Xanthurenic acid is a pigment in Junonia coenia butterfly wings

Vermilion and cinnabar are involved in ommochrome pigment biosynthesis in eyes but not wings of Bicyclus anynana butterflies

If the same pigment is found in different tissues in a body, it is natural to assume that the same metabolic pathways are deployed similarly in each tissue. Here we show that this is not the case for ommochromes, the red and orange pigments found in the eyes and wings of butterflies. We tested the expression and function of vermilion and cinnabar, two known fly genes in the ommochrome pathway, in the development of pigments in the eyes and in the wings of Bicyclus anynana butterflies, both traits having reddish/orange pigments. By using fluorescent in-situ hybridization (HCR3.0) we localized the expression of vermilion and cinnabar in the cytoplasm of pigment cells in the ommatidia but observed no clear expression for either gene on larval and pupal wings. We then disrupted the function of both genes, using CRISPR-Cas9, which resulted in the loss of pigment in the eyes but not in the wings. Using thin-layer chromatography and UV-vis spectroscopy we identified the presence of ommochrome and ommochrome precursors in the orange wing scales and in the hemolymph of pupae. We conclude that the wings either synthesize ommochromes locally, with yet unidentified enzymes or incorporate these pigments synthesized elsewhere from the hemolymph. Different metabolic pathways or transport mechanisms, thus, lead to the presence of ommochromes in the wings and eyes of B. anynana butterflies.

Properties of Cephalopod Skin Ommochromes to Inhibit Free Radicals, and the Maillard Reaction and Retino-Protective Mechanisms in Cellular Models Concerning Oxidative Stress, Angiogenesis, and Inflammation

Ommochromes are pigments of invertebrates that exhibit oxidative stress protection. The aim of this study was to investigate ommochromes extracted from cephalopod’s skin for their ability to inhibit age-related-macular degeneration (AMD)-related factors such as H₂O₂-induced and iron-dependent oxidative stress (ferroptosis and erastin), accumulation of advanced glycation end-products (AGEs), as well as vascular endothelial growth factor (VEGF), and inflammatory cytokines (interleukin 6 and interleukin 8) secretion. As cell systems, we used primary porcine retinal pigment epithelium (RPE), human retinal pigment epithelium cell line ARPE-19 and uveal melanoma cell line OMM-1. In vitro, ommochromes produced an antiglycation effect by the inhibition of fructosylation reaction. The ommochromes showed protective effects against erastin- induced cell death in ARPE-19. In addition, in long-term stimulation (7 days) ommochromes decreased constitutively secreted VEGF, as well as interleukin 6 and interleukin 8 induced by Poly I:C in primary RPE. No relevant effects were detected in OMM-1 cells. The effects are dependent on the cell system, time of exposition, and concentration. This substance is of interest for further research concerning age-related macular degeneration.

Xanthurenic Acid in the Shell Purple Patterns of Crassostrea gigas: First Evidence of an Ommochrome Metabolite in a Mollusk Shell

Ommochromes are one of the least studied groups of natural pigments, frequently confused with melanin and, so far, exclusively found in invertebrates such as cephalopods and butterflies. In this study focused on the purple color of the shells of a mollusk, Crassostrea gigas, the first evidence of a metabolite of ommochromes, xanthurenic acid (XA), was obtained by liquid chromatography combined with mass spectrometry (UPLC-MS). In addition to XA and various porphyrins previously identified, a second group of high molecular weight acid-soluble pigments (HMASP) has been identified with physicochemical and structural characteristics similar to those of ommochromes. In addition, fragmentation of HMASP by tandem mass spectrometry (MS/MS) has revealed a substructure common to XA and ommochromes of the ommatin type. Furthermore, the presence of melanins was excluded by the absence of characteristic by-products among the oxidation residues of HMASP. Altogether, these results show that the purple color of the shells of Crassostrea gigas is a complex association of porphyrins and ommochromes of potentially ommatin or ommin type.

Xanthurenic Acid Is the Main Pigment of Trichonephila clavata Gold Dragline Silk

Spider silk is a natural fiber with remarkable strength, toughness, and elasticity that is attracting attention as a biomaterial of the future. Golden orb-weaving spiders (Trichonephila clavata) construct large, strong webs using golden threads. To characterize the pigment of golden T. clavata dragline silk, we used liquid chromatography and mass spectrometric analysis. We found that the major pigment in the golden dragline silk of T. clavata was xanthurenic acid. To investigate the possible function of the pigment, we tested the effect of xanthurenic acid on bacterial growth using gram-negative Escherichia coli and gram-positive Bacillus subtilis. We found that xanthurenic acid had a slight antibacterial effect. Furthermore, to investigate the UV tolerance of the T. clavata threads bleached of their golden color, we conducted tensile deformation tests and scanning electron microscope observations. However, in these experiments, no significant effect was observed. We therefore speculate that golden orb-weaving spiders use the pigment for other purposes, such as to attract their prey in the sunlight.

Evaluation of Dosidicus gigas Skin Extract as An Antioxidant and Preservative in Tuna Pâté

A strategy for food preservation, based on a methanol–HCl squid skin extract (Dosidicus gigas) (JSSE), was evaluated at two concentrations in yellowfin tuna fish pâtés, which were stored at 4 and 8 °C for 20 day. The JSSE was characterized by determining its antioxidant and mutagenic activities. A yellowfin tuna pâté was elaborated, with and without the addition of the JSSE. An affective sensory analysis was performed to establish consumers’ preferences. During a 20-day storage period, the water activity (a_w), pH, color difference (ΔE*_ab), microbiological analysis, lipid oxidation and sensory quality attributes were evaluated, and the results were compared with the results of the butylated hydroxyanisole (BHA) and control treatments. The JSSE showed antioxidant activity against the 2,2-diphenyl-1-picrylhydrazyl (DPPH^●+) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS^●) radicals and did not induce mutation, according to the Ames’ Salmonella test, nor chromosomal abnormalities, according to the onion root-tip cell assay. The consumer analysis demonstrated a higher preference for the pâté with the added JSSE in seven out of the eight evaluated attributes. During storage, the JSSE neither had an impact on a_w nor pH, maintained lower ΔE*_ab values, inhibited the microbial activity and lipid oxidation (unlike the control pâté), and preserved the sensory quality attributes, unlike the BHA and control treatments. This study showed that the JSSE has biologically active pigments that can act as antioxidants and antimicrobials in yellowfin tuna fish pâtés.

Ommochromes in invertebrates: biochemistry and cell biology

Ommochromes are widely occurring coloured molecules of invertebrates, arising from tryptophan catabolism through the so-called Tryptophan → Ommochrome pathway. They are mainly known to mediate compound eye vision, as well as reversible and irreversible colour patterning. Ommochromes might also be involved in cell homeostasis by detoxifying free tryptophan and buffering oxidative stress. These biological functions are directly linked to their unique chromophore, the phenoxazine/phenothiazine system. The most recent reviews on ommochrome biochemistry were published more than 30 years ago, since when new results on the enzymes of the ommochrome pathway, on ommochrome photochemistry as well as on their antiradical capacities have been obtained. Ommochromasomes are the organelles where ommochromes are synthesised and stored. Hence, they play an important role in mediating ommochrome functions. Ommochromasomes are part of the lysosome-related organelles (LROs) family, which includes other pigmented organelles such as vertebrate melanosomes. Ommochromasomes are unique because they are the only LRO for which a recycling process during reversible colour change has been described. Herein, we provide an update on ommochrome biochemistry, photoreactivity and antiradical capacities to explain their diversity and behaviour both in vivo and in vitro. We also highlight new biochemical techniques, such as quantum chemistry, metabolomics and crystallography, which could lead to major advances in their chemical and functional characterisation. We then focus on ommochromasome structure and formation by drawing parallels with the well-characterised melanosomes of vertebrates. The biochemical, genetic, cellular and microscopic tools that have been applied to melanosomes should provide important information on the ommochromasome life cycle. We propose LRO-based models for ommochromasome biogenesis and recycling that could be tested in the future. Using the context of insect compound eyes, we finally emphasise the importance of an integrated approach in understanding the biological functions of ommochromes.

Different ommochrome pigment mixtures enable sexually dimorphic Batesian mimicry in disjunct populations of the common palmfly butterfly, Elymnias hypermnestra

With varied, brightly patterned wings, butterflies have been the focus of much work on the evolution and development of phenotypic novelty. However, the chemical structures of wing pigments from few butterfly species have been identified. We characterized the orange wing pigments of female Elymnias hypermnestra butterflies (Lepidoptera: Nymphalidae: Satyrinae) from two Southeast Asian populations. This species is a sexually dimorphic Batesian mimic of several model species. Females are polymorphic: in some populations, females are dark, resemble conspecific males, and mimic Euploea spp. In other populations, females differ from males and mimic orange Danaus spp. Using LC-MS/MS, we identified nine ommochrome pigments: six from a population in Chiang Mai, Thailand, and five compounds from a population in Bali, Indonesia. Two ommochromes were found in both populations, and only two of the nine compounds have been previously reported. The sexually dimorphic Thai and Balinese populations are separated spatially by monomorphic populations in peninsular Malaysia, Singapore, and Sumatra, suggesting independent evolution of mimetic female wing pigments in these disjunct populations. These results indicate that other butterfly wing pigments remain to be discovered.

Positional cloning of a Bombyx pink-eyed white egg locus reveals the major role of cardinal in ommochrome synthesis

Ommochromes are major insect pigments involved in coloration of compound eyes, eggs, epidermis and wings. In the silkworm Bombyx mori, adult compound eyes and eggs contain a mixture of the ommochrome pigments such as ommin and xanthommatin. Here, we identified the gene involved in ommochrome biosynthesis by positional cloning of B. mori egg and eye color mutant pink-eyed white egg (pe). The recessive homozygote of pe has bright red eyes and white or pale pink eggs instead of a normal dark coloration due to the decrease of dark ommochrome pigments. By genetic linkage analysis, we narrowed down the pe-linked region to ~258 kb, containing 17 predicted genes. RNA sequencing analyses showed that the expression of one candidate gene, the ortholog of Drosophila haem peroxidase cardinal, coincided with egg pigmentation timing, similar to other ommochrome-related genes such as Bm-scarlet and Bm-re. In two pe strains, a common missense mutation was found within a conserved motif of B. mori cardinal homolog (Bm-cardinal). RNA interference-mediated knockdown and transcription activator-like effector nuclease (TALEN)-mediated knockout of the Bm-cardinal gene produced the same phenotype as pe in terms of egg, adult eye and larval epidermis coloration. A complementation test of the pe mutant with the TALEN-mediated Bm-cardinal-deficient strain showed that the mutant phenotype could not be rescued, indicating that Bm-cardinal is responsible for pe. Moreover, knockdown of the cardinal homolog in Tribolium castaneum also induced red compound eyes. Our results indicate that cardinal plays a major role in ommochrome synthesis of holometabolous insects.

Extensive transcriptional response associated with seasonal plasticity of butterfly wing patterns

In the eastern United States, the buckeye butterfly, Junonia coenia, shows seasonal wing colour plasticity where adults emerging in the spring are tan, while those emerging in the autumn are dark red. This variation can be artificially induced in laboratory colonies, thus making J. coenia a useful model system to examine the mechanistic basis of plasticity. To better understand the developmental basis of seasonal plasticity, we used RNA-seq to quantify transcription profiles associated with development of alternative seasonal wing morphs. Depending on the developmental stage, between 547 and 1420 transfrags were significantly differentially expressed between morphs. These extensive differences in gene expression stand in contrast to the much smaller numbers of differentially expressed transcripts identified in previous studies of genetic wing pattern variation in other species and suggest that environmentally induced phenotypic shifts arise from very broad systemic processes. Analyses of candidate endocrine and pigmentation transcripts revealed notable genes upregulated in the red morph, including several ecdysone-associated genes, and cinnabar, an ommochrome pigmentation gene implicated in colour pattern variation in other butterflies. We also found multiple melanin-related transcripts strongly upregulated in the red morph, including tan and yellow-family genes, leading us to speculate that dark red pigmentation in autumn J. coenia may involve nonommochrome pigments. While we identified several endocrine and pigmentation genes as obvious candidates for seasonal colour morph differentiation, we speculate that the majority of observed expression differences were due to thermal stress response. The buckeye transcriptome provides a basis for further developmental studies of phenotypic plasticity.