Biology subjects

Saranathan, V.

Publications and source records attributed to Saranathan, V..

4 recordsLinked to original sources

bioRxiv2024

Ridge and crossrib height of butterfly wing scales is a toolbox for structural color diversity

The brightest and most vivid colors of butterflies usually originate from light reflecting off the cuticular scales that cover the wing membrane. These scales have an intricate architecture that consists of an upper layer, a grid of longitudinal ridges and transverse crossribs, connected to a lower lamina by pillars called trabeculae. Whereas the role of the lower lamina as a reflector has been well documented in simpler scales, this study unveils the role of the scales upper surface in generating or fine-tuning hue, brightness, and saturation. In the nymphalid Bicyclus anynana, we showed that changes in ridge and trabecula heights accompanied changes in hue of scales produced via artificial selection. We further found that this correlation between ridge height and hue can be generalized to 40 scale types from 35 species across butterfly families. By combining focused ion beam milling, microspectrophotometry, and optical modelling, we found that modifying the ridge height is sufficient to change ridge hue, notably in Morpho didius whose blue color was thought to be generated exclusively by lamella protruding from ridges, rather than ridge height. This study identifies the scales upper surface as a toolbox for structural color diversity in butterflies and proposes a geometrical model to predict color that unifies species with and without Morpho-type Christmas-tree ridges.

evolutionary biology↗

bioRxiv2021

Antennapedia regulates metallic silver wing scale development and cell shape in Bicyclus anynana butterflies

Butterfly wing scale cells can develop very intricate cuticular nanostructures that interact with light to produce structural colors including silvery hues, but the genetic basis of such nanostructures is mostly unexplored. Here, we first identified that optix is necessary for metallic scale development in the butterfly Bicyclus anynana. We then characterized different subtypes of wildtype metallic silver scales and addressed the function of five genes - apterous A, Ultrabithorax, doublesex, Antennapedia, and optix - in the differentiation of silver scales, at a single-cell resolution, by leveraging crispants that exhibited either ectopic gains or losses of silver scales. Wildtype silver scales were generally rounded and had low amounts of pigmentation, exhibiting a common ultrastructural modification for metallic broadband reflectance, i.e., an undulatory air layer enclosed by an upper and lower lamina. Our results indicated that the varying air layer thickness was the important parameter of the bilaminate scale for producing a broadband reflectance across visible wavelengths. While a single lamina of the appropriate thickness could also produce broadband colors, the bilaminate structure is advantageous as it increases the overall reflectivity. Crispant brown scales differed from wildtype silver scales via the loss of the continuous upper lamina, increased lower lamina thickness, and increased pigmentation. The reverse was seen when brown scales became silver. We identified Antennapedia and optix as high-level regulators in the network differentiating different silver scale subtypes and determining overall cell shape in both sexes. In addition, Antp exhibits a novel, post-embryonic role in the determination of ridge and crossrib orientation.

developmental biology↗

bioRxiv2020

F-actin Re-organization Mediates Hierarchical Morphogenesis of Swallowtail Butterfly Wing Scale Nanostructures

The study of color patterns in the animal integument is a fundamental question in biology, with many lepidopteran species being exemplary models in this endeavor due to their relative simplicity and elegance. While significant advances have been made in unravelling the cellular and molecular basis of lepidopteran pigmentary coloration, the morphogenesis of wing scale nanostructures involved in structural color production is not well understood. Contemporary research in this topic largely focuses on a few nymphalid model taxa (e.g., Bicyclus, Heliconius), despite an overwhelming diversity in the hierarchical nanostructural organization of lepidopteran wing scales. Here, we present a time-resolved, comparative developmental study of hierarchical scale nanostructures in Parides eurimedes and five other papilionid species. Our results uphold the putative conserved role of F-actin bundles in acting as spacers between developing ridges, as previously documented in several nymphalid species. Interestingly, while ridges are developing in P. eurimedes, plasma membrane manifests irregular mesh-like crossribs characteristic of Papilionidae, which delineate the accretion of cuticle into rows of planar disks in between ridges. Once the ridges have grown, disintegrating F-actin bundles appear to reorganize into a network that supports the invagination of plasma membrane underlying the disks, subsequently forming an extruded honeycomb lattice. Our results uncover a previously undocumented role for F-actin in the morphogenesis of complex wing scale nanostructures, likely specific to Papilionidae.

developmental biology↗

bioRxiv2020

The Evolution Of Colouration And Opsins In Tarantulas

Tarantulas paradoxically exhibit a diverse palette of vivid colouration despite their crepuscular to nocturnal habits. The evolutionary origin and maintenance of these colours remains a mystery. In this study, we reconstructed the ancestral states of both blue and green colouration in tarantula setae, and tested how these colours correlate with the presence of stridulation, urtication, and arboreality. Green colouration has likely evolved at least eight times, and blue colouration is likely an ancestral condition that appears to be lost more frequently than gained. While our results indicate that neither colour correlates with the presence of stridulation or urtication, the evolution of green colouration appears to depend upon the presence of arboreality, suggesting that it likely originated for, and functions in, crypsis through substrate matching among leaves. We also constructed a network of opsin homologs across tarantula transcriptomes. Despite their crepuscular tendencies, tarantulas express a full suite of opsin genes - a finding that contradicts current consensus that tarantulas have poor colour vision on the basis of low opsin diversity. Overall, our results support the intriguing hypotheses that blue colouration may have ultimately evolved via sexual selection and perhaps proximately be used in mate choice or predation avoidance due to possible sex differences in mate-searching.

evolutionary biology↗