Insights into eyespot color-pattern formation mechanisms from color gradients, boundary scales, and rudimentary eyespots in butterfly wings

Wound Healing in Butterfly Pupal Wing Tissues: Real-Time In Vivo Imaging of Long-Range Cell Migration, Cluster Formation, and Calcium Oscillations

Insects can repair wounds and regenerate body parts in response to physical damage. Wound healing in butterfly pupal wing tissues is developmentally interesting because ectopic color patterns develop during healing, suggesting that normal and damage-induced color patterns may use similar mechanisms. Here we physiologically investigated wound healing and ectopic color pattern formation in butterfly pupal wing tissues using the blue pansy butterfly Junonia orithya. In response to physical puncture damage, various ectopic color patterns are formed around the damage site. After the wounding operation, we observed hemocytes migrating over long distances along the wing veins (lacunae) toward the damage site, where hemocytes and epidermal cells formed cellular clusters. Calcium oscillations were observed in cells at and near the damage site. Calcium oscillations were transiently affected by ruthenium red, an inhibitor of calcium transporters and channels, and ruthenium red caused various abnormalities in the scales of adult wings. These results suggest that cell migration, cluster formation, and calcium oscillations play important roles in wound healing and scale development at and near the damage site. Ectopic color patterns may develop in response to local calcium oscillations as a consequence of the evolutionary co-option of the healing process for normal 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.

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.

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.

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.

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.

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.

Real-Time In Vivo Imaging of the Developing Pupal Wing Tissues in the Pale Grass Blue Butterfly Zizeeria maha: Establishing the Lycaenid System for Multiscale Bioimaging

To systematically analyze biological changes with spatiotemporal dynamics, it is important to establish a system that is amenable for real-time in vivo imaging at various size levels. Herein, we focused on the developing pupal wing tissues in the pale grass blue butterfly, Zizeeria maha, as a system of choice for a systematic multiscale approach in vivo in real time. We showed that the entire pupal wing could be monitored throughout development using a high-resolution bright-field time-lapse imaging system under the forewing-lift configuration; we recorded detailed dynamics of the dorsal and ventral epithelia that behaved independently for peripheral adjustment. We also monitored changes in the dorsal hindwing at the compartmental level and directly observed evaginating scale buds. We also employed a confocal laser microscopy system with multiple fluorescent dyes for three-dimensional observations at the tissue and cellular levels. We discovered extensive cellular clusters that may be functionally important as a unit of cellular communication and differentiation. We also identified epithelial discal and marginal dents that may function during development. Together, this lycaenid forewing system established a foundation to study the differentiation process of epithelial cells and can be used to study biophysically challenging mechanisms such as the determination of color patterns and scale nanoarchitecture at the multiscale levels.