Wnt signaling underlies evolution and development of the butterfly wing pattern symmetry systems

Butterfly pupal wing tissue with an eyespot organizer

Butterfly wing eyespots are developmentally determined at the early pupal stage, when prospective eyespot focal cells underneath the pupal cuticle focal spot function as eyespot organizers in the pupal wing tissue. Here, we performed light microscopy and transmission electron microscopy (TEM) to describe cellular structures of pupal wing tissue with an eyespot organizer immediately after pupation using the Blue Pansy butterfly Junonia orithya. The pupal forewing dorsal epidermis was a pseudostratified monolayer of vertically elongated epidermal cells. The apical portion of the cells adhered laterally to one another, but their medial and basal portions were thinner than the apical portion and were tilted to enclose cells at the center, forming a cellular cluster. The cellular cluster at the organizer was relatively large laterally and vertically. The apical portion of the cells and its corresponding cuticle at the organizer were thicker than those in the surroundings. The innermost cuticle layer was being synthesized, indicating high cuticle synthesis and secretion activity of the cells. At the medial and basal portions of the dorsal epidermis, there were many intracellular and extracellular vacuole-like globules, most likely containing extracellular matrix molecules. Some of the basal processes from epidermal cells extended to form protrusions of the basement membrane, which was often attended by hemocytes. These results suggest that the butterfly eyespot organizer is composed of a single or a few cellular clusters that secrete more cuticle than surrounding clusters, supporting the pupal cuticle hypothesis that cuticle formation is critical for eyespot color pattern determination in butterflies.

Morphogens in the evolution of size, shape and patterning

Much of the striking diversity of life on Earth has arisen from variations in the way that the same molecules and networks operate during development to shape and pattern tissues and organs into different morphologies. However, we still understand very little about the potential for diversification exhibited by different, highly conserved mechanisms during evolution, or, conversely, the constraints that they place on evolution. With the aim of steering the field in new directions, we focus on morphogen-mediated patterning and growth as a case study to demonstrate how conserved developmental mechanisms can adapt during evolution to drive morphological diversification and optimise functionality, and to illustrate how evolution algorithms and computational tools can be used alongside experiments to provide insights into how these conserved mechanisms can evolve. We first introduce key conserved properties of morphogen-driven patterning mechanisms, before summarising comparative studies that exemplify how changes in the spatiotemporal expression and signalling levels of morphogens impact the diversification of organ size, shape and patterning in nature. Finally, we detail how theoretical frameworks can be used in conjunction with experiments to probe the role of morphogen-driven patterning mechanisms in evolution. We conclude that morphogen-mediated patterning is an excellent model system and offers a generally applicable framework to investigate the evolution of developmental mechanisms.

Antibody-Mediated Protein Knockdown Reveals Distal-less Functions for Eyespots and Parafocal Elements in Butterfly Wing Color Pattern Development

One of the important genes for eyespot development in butterfly wings is Distal-less. Its function has been evaluated via several methods, including CRISPR/Cas9 genome editing. However, functional inhibition may be performed at the right time at the right place using a different method. Here, we used a novel protein delivery method for pupal wing tissues in vivo to inactivate a target protein, Distal-less, with a polyclonal anti-Distal-less antibody using the blue pansy butterfly Junonia orithya. We first demonstrated that various antibodies including the anti-Distal-less antibody were delivered to wing epithelial cells in vivo in this species. Treatment with the anti-Distal-less antibody reduced eyespot size, confirming the positive role of Distal-less in eyespot development. The treatment eliminated or deformed a parafocal element, suggesting a positive role of Distal-less in the development of the parafocal element. This result also suggested the integrity of an eyespot and its corresponding parafocal element as the border symmetry system. Taken together, these findings demonstrate that the antibody-mediated protein knockdown method is a useful tool for functional assays of proteins, such as Distal-less, expressed in pupal wing tissues, and that Distal-less functions for eyespots and parafocal elements in butterfly wing color pattern development.

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.

Potential and progress of studying mountain biodiversity by means of butterfly genetics and genomics

Mountains are rich in biodiversity, and butterflies are species-rich and have a good ecological and evolutionary research foundation. This review addresses the potential and progress of studying mountain biodiversity using butterflies as a model. We discuss the uniqueness of mountain ecosystems, factors influencing the distribution of mountain butterflies, representative genetic and evolutionary models in butterfly research, and evolutionary studies of mountain biodiversity involving butterfly genetics and genomics. Finally, we demonstrate the necessity of studying mountain butterflies and propose future perspectives. This review provides insights for studying the biodiversity of mountain butterflies as well as a summary of research methods for reference.

Transposable Element Insertions Are Associated with Batesian Mimicry in the Pantropical Butterfly Hypolimnas misippus

Hypolimnas misippus is a Batesian mimic of the toxic African Queen butterfly (Danaus chrysippus). Female H. misippus butterflies use two major wing patterning loci (M and A) to imitate three color morphs of D. chrysippus found in different regions of Africa. In this study, we examine the evolution of the M locus and identify it as an example of adaptive atavism. This phenomenon involves a morphological reversion to an ancestral character that results in an adaptive phenotype. We show that H. misippus has re-evolved an ancestral wing pattern present in other Hypolimnas species, repurposing it for Batesian mimicry of a D. chrysippus morph. Using haplotagging, a linked-read sequencing technology, and our new analytical tool, Wrath, we discover two large transposable element insertions located at the M locus and establish that these insertions are present in the dominant allele responsible for producing mimetic phenotype. By conducting a comparative analysis involving additional Hypolimnas species, we demonstrate that the dominant allele is derived. This suggests that, in the derived allele, the transposable elements disrupt a cis-regulatory element, leading to the reversion to an ancestral phenotype that is then utilized for Batesian mimicry of a distinct model, a different morph of D. chrysippus. Our findings present a compelling instance of convergent evolution and adaptive atavism, in which the same pattern element has independently evolved multiple times in Hypolimnas butterflies, repeatedly playing a role in Batesian mimicry of diverse model species.

Thermal plasticity of wing size and wing spot size in Drosophila guttifera

Thermal plasticity of melanin pigmentation patterns in Drosophila species has been studied as a model to investigate developmental mechanisms of phenotypic plasticity. The developmental process of melanin pigmentation patterns on wings of Drosophila is divided into two parts, prepattern specification during the pupal period and wing vein-dependent transportation of melanin precursors after eclosion. Which part can be affected by thermal changes? To address this question, we used polka-dotted melanin spots on wings of Drosophila guttifera, whose spot areas are specified by wingless morphogen. In this research, we reared D. guttifera at different temperatures to test whether wing spots show thermal plasticity. We found that wing size becomes larger at lower temperature and that different spots have different reaction norms. Furthermore, we changed the rearing temperature in the middle of the pupal period and found that the most sensitive developmental periods for wing size and spot size are different. The results suggest that the size control mechanisms for the thermal plasticity of wing size and spot size are independent. We also found that the most sensitive stage for spot size was part of the pupal period including stages at which wingless is expressed in the polka-dotted pattern. Therefore, it is suggested that temperature change might affect the prepattern specification process and might not affect transportation through wing veins.

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.

Acute and Long-Term Consequences of Co-opted doublesex on the Development of Mimetic Butterfly Color Patterns

Novel phenotypes are increasingly recognized to have evolved by co-option of conserved genes into new developmental contexts, yet the process by which co-opted genes modify existing developmental programs remains obscure. Here, we provide insight into this process by characterizing the role of co-opted doublesex in butterfly wing color pattern development. dsx is the master regulator of insect sex differentiation but has been co-opted to control the switch between discrete nonmimetic and mimetic patterns in Papilio alphenor and its relatives through the evolution of novel mimetic alleles. We found dynamic spatial and temporal expression pattern differences between mimetic and nonmimetic butterflies throughout wing development. A mimetic color pattern program is switched on by a pulse of dsx expression in early pupal development that causes acute and long-term differential gene expression, particularly in Wnt and Hedgehog signaling pathways. RNAi suggested opposing, novel roles for these pathways in mimetic pattern development. Importantly, Dsx co-option caused Engrailed, a primary target of Hedgehog signaling, to gain a novel expression domain early in pupal wing development that is propagated through mid-pupal development to specify novel mimetic patterns despite becoming decoupled from Dsx expression itself. Altogether, our findings provide multiple views into how co-opted genes can both cause and elicit changes to conserved networks and pathways to result in development of novel, adaptive phenotypes.

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.

Protein Delivery to Insect Epithelial Cells In Vivo: Potential Application to Functional Molecular Analysis of Proteins in Butterfly Wing Development

Protein delivery to cells in vivo has great potential for the functional analysis of proteins in nonmodel organisms. In this study, using the butterfly wing system, we investigated a method of protein delivery to insect epithelial cells that allows for easy access, treatment, and observation in real time in vivo. Topical and systemic applications (called the sandwich and injection methods, respectively) were tested. In both methods, green/orange fluorescent proteins (GFP/OFP) were naturally incorporated into intracellular vesicles and occasionally into the cytosol from the apical surface without any delivery reagent. However, the antibodies were not delivered by the sandwich method at all, and were delivered only into vesicles by the injection method. A membrane-lytic peptide, L17E, appeared to slightly improve the delivery of GFP/OFP and antibodies. A novel peptide reagent, ProteoCarry, successfully promoted the delivery of both GFP/OFP and antibodies into the cytosol via both the sandwich and injection methods. These protein delivery results will provide opportunities for the functional molecular analysis of proteins in butterfly wing development, and may offer a new way to deliver proteins into target cells in vivo in nonmodel organisms.

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.

High level of novelty under the hood of convergent evolution

Little is known about the extent to which species use homologous regulatory architectures to achieve phenotypic convergence. By characterizing chromatin accessibility and gene expression in developing wing tissues, we compared the regulatory architecture of convergence between a pair of mimetic butterfly species. Although a handful of color pattern genes are known to be involved in their convergence, our data suggest that different mutational paths underlie the integration of these genes into wing pattern development. This is supported by a large fraction of accessible chromatin being exclusive to each species, including the de novo lineage-specific evolution of a modular optix enhancer. These findings may be explained by a high level of developmental drift and evolutionary contingency that occurs during the independent evolution of mimicry.

Dominance mechanisms in supergene alleles controlling butterfly wing pattern variation: insights from gene expression in Heliconius numata

Loci under balancing selection, where multiple alleles are maintained, offer a relevant opportunity to investigate the role of natural selection in shaping genetic dominance: the high frequency of heterozygotes at these loci has been shown to enable the evolution of dominance among alleles. In the butterfly Heliconius numata, mimetic wing color variations are controlled by an inversion polymorphism of a circa 2 Mb genomic region (supergene P), with strong dominance between sympatric alleles. To test how differences in dominance observed on wing patterns correlate with variations in expression levels throughout the supergene region, we sequenced the complete transcriptome of heterozygotes at the prepupal stage and compared it to corresponding homozygotes. By defining dominance based on non-overlapping ranges of transcript expression between genotypes, we found contrasting patterns of dominance between the supergene and the rest of the genome; the patterns of transcript expression in the heterozygotes were more similar to the expression observed in the dominant homozygotes in the supergene region. Dominance also differed among the three subinversions of the supergene, suggesting possible epistatic interactions among their gene contents underlying dominance evolution. We found the expression pattern of the melanization gene cortex located in the P-region to predict wing pattern phenotype in the heterozygote. We also identify new candidate genes that are potentially involved in mimetic color pattern variations highlighting the relevance of transcriptomic analyses in heterozygotes to pinpoint candidate genes in non-recombining regions.

The regulation of a pigmentation gene in the formation of complex color patterns in Drosophila abdomens

Changes in the control of developmental gene expression patterns have been implicated in the evolution of animal morphology. However, the genetic mechanisms underlying complex morphological traits remain largely unknown. Here we investigated the molecular mechanisms that induce the pigmentation gene yellow in a complex color pattern on the abdomen of Drosophila guttifera. We show that at least five developmental genes may collectively activate one cis-regulatory module of yellow in distinct spot rows and a dark shade to assemble the complete abdominal pigment pattern of Drosophila guttifera. One of these genes, wingless, may play a conserved role in the early phase of spot pattern development in several species of the quinaria group. Our findings shed light on the evolution of complex animal color patterns through modular changes of gene expression patterns.

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.

CRISPR in butterflies: An undergraduate lab experience to inactivate wing patterning genes during development

CRISPR is a technique increasingly used in the laboratory for both fundamental and applied research. We designed and implemented a lab experience for undergraduates to carry out CRISPR technology in the lab, and knockout the wing patterning genes optix and WntA in Vanessa cardui butterflies. Students obtained spectacular phenotypic mutants of butterfly wings color and patterns, awakening curiosity about how genomes encode morphology. In addition, students successfully used molecular techniques to genotype and screen wild-type caterpillar larvae and butterflies for CRISPR edits in genes. Student feedback suggests that they experienced a meaningful process of scientific inquiry by carrying out the whole CRISPR workflow process, from the design and delivery of CRISPR components through microinjection of butterfly eggs, the rearing of live animals through their complete life cycle, and molecular and phenotypic analyses of the resulting mutants. We discuss our experience using CRISP genome editing experiments in butterflies to expose students to hands-on research experiences probing gene-to-phenotype relationships in a charismatic and live organism.

How butterfly wings got their pattern

Gene regulatory elements play a crucial role in the pattern formation of butterfly wings.

Deep cis-regulatory homology of the butterfly wing pattern ground plan

Butterfly wing patterns derive from a deeply conserved developmental ground plan yet are diverse and evolve rapidly. It is poorly understood how gene regulatory architectures can accommodate both deep homology and adaptive change. To address this, we characterized the cis-regulatory evolution of the color pattern gene WntA in nymphalid butterflies. Comparative assay for transposase-accessible chromatin using sequencing (ATAC-seq) and in vivo deletions spanning 46 cis-regulatory elements across five species revealed deep homology of ground plan-determining sequences, except in monarch butterflies. Furthermore, noncoding deletions displayed both positive and negative regulatory effects that were often broad in nature. Our results provide little support for models predicting rapid enhancer turnover and suggest that deeply ancestral, multifunctional noncoding elements can underlie rapidly evolving trait systems.

LepTraits 1.0 A globally comprehensive dataset of butterfly traits

Here, we present the largest, global dataset of Lepidopteran traits, focusing initially on butterflies (ca. 12,500 species records). These traits are derived from field guides, taxonomic treatments, and other literature resources. We present traits on wing size, phenology,voltinism, diapause/overwintering stage, hostplant associations, and habitat affinities (canopy, edge, moisture, and disturbance). This dataset will facilitate comparative research on butterfly ecology and evolution and our goal is to inspire future research collaboration and the continued development of this dataset.

Measurement(s)	Wingspan • Habitat Affinity • oviposition • voltinism • phenology • hostplant association
Technology Type(s)	natural language processing
Sample Characteristic - Organism	Lepidoptera
Sample Characteristic - Location	Global

Whole-genome sequencing reveals the artificial selection and local environmental adaptability of pigeons (Columba livia)

To meet human needs, domestic pigeons (Columba livia) with various phenotypes have been bred to provide genetic material for our research on artificial selection and local environmental adaptation. Seven pigeon breeds were resequenced and can be divided into commercial varieties (Euro-pigeon, Shiqi, Shen King, Taishen, and Silver King), ornamental varieties (High Fliers), and local varieties (Tarim pigeon). Phylogenetic analysis based on population resequencing showed that one group contained local breeds and ornamental pigeons from China, whereas all commercial varieties were clustered together. It is revealed that the traditional Chinese ornamental pigeon is a branch of Tarim pigeon. Runs of homozygosity (ROH) and linkage disequilibrium (LD) analyses revealed significant differences in the genetic diversity of the three types of pigeons. Genome sweep analysis revealed that the selected genes of commercial breeds were related to body size, reproduction, and plumage color. The genomic imprinting genes left by the ornamental pigeon breeds were mostly related to special human facial features and muscular dystrophy. The Tarim pigeon has evolved genes related to chemical ion transport, photoreceptors, oxidative stress, organ development, and olfaction in order to adapt to local environmental stress. This research provides a molecular basis for pigeon genetic resource evaluation and genetic improvement and suggests that the understanding of adaptive evolution should integrate the effects of various natural environmental characteristics.

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.

Novel doublesex duplication associated with sexually dimorphic development of dogface butterfly wings

Sexually dimorphic development is responsible for some of the most remarkable phenotypic variation found in nature. Alternative splicing of the transcription factor gene doublesex (dsx) is a highly conserved developmental switch controlling the expression of sex-specific pathways. Here, we leverage sex-specific differences in butterfly wing color pattern to characterize the genetic basis of sexually dimorphic development. We use RNA-seq, immunolocalization, and motif binding site analysis to test specific predictions about the role of dsx in the development of structurally based ultraviolet (UV) wing patterns in Zerene cesonia (Southern Dogface). Unexpectedly, we discover a novel duplication of dsx that shows a sex-specific burst of expression associated with the sexually dimorphic UV coloration. The derived copy consists of a single exon that encodes a DNA binding but no protein-binding domain and has experienced rapid amino-acid divergence. We propose the novel dsx paralog may suppress UV scale differentiation in females, which is supported by an excess of Dsx-binding sites at cytoskeletal and chitin-related genes with sex-biased expression. These findings illustrate the molecular flexibility of the dsx gene in mediating the differentiation of secondary sexual characteristics.

Genomics Reveals the Origins of Historical Specimens

Centuries of zoological studies have amassed billions of specimens in collections worldwide. Genomics of these specimens promises to reinvigorate biodiversity research. However, because DNA degrades with age in historical specimens, it is a challenge to obtain genomic data for them and analyze degraded genomes. We developed experimental and computational protocols to overcome these challenges and applied our methods to resolve a series of long-standing controversies involving a group of butterflies. We deduced the geographical origins of several historical specimens of uncertain provenance that are at the heart of these debates. Here, genomics tackles one of the greatest problems in zoology: countless old specimens that serve as irreplaceable embodiments of species concepts cannot be confidently assigned to extant species or population due to the lack of diagnostic morphological features and clear documentation of the collection locality. The ability to determine where they were collected will resolve many on-going disputes. More broadly, we show the utility of applying genomics to historical museum specimens to delineate the boundaries of species and populations, and to hypothesize about genotypic determinants of phenotypic traits.

Cortex cis-regulatory switches establish scale colour identity and pattern diversity in Heliconius

In Heliconius butterflies, wing colour pattern diversity and scale types are controlled by a few genes of large effect that regulate colour pattern switches between morphs and species across a large mimetic radiation. One of these genes, cortex, has been repeatedly associated with colour pattern evolution in butterflies. Here we carried out CRISPR knockouts in multiple Heliconius species and show that cortex is a major determinant of scale cell identity. Chromatin accessibility profiling and introgression scans identified cis-regulatory regions associated with discrete phenotypic switches. CRISPR perturbation of these regions in black hindwing genotypes recreated a yellow bar, revealing their spatially limited activity. In the H. melpomene/timareta lineage, the candidate CRE from yellow-barred phenotype morphs is interrupted by a transposable element, suggesting that cis-regulatory structural variation underlies these mimetic adaptations. Our work shows that cortex functionally controls scale colour fate and that its cis-regulatory regions control a phenotypic switch in a modular and pattern-specific fashion.

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 color pattern inducing gene wingless is expressed in specific cell types of campaniform sensilla of a polka-dotted fruit fly, Drosophila guttifera

A polka-dotted fruit fly, Drosophila guttifera, has a unique pigmentation pattern on its wings and is used as a model for evo-devo studies exploring the mechanism of evolutionary gain of novel traits. In this species, a morphogen-encoding gene, wingless, is expressed in species-specific positions and induces a unique pigmentation pattern. To produce some of the pigmentation spots on wing veins, wingless is thought to be expressed in developing campaniform sensillum cells, but it was unknown which of the four cell types there express(es) wingless. Here we show that two of the cell types, dome cells and socket cells, express wingless, as indicated by in situ hybridization together with immunohistochemistry. This is a unique case in which non-neuronal SOP (sensory organ precursor) progeny cells produce Wingless as an inducer of pigmentation pattern formation. Our finding opens a path to clarifying the mechanism of evolutionary gain of a unique wingless expression pattern by analyzing gene regulation in dome cells and socket cells.

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.

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.

R2R3-MYB genes control petal pigmentation patterning in Clarkia gracilis ssp. sonomensis (Onagraceae)

Petal pigmentation patterning is widespread in flowering plants. The genetics of these pattern elements has been of great interest for understanding the evolution of phenotypic diversification. Here, we investigate the genetic changes responsible for the evolution of an unpigmented petal element on a colored background. We used transcriptome analysis, gene expression assays, cosegregation in F₂ plants and functional tests to identify the gene(s) involved in petal coloration in Clarkia gracilis ssp. sonomensis. We identified an R2R3-MYB transcription factor (CgsMYB12) responsible for anthocyanin pigmentation of the basal region ('cup') in the petal of C. gracilis ssp. sonomensis. A functional mutation in CgsMYB12 creates a white cup on a pink petal background. Additionally, we found that two R2R3-MYB genes (CgsMYB6 and CgsMYB11) are also involved in petal background pigmentation. Each of these three R2R3-MYB genes exhibits a different spatiotemporal expression pattern. The functionality of these R2R3-MYB genes was confirmed through stable transformation of Arabidopsis. Distinct spatial patterns of R2R3-MYB expression have created the possibility that pigmentation in different sections of the petal can evolve independently. This finding suggests that recent gene duplication has been central to the evolution of petal pigmentation patterning in C. gracilis ssp. sonomensis.

Speciation in North American Junonia from a genomic perspective

Delineating species boundaries in phylogenetic groups undergoing recent radiation is a daunting challenge akin to discretizing continuity. Here, we propose a general approach exemplified by American butterflies from the genus Junonia Hübner notorious for the variety of similar phenotypes, ease of hybridization, and the lack of consensus about their classification. We obtain whole-genome shotgun sequences of about 200 specimens. We reason that discreteness emerges from continuity by means of a small number of key players, and search for the proteins that diverged markedly between sympatric populations of different species, while keeping low polymorphism within these species. Being 0.25% of the total number, these three dozen 'speciation' proteins indeed partition pairs of Junonia populations into two clusters with a prominent break in between, while all proteins taken together fail to reveal this discontinuity. Populations with larger divergence from each other, comparable to that between two sympatric species, form the first cluster and correspond to different species. The other cluster is characterized by smaller divergence, similar to that between allopatric populations of the same species and comprise conspecific pairs. Using this method, we conclude that J. genoveva (Cramer), J. litoralis Brévignon, J. evarete (Cramer), and J. divaricata C. & R. Felder are restricted to South America. We find that six species of Junonia are present in the United States, one of which is new: Junonia stemosa Grishin, sp.n. (i), found in south Texas and phenotypically closest to J. nigrosuffusa W. Barnes & McDunnough (ii) in its dark appearance. In the pale nudum of the antennal club, these two species resemble J. zonalis C. & R. Felder (iii) from Florida and the Caribbean Islands. The pair of sister species, J. grisea Austin & J. Emmel (iv) and J. coenia Hübner (v), represent the classic west/east U.S.A. split. The mangrove feeder (as caterpillar), dark nudum J. neildi Brévignon (vi) enters south Texas as a new subspecies Junonia neildi varia Grishin ssp.n. characterized by more extensive hybridization with and introgression from J. coenia, and, as a consequence, more variable wing patterns compared with the nominal J. n. neildi in Florida. Furthermore, a new mangrove-feeding species from the Pacific Coast of Mexico is described as Junonia pacoma Grishin sp.n. Finally, genomic analysis suggests that J. nigrosuffusa may be a hybrid species formed by the ancestors of J. grisea and J. stemosa sp.n.

Morphological and Spatial Diversity of the Discal Spot on the Hindwings of Nymphalid Butterflies: Revision of the Nymphalid Groundplan

Simple Summary

Butterfly wing color patterns are diverse, but they can be understood as modifications of the common scheme called the nymphalid groundplan. The discal spot is relatively small, but it is one of the important components of the nymphalid groundplan. Using many hindwing specimens of the family Nymphalidae, the morphological and spatial diversity of the discal spot was studied. The discal spot is expressed as a small or narrow spot, a pair of parallel bands, a diamond or oval structure, a large dark spot, a few fragmented spots, or a white structure. The discal spot is always located in a central portion of the wing defined by the wing veins, and this portion is sandwiched by a pair of bands of the central symmetry system, another important component of the nymphalid groundplan. On the basis of these results, the present study revises the nymphalid groundplan in minor points; the discal spot is an independent and diverse miniature symmetry system nested within the central symmetry system. Due to the involvement of wing veins to define the locations of the discal spot, the present study suggests the possible developmental dynamics of butterfly color pattern formation that produces color pattern diversity.

Abstract

Diverse butterfly wing color patterns are understood through the nymphalid groundplan, which mainly consists of central, border, and basal symmetry systems and a discal spot. However, the status of the discal spot remains unexplored. Here, the morphological and spatial diversity of the discal spot was studied in nymphalid hindwings. The discal spot is expressed as a small or narrow spot, a pair of parallel bands, a diamond or oval structure, a large dark spot, a few fragmented spots, or a white structure. In some cases, the discal spot is morphologically similar to and integrated with the central symmetry system (CSS). The discal spot is always located in a distal portion of the discal cell defined by the wing veins, which is sandwiched by the distal and proximal bands of the CSS (dBC and pBC) and is rarely occupied by border ocelli. The CSS occasionally has the central band (cBC), which differs from the discal spot. These results suggest that the discal spot is an independent and diverse miniature symmetry system nested within the CSS and that the locations of the discal spot and the CSS are determined by the wing veins at the early stage of wing development.

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.

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.

Multicomponent structures in camouflage and mimicry in butterfly wing patterns

Understanding how morphological structures are built is essential for appreciating the morphological complexity and divergence of organisms. One representative case of morphological structures is the camouflage and mimicry of butterfly wing patterns. Some previous studies have questioned whether camouflage and mimicry are truly structures, considering that they rely on coloration. Nevertheless, our recent study revealed that the leaf pattern of Kallima inachus butterfly wings evolved through the combination of changes in several pigment components in a block-wise manner; it remains unclear whether such block-wise structures are common in other cases of camouflage and mimicry in butterflies and how they come about. Previous studies focused solely on a set of homologous components, termed the nymphalid ground plan. In the present study, we extended the scope of the description of components by including not only the nymphalid ground plan but also other common components (i.e., ripple patterns, dependent patterns, and color fields). This extension allowed us to analyze the combinatorial building logic of structures and examine multicomponent structures of camouflage and mimicry in butterfly wing patterns. We investigated various patterns of camouflage and mimicry (e.g., masquerade, crypsis, Müllerian mimicry, Batesian mimicry) in nine species and decomposed them into an assembly of multiple components. These structural component analyses suggested that camouflage and mimicry in butterfly wing patterns are built up by combining multiple types of components. We also investigated associations between components and the kinds of camouflage and mimicry. Several components are statistically more often used to produce specific types of camouflage or mimicry. Thus, our work provides empirical evidence that camouflage and mimicry patterns of butterfly wings are mosaic structures, opening up a new avenue of studying camouflage, and mimicry from a structural perspective.

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.

Contrasting genomic and phenotypic outcomes of hybridization between pairs of mimetic butterfly taxa across a suture zone

Hybrid zones, whereby divergent lineages come into contact and eventually hybridize, can provide insights on the mechanisms involved in population differentiation and reproductive isolation, and ultimately speciation. Suture zones offer the opportunity to compare these processes across multiple species. In this paper we use reduced-complexity genomic data to compare the genetic and phenotypic structure and hybridization patterns of two mimetic butterfly species, Ithomia salapia and Oleria onega (Nymphalidae: Ithomiini), each consisting of a pair of lineages differentiated for their wing colour pattern and that come into contact in the Andean foothills of Peru. Despite similarities in their life history, we highlight major differences, both at the genomic and phenotypic level, between the two species. These differences include the presence of hybrids, variations in wing phenotype, and genomic patterns of introgression and differentiation. In I. salapia, the two lineages appear to hybridize only rarely, whereas in O. onega the hybrids are not only more common, but also genetically and phenotypically more variable. We also detected loci statistically associated with wing colour pattern variation, but in both species these loci were not over-represented among the candidate barrier loci, suggesting that traits other than wing colour pattern may be important for reproductive isolation. Our results contrast with the genomic patterns observed between hybridizing lineages in the mimetic Heliconius butterflies, and call for a broader investigation into the genomics of speciation in Ithomiini - the largest radiation of mimetic butterflies.

Butterfly Mimicry Polymorphisms Highlight Phylogenetic Limits of Gene Reuse in the Evolution of Diverse Adaptations

Some genes have repeatedly been found to control diverse adaptations in a wide variety of organisms. Such gene reuse reveals not only the diversity of phenotypes these unique genes control but also the composition of developmental gene networks and the genetic routes available to and taken by organisms during adaptation. However, the causes of gene reuse remain unclear. A small number of large-effect Mendelian loci control a huge diversity of mimetic butterfly wing color patterns, but reasons for their reuse are difficult to identify because the genetic basis of mimicry has primarily been studied in two systems with correlated factors: female-limited Batesian mimicry in Papilio swallowtails (Papilionidae) and non-sex-limited Müllerian mimicry in Heliconius longwings (Nymphalidae). Here, we break the correlation between phylogenetic relationship and sex-limited mimicry by identifying loci controlling female-limited mimicry polymorphism Hypolimnas misippus (Nymphalidae) and non-sex-limited mimicry polymorphism in Papilio clytia (Papilionidae). The Papilio clytia polymorphism is controlled by the genome region containing the gene cortex, the classic P supergene in Heliconius numata, and loci controlling color pattern variation across Lepidoptera. In contrast, female-limited mimicry polymorphism in Hypolimnas misippus is associated with a locus not previously implicated in color patterning. Thus, although many species repeatedly converged on cortex and its neighboring genes over 120 My of evolution of diverse color patterns, female-limited mimicry polymorphisms each evolved using a different gene. Our results support conclusions that gene reuse occurs mainly within ∼10 My and highlight the puzzling diversity of genes controlling seemingly complex female-limited mimicry polymorphisms.

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.

Morphological Murals: The Scaling and Allometry of Butterfly Wing Patterns

The color patterns of butterflies moths are exceptionally diverse, but are very stable within a species, so that most species can be identified on the basis of their color pattern alone. The color pattern is established in the wing imaginal disc during a prolonged period of growth and differentiation, beginning during the last larval instar and ending during the first few days of the pupal stage. During this period, a variety of diffusion and reaction-diffusion signaling mechanisms determine the positions and sizes of the various elements that make up the overall color pattern. The patterning occurs while the wing is growing from a small imaginal disc to a very large pupal wing. One would therefore expect that some or all aspects of the color pattern would be sensitive to the size of the developmental field on which pattern formation takes place. To study this possibility, we analyzed the color patterns of Junonia coenia from animals whose growth patterns were altered by periodic starvation during larval growth, which produced individuals with a large range of variation in body size and wing size. Analyses of the color patterns showed that the positions and size of the pattern elements scaled perfectly isometrically with wing size. This is a puzzling finding and suggests the operation of a homeostatic or robustness mechanism that stabilizes pattern in spite of variation in the growth rate and final size of the wing.

Comparative analysis of integument transcriptomes identifies genes that participate in marking pattern formation in three allelic mutants of silkworm, Bombyx mori

The diversity markings and pigment patterns in insects are outcomes of adaptive evolution. The elucidation of the molecular mechanism underlying variations in pigment patterns may improve our understanding of the origin and evolution of these spectacular diverse phenotypes. Melanin, ommochrome, and pteridine are the three main types of insect pigments, and the genes that directly participate in pigment biosynthesis have been extensively studied. However, available information on gene interactions and the whole pigment regulatory network is limited. In this study, we performed integument transcriptome sequencing to analyze three larval marking allelic mutants, namely, multi lunar (L), L^C, and L^Ca, which have similar twin-spot markings on the dorsal side of multiple segments. Further analysis identified 336 differentially expressed genes (DEGs) between L and Dazao (wild type which exhibits normal markings), 68 DEGs between L^C/+ and +^LC/+^LC, and 188 DEGs between L^Ca/+ and +^LCa/+^LCa. Gene Ontology (GO) analysis indicated a significant DEG enrichment of the functional terms catalytic activity, binding, metabolic process, and cellular process. Furthermore, three mutants share six common enriched KEGG pathways. We finally identified eight common DEGs among three pairwise comparisons, including Krueppel-like factor, TATA-binding protein, protein patched, UDP-glycosyltransferase, an unknown secreted protein, and three cuticular proteins. Microarray-based gene expression analysis revealed that the eight genes are upregulated during molting, which coincides with marking formation, and are significantly differentially expressed between marking and non-marking regions. The results suggest that the eight common genes are involved in the construction of the multiple twin-spot marking patterns in the three mutants.

Unravelling the genes forming the wing pattern supergene in the polymorphic butterfly Heliconius numata

Background

Unravelling the genetic basis of polymorphic characters is central to our understanding of the origins and diversification of living organisms. Recently, supergenes have been implicated in a wide range of complex polymorphisms, from adaptive colouration in butterflies and fish to reproductive strategies in birds and plants. The concept of a supergene is now a hot topic in biology, and identification of its functional elements is needed to shed light on the evolution of highly divergent adaptive traits. Here, we apply different gene expression analyses to study the supergene P that controls polymorphism of mimetic wing colour patterns in the neotropical butterfly Heliconius numata.

Results

We performed de novo transcriptome assembly and differential expression analyses using high-throughput Illumina RNA sequencing on developing wing discs of different H. numata morphs. Within the P interval, 30 and 17 of the 191 transcripts were expressed differentially in prepupae and day-1 pupae, respectively. Among these is the gene cortex, known to play a role in wing pattern formation in Heliconius and other Lepidoptera. Our in situ hybridization experiments confirmed the relationship between cortex expression and adult wing patterns.

Conclusions

This study found the majority of genes in the P interval to be expressed in the developing wing discs during the critical stages of colour pattern formation, and detect drastic changes in expression patterns in multiple genes associated with structural variants. The patterns of expression of cortex only partially recapitulate the variation in adult phenotype, suggesting that the remaining phenotypic variation could be controlled by other genes within the P interval. Although functional studies on cortex are now needed to determine its exact developmental role, our results are in accordance with the classical supergene hypothesis, whereby several genes inherited together due to tight linkage control a major developmental switch.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0129-2) contains supplementary material, which is available to authorized users.