present in most other Wnts, suggesting WntA requires the core Wnt secretory pathway. WntA proteins are distinguishable from other Wnts by a synapomorphic Iso/Val/Ala216 amino-acid residue that replaces the otherwise ubiquitous Thr216 position. WntA embryonic expression is conserved between beetles and butterflies, suggesting functionality, but the WntA gene was lost three times within arthropods, in podoplean copepods, in the cyclorrhaphan fly radiation, and in ensiferan crickets and katydids. Finally, CRISPR mosaic knockouts (KOs) of porcupine and wntless phenocopied the pattern-specific effects of WntA KOs in the wings of Vanessa cardui butterflies. These results highlight the molecular conservation of the WntA protein across invertebrates, and imply it functions as a typical Wnt ligand that is acylated and secreted through the Porcupine/Wntless secretory pathway.

The seven ways eukaryotes produce repeated colour motifs on external tissues

The external tissues of numerous eukaryote species show repeated colour patterns, usually characterized by units that are present at least twice on the body. These dotted, striped or more complex phenotypes carry out crucial biological functions, such as partner recognition, aposematism or camouflage. Very diverse mechanisms explaining the formation of repeated colour patterns in eukaryotes have been identified and described, and it is timely to review this field from an evolutionary and developmental biology perspective. We propose a novel classification consisting of seven families of primary mechanisms: Turing(-like), cellular automaton, multi-induction, physical cracking, random, neuromuscular and printing. In addition, we report six pattern modifiers, acting synergistically with these primary mechanisms to enhance the spectrum of repeated colour patterns. We discuss the limitations of our classification in light of currently unexplored extant diversity. As repeated colour patterns require both the production of a repetitive structure and colouration, we also discuss the nature of the links between these two processes. A more complete understanding of the formation of repeated colour patterns in eukaryotes will require (i) a deeper exploration of biological diversity, tackling the issue of pattern elaboration during the development of non-model taxa, and (ii) exploring some of the most promising ways to discover new families of mechanisms. Good starting points include evaluating the role of mechanisms known to produce non-repeated colour patterns and that of mechanisms responsible for repeated spatial patterns lacking colouration.

Ultrabithorax Is a Micromanager of Hindwing Identity in Butterflies and Moths

The forewings and hindwings of butterflies and moths (Lepidoptera) are differentiated from each other, with segment-specific morphologies and color patterns that mediate a wide range of functions in flight, signaling, and protection. The Hox geneUltrabithorax(Ubx) is a master selector gene that differentiates metathoracic from mesothoracic identities across winged insects, and previous work has shown this role extends to at least some of the color patterns from the butterfly hindwing. Here we used CRISPR targeted mutagenesis to generateUbxloss-of-function somatic mutations in two nymphalid butterflies (Junonia coenia,Vanessa cardui) and a pyralid moth (Plodia interpunctella). The resulting mosaic clones yielded hindwing-to-forewing transformations, showingUbxis necessary for specifying many aspects of hindwing-specific identities, including scale morphologies, color patterns, and wing venation and structure. These homeotic phenotypes showed cell-autonomous, sharp transitions between mutant and non-mutant scales, except for clones that encroached into the border ocelli (eyespots) and resulted in composite and non-autonomous effects on eyespot ring determination. In the pyralid moth, homeotic clones converted the folding and depigmented hindwing into rigid and pigmented composites, affected the wing-coupling frenulum, and induced ectopic scent-scales in male androconia. These data confirmUbxis a master selector of lepidopteran hindwing identity and suggest it acts on many gene regulatory networks involved in wing development and patterning.

Growing Up in a Changing World: Environmental Regulation of Development in Insects

All organisms are exposed to changes in their environment throughout their life cycle. When confronted with these changes, they adjust their development and physiology to ensure that they can produce the functional structures necessary for survival and reproduction. While some traits are remarkably invariant, or robust, across environmental conditions, others show high degrees of variation, known as plasticity. Generally, developmental processes that establish cell identity are thought to be robust to environmental perturbation, while those relating to body and organ growth show greater degrees of plasticity. However, examples of plastic patterning and robust organ growth demonstrate that this is not a hard-and-fast rule.In this review, we explore how the developmental context and the gene regulatory mechanisms underlying trait formation determine the impacts of the environment on development in insects. Furthermore, we outline future issues that need to be resolved to understand how the structure of signaling networks defines whether a trait displays plasticity or robustness.

The Fractal Geometry of the Nymphalid Groundplan: Self-Similar Configuration of Color Pattern Symmetry Systems in Butterfly Wings

Simple Summary

Highly diverse color patterns of butterfly wings can be explained as modifications of an archetypical color pattern of nymphalid butterflies called the nymphalid groundplan. The nymphalid groundplan contains three major symmetry systems and a discal symmetry system, but their relationships have been elusive. Here, the morphological and spatial relationships among these symmetry systems were studied based on cross-species color-pattern comparisons of the hindwings in nymphalid butterflies. It was shown that all symmetry systems can be expressed as various structures, suggesting the equivalence (homology) of these systems in developmental potential. In some cases, the discal symmetry system is circularly surrounded by the central symmetry system, which may then be surrounded by the border and basal symmetry systems, indicating a unified supersymmetry system covering the entire wing. These results suggest that butterfly color patterns are hierarchically constructed; one system is nested within another system, which is a self-similar relationship that achieves the fractal geometry. This self-similarity is likely mediated by the serial induction of organizers during development, and a possible mechanism is proposed for symmetry breaking of the system morphology, which contributes to the diversity of butterfly wing color patterns.

Abstract

The nymphalid groundplan is an archetypical color pattern of nymphalid butterflies involving three major symmetry systems and a discal symmetry system, which share the basic morphogenesis unit. Here, the morphological and spatial relationships among these symmetry systems were studied based on cross-species comparisons of nymphalid hindwings. Based on findings in Neope and Symbrenthia, all three major symmetry systems can be expressed as bands, spots, or eyespot-like structures, suggesting equivalence (homology) of these systems in developmental potential. The discal symmetry system can also be expressed as various structures. The discal symmetry system is circularly surrounded by the central symmetry system, which may then be surrounded by the border and basal symmetry systems, based mainly on findings in Agrias, indicating a unified supersymmetry system covering the entire wing. The border symmetry system can occupy the central part of the wing when the central symmetry system is compromised, as seen in Callicore. These results suggest that butterfly color patterns are hierarchically constructed in a self-similar fashion, as the fractal geometry of the nymphalid groundplan. This self-similarity is likely mediated by the serial induction of organizers, and symmetry breaking of the system morphology may be generated by the collision of opposing signals during development.

Distinguishing serial homologs from novel traits: Experimental limitations and ideas for improvements

One of the central but yet unresolved problems in evolutionary biology concerns the origin of novel complex traits. One hypothesis is that complex traits derive from pre-existing gene regulatory networks (GRNs) reused and modified to specify a novel trait somewhere else in the body. This simple explanation encounters problems when the novel trait that emerges in a body is in a region that is known to harbor a latent or repressed trait that has been silent for millions of years. Is the novel trait merely a re-emerged de-repressed trait or a truly novel trait that emerged via a novel deployment of an old GRN? A couple of new studies sided on opposite sides of this question when investigating the origin of horns in dung beetles and helmets in treehoppers that develop in the first thoracic segment (T1) of their bodies, a segment known to harbor a pair of repressed/modified wings in close relatives. Here, I point to some key limitations of the experimental approaches used and highlight additional experiments that could be done in future to resolve the developmental origin of these and other traits.

A release from developmental bias accelerates morphological diversification in butterfly eyespots

Development can bias the independent evolution of traits sharing ontogenetic pathways, making certain evolutionary changes less likely. The eyespots commonly found on butterfly wings each have concentric rings of differing colors, and these serially repeated pattern elements have been a focus for evo-devo research. In the butterfly family Nymphalidae, eyespots have been shown to function in startling or deflecting predators and to be involved in sexual selection. Previous work on a model species of Mycalesina butterfly, Bicyclus anynana, has provided insights into the developmental control of the size and color composition of individual eyespots. Experimental evolution has also shown that the relative size of a pair of eyespots on the same wing surface is highly flexible, whereas they are resistant to diverging in color composition, presumably due to the underlying shared developmental process. This fixed color composition has been considered as a prime example of developmental bias with significant consequences for wing pattern evolution. Here, we test this proposal by surveying eyespots across the whole subtribe of Mycalesina butterflies and demonstrate that developmental bias shapes evolutionary diversification except in the genus Heteropsis which has gained independent control of eyespot color composition. Experimental manipulations of pupal wings reveal that the bias has been released through a novel regional response of the wing tissue to a conserved patterning signal. Our study demonstrates that development can bias the evolutionary independence of traits, but it also shows how bias can be released through developmental innovations, thus, allowing rapid morphological change, facilitating evolutionary diversification.

Transcription factors underlying wing margin color patterns and pupal cuticle markings in butterflies

Background

The diversity of butterfly color patterns can be attributed to a relatively small number of pattern elements that are homologous across Lepidoptera. Although genes involved in patterning some of these elements have been identified, the development of several major elements remains poorly understood. To identify genes underlying wing pupal cuticle markings and wing margin color patterns, we examined expression of the candidate transcription factors Engrailed/Invected (En/Inv), Distal-less (Dll), Cubitus interruptus (Ci), and Spalt in two nymphalids: Junonia coenia and Bicyclus anynana.

Results

We found that En/Inv, Dll, and Ci mark domains on the J. coenia last-instar forewing disc that closely correspond to the position and shape of pupal cuticle markings. We also found that Spalt demarcates wing margin color patterns in both J. coenia and B. anynana, and that CRISPR/Cas9 deletions in the spalt gene result in reduction and loss of wing margin color patterns in J. coenia. These data demonstrate a role for spalt in promoting wing margin color patterning, in addition to its previously described role in eyespot patterning.

Conclusion

Our observations support the model that a core set of regulatory genes are redeployed multiple times, and in multiple roles, during butterfly wing pattern development. Of these genes, spalt is of special interest as it plays a dual role in both eyespot and margin color pattern development.

Temporal flexibility of gene regulatory network underlies a novel wing pattern in flies

Developmental genes can be coopted to generate evolutionary novelties by changing their spatial regulation. However, developmental genes seldom act independently, but rather work in a gene regulatory network (GRN). How is it possible to recruit a single gene from a whole GRN? What are the properties that allow parallel cooptions of the same genes during evolution? Here, we show that a novel engrailed gene expression underlies a novel wing color pattern in flies. We show that cooption is facilitated 1) because of GRN flexibility over development and 2) because every single gene of the GRN has its own functional time window. We suggest these two temporal properties could explain why the same gene can be independently recruited several times during evolution.

Organisms have evolved endless morphological, physiological, and behavioral novel traits during the course of evolution. Novel traits were proposed to evolve mainly by orchestration of preexisting genes. Over the past two decades, biologists have shown that cooption of gene regulatory networks (GRNs) indeed underlies numerous evolutionary novelties. However, very little is known about the actual GRN properties that allow such redeployment. Here we have investigated the generation and evolution of the complex wing pattern of the fly Samoaia leonensis. We show that the transcription factor Engrailed is recruited independently from the other players of the anterior–posterior specification network to generate a new wing pattern. We argue that partial cooption is made possible because 1) the anterior–posterior specification GRN is flexible over time in the developing wing and 2) this flexibility results from the fact that every single gene of the GRN possesses its own functional time window. We propose that the temporal flexibility of a GRN is a general prerequisite for its possible cooption during the course of evolution.

Transcriptome analysis reveals wingless regulates neural development and signaling genes in the region of wing pigmentation of a polka-dotted fruit fly

How evolutionary novelties have arisen is one of the central questions in evolutionary biology. Preexisting gene regulatory networks or signaling pathways have been shown to be co-opted for building novel traits in several organisms. However, the structure of entire gene regulatory networks and evolutionary events of gene co-option for emergence of a novel trait are poorly understood. In this study, to explore the genetic and molecular bases of the novel wing pigmentation pattern of a polka-dotted fruit fly (Drosophila guttifera), we performed de novo genome sequencing and transcriptome analyses. As a result, we comprehensively identified the genes associated with the pigmentation pattern. Furthermore, we revealed that 151 of these associated genes were positively or negatively regulated by wingless, a master regulator of wing pigmentation. Genes for neural development, Wnt signaling, Dpp signaling, and effectors (such as enzymes) for melanin pigmentation were included among these 151 genes. None of the known regulatory genes that regulate pigmentation pattern formation in other fruit fly species were included. Our results suggest that the novel pigmentation pattern of a polka-dotted fruit fly might have emerged through multistep co-options of multiple gene regulatory networks, signaling pathways, and effector genes, rather than recruitment of one large gene circuit.

Evolution of wing pigmentation in Drosophila: Diversity, physiological regulation, and cis-regulatory evolution

Fruit flies (Drosophila and its close relatives, or “drosophilids”) are a group that includes an important model organism, Drosophila melanogaster, and also very diverse species distributed worldwide. Many of these species have black or brown pigmentation patterns on their wings, and have been used as material for evo‐devo research. Pigmentation patterns are thought to have evolved rapidly compared with body plans or body shapes; hence they are advantageous model systems for studying evolutionary gains of traits and parallel evolution. Various groups of drosophilids, including genus Idiomyia (Hawaiian Drosophila), have a variety of pigmentations, ranging from simple black pigmentations around crossveins to a single antero‐distal spot and a more complex mottled pattern. Pigmentation patterns are sometimes obviously used for sexual displays; however, in some cases they may have other functions. The process of wing formation in Drosophila, the general mechanism of pigmentation formation, and the transport of substances necessary for pigmentation, including melanin precursors, through wing veins are summarized here. Lastly, the evolution of the expression of genes regulating pigmentation patterns, the role of cis‐regulatory regions, and the conditions required for the evolutionary emergence of pigmentation patterns are discussed. Future prospects for research on the evolution of wing pigmentation pattern formation in drosophilids are presented, particularly from the point of view of how they compare with other studies of the evolution of new traits.

Multiple Loci Control Eyespot Number Variation on the Hindwings of Bicyclus anynana Butterflies

The underlying genetic changes that regulate the appearance and disappearance of repeated traits, or serial homologs, remain poorly understood. One hypothesis is that variation in genomic regions flanking master regulatory genes, also known as input-output genes, controls variation in trait number, making the locus of evolution almost predictable. Another hypothesis implicates genetic variation in up- or downstream loci of master control genes. Here, we use the butterfly Bicyclus anynana, a species that exhibits natural variation in eyespot number on the dorsal hindwing, to test these two hypotheses. We first estimated the heritability of dorsal hindwing eyespot number by breeding multiple butterfly families differing in eyespot number and regressing eyespot numbers of offspring on midparent values. We then estimated the number and identity of independent genetic loci contributing to eyespot number variation by performing a genome-wide association study with restriction site-associated DNA sequencing from multiple individuals varying in number of eyespots sampled across a freely breeding laboratory population. We found that dorsal hindwing eyespot number has a moderately high heritability of ∼0.50 and is characterized by a polygenic architecture. Previously identified genomic regions involved in eyespot development, and novel ones, display high association with dorsal hindwing eyespot number, suggesting that homolog number variation is likely determined by regulatory changes at multiple loci that build the trait, and not by variation at single master regulators or input-output genes.

Butterfly eyespot color pattern formation requires physical contact of the pupal wing epithelium with extracellular materials for morphogenic signal propagation

BACKGROUND

Eyespot color pattern formation on butterfly wings is sensitive to physical damage and physical distortion as well as physical contact with materials on the surface of wing epithelial tissue at the pupal stage. Contact-mediated eyespot color pattern changes may imply a developmental role of the extracellular matrix in morphogenic signal propagation. Here, we examined eyespot responses to various contact materials, focusing on the hindwing posterior eyespots of the blue pansy butterfly, Junonia orithya.

RESULTS

Contact with various materials, including both nonbiological and biological materials, induced eyespot enlargement, reduction, or no change in eyespot size, and each material was characterized by a unique response profile. For example, silicone glassine paper almost always induced a considerable reduction, while glass plates most frequently induced enlargement, and plastic plates generally produced no change. The biological materials tested here (fibronectin, polylysine, collagen type I, and gelatin) resulted in various responses, but polylysine induced more cases of enlargement, similar to glass plates. The response profile of the materials was not readily predictable from the chemical composition of the materials but was significantly correlated with the water contact angle (water repellency) of the material surface, suggesting that the surface physical chemistry of materials is a determinant of eyespot size. When the proximal side of a prospective eyespot was covered with a size-reducing material (silicone glassine paper) and the distal side and the organizer were covered with a material that rarely induced size reduction (plastic film), the proximal side of the eyespot was reduced in size in comparison with the distal side, suggesting that signal propagation but not organizer activity was inhibited by silicone glassine paper.

CONCLUSIONS

These results suggest that physical contact with an appropriate hydrophobic surface is required for morphogenic signals from organizers to propagate normally. The binding of the apical surface of the epithelium with an opposing surface may provide mechanical support for signal propagation. In addition to conventional molecular morphogens, there is a possibility that mechanical distortion of the epithelium that is propagated mechanically serves as a nonmolecular morphogen to induce subsequent molecular changes, in accordance with the distortion hypothesis for butterfly wing color pattern formation.

Developing new associations

Summary: In this Editorial, we announce the recruitment of several Associate Editors to cover new and expanding areas of developmental biology. We also discuss various policies and initiatives to improve transparency and efficiency in our editorial processes.

Bringing immersive science to undergraduate laboratory courses using CRISPR gene knockouts in frogs and butterflies

The use of CRISPR/Cas9 for gene editing offers new opportunities for biology students to perform genuine research exploring the gene-to-phenotype relationship. It is important to introduce the next generation of scientists, health practitioners and other members of society to the technical and ethical aspects of gene editing. Here, we share our experience leading hands-on undergraduate laboratory classes, where students formulate hypotheses regarding the roles of candidate genes involved in development, perform loss-of-function experiments using programmable nucleases and analyze the phenotypic effects of mosaic mutant animals. This is enabled by the use of the amphibian Xenopus laevis and the butterfly Vanessa cardui, two organisms that reliably yield hundreds of large and freshly fertilized eggs in a scalable manner. Frogs and butterflies also present opportunities to teach key biological concepts about gene regulation and development. To complement these practical aspects, we describe learning activities aimed at equipping students with a broad understanding of genome editing techniques, their application in fundamental and translational research, and the bioethical challenges they raise. Overall, our work supports the introduction of CRISPR technology into undergraduate classrooms and, when coupled with classroom undergraduate research experiences, enables hypothesis-driven research by undergraduates.

How to study enhancers in non-traditional insect models

Transcriptional enhancers are central to the function and evolution of genes and gene regulation. At the organismal level, enhancers play a crucial role in coordinating tissue- and context-dependent gene expression. At the population level, changes in enhancers are thought to be a major driving force that facilitates evolution of diverse traits. An amazing array of diverse traits seen in insect morphology, physiology and behavior has been the subject of research for centuries. Although enhancer studies in insects outside of Drosophila have been limited, recent advances in functional genomic approaches have begun to make such studies possible in an increasing selection of insect species. Here, instead of comprehensively reviewing currently available technologies for enhancer studies in established model organisms such as Drosophila, we focus on a subset of computational and experimental approaches that are likely applicable to non-Drosophila insects, and discuss the pros and cons of each approach. We discuss the importance of validating enhancer function and evaluate several possible validation methods, such as reporter assays and genome editing. Key points and potential pitfalls when establishing a reporter assay system in non-traditional insect models are also discussed. We close with a discussion of how to advance enhancer studies in insects, both by improving computational approaches and by expanding the genetic toolbox in various insects. Through these discussions, this Review provides a conceptual framework for studying the function and evolution of enhancers in non-traditional insect models.

Dissection of Larval and Pupal Wings of Bicyclus anynana Butterflies

The colorful wings of butterflies are emerging as model systems for evolutionary and developmental studies. Some of these studies focus on localizing gene transcripts and proteins in wings at the larval and pupal stages using techniques such as immunostaining and in situ hybridization. Other studies quantify mRNA expression levels or identify regions of open chromatin that are bound by proteins at different stages of wing development. All these techniques require dissection of the wings from the animal but a detailed video protocol describing this procedure has not been available until now. Here, we present a written and accompanying video protocol where we describe the tools and the method we use to remove the larval and pupal wings of the African Squinting Bush Brown butterfly Bicyclus anynana. This protocol should be easy to adapt to other species.

Molecular mechanisms underlying simplification of venation patterns in holometabolous insects

How mechanisms of pattern formation evolve has remained a central research theme in the field of evolutionary and developmental biology. The mechanism of wing vein differentiation in Drosophila is a classic text-book example of pattern formation using a system of positional-information, yet very little is known about how species with a different number of veins pattern their wings, and how insect venation patterns evolved. Here, we examine the expression pattern of genes previously implicated in vein differentiation in Drosophila in two butterfly species with more complex venation Bicyclus anynana and Pieris canidia. We also test the function of some of these genes in B. anynana. We identify both conserved as well as new domains of decapentaplegic, engrailed, invected, spalt, optix, wingless, armadillo, blistered, and rhomboid gene expression in butterflies, and propose how the simplified venation in Drosophila might have evolved via loss of decapentaplegic, spalt and optix gene expression domains, silencing of vein inducing programs at Spalt-expression boundaries, and changes in gene expression of vein maintenance genes.

Tracking genome-editing and associated molecular perturbations by SWATH mass spectrometry

Advances in gene editing now allow reverse genetics to be applied to a broad range of biological systems. Ultimately, any modification to coding sequences requires confirmation at the protein level, although immunoblotting is often hampered by antibody quality or availability especially in non-model species. Sequential Window Acquisition of All Theoretical Spectra (SWATH), a mass spectrometry (MS) technology with exceptional quantitative reproducibility and accuracy, offers an ideal alternative for protein-based confirmation. Here, using genome edits in mouse, zebrafish and Bicyclus anynana butterflies produced using either homologous recombination or targeted nucleases, we demonstrate absence of the targeted proteins using SWATH, thus confirming successful editing. We show that SWATH is a robust antibody-independent alternative for monitoring gene editing at the protein level and broadly applicable across diverse organisms and targeted genome manipulation techniques. Moreover, SWATH concomitantly defines the global proteome response in the edited organism, which may provide pertinent biological insights.

Conservation and flexibility in the gene regulatory landscape of heliconiine butterfly wings

BACKGROUND

Many traits evolve by cis-regulatory modification, by which changes to noncoding sequences affect the binding affinity for available transcription factors and thus modify the expression profile of genes. Multiple examples of cis-regulatory evolution have been described at pattern switch genes responsible for butterfly wing pattern polymorphism, including in the diverse neotropical genus Heliconius, but the identities of the factors that can regulate these switch genes have not been identified.

RESULTS

We investigated the spatial transcriptomic landscape across the wings of three closely related butterfly species, two of which have a convergently evolved co-mimetic pattern and the other having a divergent pattern. We identified candidate factors for regulating the expression of wing patterning genes, including transcription factors with a conserved expression profile in all three species, and others, including both transcription factors and Wnt pathway genes, with markedly different profiles in each of the three species. We verified the conserved expression profile of the transcription factor homothorax by immunofluorescence and showed that its expression profile strongly correlates with that of the selector gene optix in butterflies with the Amazonian forewing pattern element 'dennis.'

CONCLUSION

Here we show that, in addition to factors with conserved expression profiles like homothorax, there are also a variety of transcription factors and signaling pathway components that appear to vary in their expression profiles between closely related butterfly species, highlighting the importance of genome-wide regulatory evolution between species.

Sub-micrometer insights into the cytoskeletal dynamics and ultrastructural diversity of butterfly wing scales

BACKGROUND

The color patterns that adorn lepidopteran wings are ideal for studying cell type diversity using a phenomics approach. Color patterns are made of chitinous scales that are each the product of a single precursor cell, offering a 2D system where phenotypic diversity can be studied cell by cell, both within and between species. Those scales reveal complex ultrastructures in the sub-micrometer range that are often connected to a photonic function, including iridescent blues and greens, highly reflective whites, or light-trapping blacks.

RESULTS

We found that during scale development, Fascin immunostainings reveal punctate distributions consistent with a role in the control of actin patterning. We quantified the cytoskeleton regularity as well as its relationship to chitin deposition sites, and confirmed a role in the patterning of the ultrastructures of the adults scales. Then, in an attempt to characterize the range and variation in lepidopteran scale ultrastructures, we devised a high-throughput method to quickly derive multiple morphological measurements from fluorescence images and scanning electron micrographs. We imaged a multicolor eyespot element from the butterfly Vanessa cardui (V. cardui), taking approximately 200 000 individual measurements from 1161 scales. Principal component analyses revealed that scale structural features cluster by color type, and detected the divergence of non-reflective scales characterized by tighter cross-rib distances and increased orderedness.

CONCLUSION

We developed descriptive methods that advance the potential of butterfly wing scales as a model system for studying how a single cell type can differentiate into a multifaceted spectrum of complex morphologies. Our data suggest that specific color scales undergo a tight regulation of their ultrastructures, and that this involves cytoskeletal dynamics during scale growth.

Activation of butterfly eyespots by Distal-less is consistent with a reaction-diffusion process

Eyespots on the wings of nymphalid butterflies represent colorful examples of pattern formation, yet the developmental origins and mechanisms underlying eyespot center differentiation are still poorly understood. Using CRISPR-Cas9 we re-examine the function of Distal-less (Dll) as an activator or repressor of eyespots, a topic that remains controversial. We show that the phenotypic outcome of CRISPR mutations depends upon which specific exon is targeted. In Bicyclus anynana, exon 2 mutations are associated with both missing and ectopic eyespots, and also exon skipping. Exon 3 mutations, which do not lead to exon skipping, produce only null phenotypes, including missing eyespots, lighter wing coloration and loss of scales. Reaction-diffusion modeling of Dll function, using Wnt and Dpp as candidate morphogens, accurately replicates these complex crispant phenotypes. These results provide new insight into the function of Dll as a potential activator of eyespot development, scale growth and melanization, and suggest that the tuning of Dll expression levels can generate a diversity of eyespot phenotypes, including their appearance on the wing.This article has an associated 'The people behind the papers' interview.

The people behind the papers – Heidi Connahs, Sham Tlili, Timothy Saunders and Antónia Monteiro

Butterfly eyespots are striking examples of animal patterning, but their developmental origins are still relatively poorly understood. A new paper in Development – the result of a collaboration between two Singapore-based labs – now combines CRISPR-Cas9 gene targeting with theoretical modelling to address the role of the Distal-less transcription factor in eyespot patterning. We caught up with co-first authors Heidi Connahs and Sham Tlili, and their respective supervisors Timothy Saunders (Assistant Professor at the Mechanobiology Institute, National University of Singapore) and Antónia Monteiro (Associate Professor at the Department of Biological Sciences, National University of Singapore and Yale-NUS College) to find out more about the story.