Insect cuticular melanins are distinctly different from those of mammalian epidermal melanins

Interactions of Melanin with Electromagnetic Radiation: From Fundamentals to Applications

Melanin, especially integumentary melanin, interacts in numerous ways with electromagnetic radiation, leading to a set of critical functions, including radiation protection, UV-protection, pigmentary and structural color productions, and thermoregulation. By harnessing these functions, melanin and melanin-like materials can be widely applied to diverse applications with extraordinary performance. Here we provide a unified overview of the melanin family (all melanin and melanin-like materials) and their interactions with the complete electromagnetic radiation spectrum (X-ray, Gamma-ray, UV, visible, near-infrared), which until now has been absent from the literature and is needed to establish a solid fundamental base to facilitate their future investigation and development. We begin by discussing the chemistries and morphologies of both natural and artificial melanin, then the fundamentals of melanin-radiation interactions, and finally the exciting new developments in high-performance melanin-based functional materials that exploit these interactions. This Review provides both a comprehensive overview and a discussion of future perspectives for each subfield of melanin that will help direct the future development of melanin from both fundamental and applied perspectives.

Tanning of the tarsal and mandibular cuticle in adult Anax imperator (Insecta: Odonata) during the emergence sequence

The arthropod cuticle offers strength, protection, and lightweight. Due to its limit in expandability, arthropods have to moult periodically to grow. While moulting is beneficial in terms of parasite or toxin control, growth and adaptation to environmental conditions, it costs energy and leaves the soft animal's body vulnerable to injuries and desiccation directly after ecdysis. To investigate the temporal change in sclerotization and pigmentation during and after ecdysis, we combined macrophotography, confocal laser scanning microscopy, scanning electron microscopy and histological sectioning. We analysed the tarsal and mandibular cuticle of the blue emperor dragonfly to compare the progress of tanning for structures that are functionally involved during emergence (tarsus/tarsal claws) with structures whose functionality is required much later (mandibles). Our results show that: (i) the tanning of the tarsal and mandibular cuticle increases during emergence; (ii) the tarsal cuticle tans faster than the mandibular cuticle; (iii) the mandibles tan faster on the aboral than on the oral side; and (iv) both the exo- and the endocuticle are tanned. The change in the cuticle composition of the tarsal and mandibular cuticle reflects the demand for higher mechanical stability of these body parts when holding on to the substrate during emergence and during first walking or hunting attempts.

The melanin pigment gene black mediates body pigmentation and courtship behaviour in the German cockroach Blattella germanica

Genes involved in melanin production directly impact insect pigmentation and can affect diverse physiology and behaviours. The role these genes have on sex behaviour, however, is unclear. In the present study, the crucial melanin pigment gene black was functionally characterised in an urban pest, the German cockroach, Blattella germanica. RNAi knockdown of B. germanica black (Bgblack) had no effect on survival, but did result in black pigmentation of the thoraxes, abdomens, heads, wings, legs, antennae, and cerci due to cuticular accumulation of melanin. Sex-specific variation in the pigmentation pattern was apparent, with females exhibiting darker coloration on the abdomen and thorax than males. Bgblack knockdown also resulted in wing deformation and negatively impacted the contact sex pheromone-based courtship behaviour of males. This study provides evidence for black function in multiple aspects of B. germanica biology and opens new avenues of exploration for novel pest control strategies.

Colour polymorphism associated with a gene duplication in male wood tiger moths

Colour is often used as an aposematic warning signal, with predator learning expected to lead to a single colour pattern within a population. However, there are many puzzling cases where aposematic signals are also polymorphic. The wood tiger moth, Arctia plantaginis, displays bright hindwing colours associated with unpalatability, and males have discrete colour morphs which vary in frequency between localities. In Finland, both white and yellow morphs can be found, and these colour morphs also differ in behavioural and life-history traits. Here, we show that male colour is linked to an extra copy of a yellow family gene that is only present in the white morphs. This white-specific duplication, which we name valkea, is highly upregulated during wing development. CRISPR targeting valkea resulted in editing of both valkea and its paralog, yellow-e, and led to the production of yellow wings. We also characterise the pigments responsible for yellow, white, and black colouration, showing that yellow is partly produced by pheomelanins, while black is dopamine-derived eumelanin. Our results add to a growing number of studies on the genetic architecture of complex and seemingly paradoxical polymorphisms, and the role of gene duplications and structural variation in adaptive evolution.

Catecholamine Derivatives as Novel Crosslinkers for the Synthesis of Versatile Biopolymers

Catecholamine metabolites are not only involved in primary metabolism, but also in secondary metabolism, serving a diverse array of physiologically and biochemically important functions. Melanin, which originates from dopa and dopamine, found in the hair, eye, and skin of all animals, is an important biopolymeric pigment. It provides protection against damaging solar radiation to animals. N-Acetyldopamine and N-β-alanyldopamine play a crucial role in the hardening of the exoskeletons of all insects. In addition, insects and other arthropods utilize the melanogenic process as a key component of their defense systems. Many marine organisms utilize dopyl peptides and proteins as bonding materials to adhere to various substrata. Moreover, the complex dopa derivatives that are precursors to the formation of the exoskeletons of numerous marine organisms also exhibit antibiotic properties. The biochemistry and mechanistic transformations of different catecholamine derivatives to produce various biomaterials with antioxidant, antibiotic, crosslinking, and gluing capabilities are highlighted. These reactivities are exhibited through the transient and highly reactive quinones, quinone methides, and quinone methide imine amide intermediates, as well as chelation to metal ions. A careful consideration of the reactivities summarized in this review will inspire numerous strategies for synthesizing novel biomaterials for future medical and industrial use.

Effects of Dietary Cystine and Tyrosine Supplementation on Melanin Synthesis in the Pacific Oyster (Crassostrea gigas)

Melanogenesis is a multistep process to produce melanin for dark pigmentation in skin coloration. Previous studies in vertebrates demonstrated that cystine and tyrosine amino acids are involved in the melanin synthesis. However, very little is known about the melanogenesis in bivalve. In this study, cystine supplementation for 30 days significantly upregulated the expression of CgB-aat1, CgCbs and CgTyr and pheomelanin content in the Pacific oyster Crassostrea gigas. Transmission electron microscope (TEM) results revealed more melanosomes in the connective tissue and melanin granules were secreted in epithelium of mantle. In contrast, tyrosine supplementation had no clear effect on melanogenesis except the gene expression changes of CgB-aat1 and CgCbs. In addition, prolonged supplementation of cystine or tyrosine for 60 days had a negative impact on melanogenesis. Indeed, after 60 days, expression of most of the melanin synthesis-related genes under study was decreased, and melanin content was significantly reduced, indicating that cystine and tyrosine might inhibit production of eumelanin and pheomelanin, respectively. In addition, in vitro analysis using primary cell culture from mantle tissue indicated that incubation with cystine, tyrosine, or B-AAT1 polypeptide, CBS/TYR recombinant proteins induced the increase of CgB-aat1 and CgCbs expression in a dose-dependent manner, suggesting the presence of a regulatory network in response to cystine and tyrosine amino acids intakes in pheomelanin synthesis-related gene expression. Taken together, these data indicate that cystine-CgB-aat1-CgCbs-CgTyr axis is a potential regulator of the pheomelanin biosynthesis pathway, and thus plays an important role in the mantle pigmentation in C. gigas. This work provides a new clue for selective cultivation of oyster strains with specific shell colors in bivalve breeding.

Negative phototaxis of jumping cocooned parasitoid wasp larvae against short wavelengths and physicochemical properties of the cocoon shell

The parasitoid wasp Bathyplectes anurus (Hymenoptera: Ichneumonidae: Campopleginae) is a successful biocontrol agent against the alfalfa weevil, Hypera postica. This weevil is a serious pest of beneficial fabaceous plants such as alfalfa and Chinese milk vetch. One of the possible reasons for the success of this wasp in hot climates may be the ability of its cocooned larvae to repeatedly jump and roll until they relocate themselves away from detrimental sunlight and heat. It is not yet known which wavelengths of light trigger this avoidance behavior or the microstructure of the cocoon shell that might allow light transmission. Here, the response of the cocooned larvae to different wavelengths, and the microstructure, hardness, and elemental components of the cocoon shell were studied. A population of cocooned larvae were introduced on the boundary line between illuminated and shaded areas with blue, green, red, or near-infrared light-emitting diodes. The cocoons moved away from the blue and green light. The distance from the boundary to the cocoons in the shaded area was longer under these long wavelengths, followed by the red light and shortest under the near-infrared light and nil under darkness. No difference was found in mortality between different wavelengths after three days of illumination. Scanning electron microscope observations of the surface of the cocoon shell revealed that the belt-like central ridge was porous, which likely allows ventilation and light transmission. The surface of the cocoon shell showed a uniform distribution of sulfur, potentially aiding in the capture of green wavelengths. The ridge had twice the thickness of the main body and was 1.9 times harder than the main body. These results may be applied to better understand the individual responses of this biological control agent to modifications to their environment, including light pollution.

Recent Advances in Characterization of Melanin Pigments in Biological Samples

The melanin pigments eumelanin (EM) and pheomelanin (PM), which are dark brown to black and yellow to reddish-brown, respectively, are widely found among vertebrates. They are produced in melanocytes in the epidermis, hair follicles, the choroid, the iris, the inner ear, and other tissues. The diversity of colors in animals is mainly caused by the quantity and quality of their melanin, such as by the ratios of EM versus PM. We have developed micro-analytical methods to simultaneously measure EM and PM and used these to study the biochemical and genetic fundamentals of pigmentation. The photoreactivity of melanin has become a major focus of research because of the postulated relevance of EM and PM for the risk of UVA-induced melanoma. Our biochemical methods have found application in many clinical studies on genetic conditions associated with alterations in pigmentation. Recently, besides chemical degradative methods, other methods have been developed for the characterization of melanin, and these are also discussed here.

Transcription factor CgPOU3F4-like regulates expression of pheomelanin synthesis related gene CgB-aat1 in the Pacific oyster (Crassostrea gigas)

Previous study has found that b (0, +) -type amino acid transporter 1 (CgB-aat1) plays an essential role on mantle pigmentation in the Pacific oyster Crassostrea gigas. However, the molecular regulation of CgB-aat1 gene expression remains unclear. Herein, three POU domain family members, CgPOU2F1, CgPOU3F4-like and CgPOU4F3-X1 were characterized and they all had POUs and HOX domains, respectively, which were important in transcriptional regulation. CgPOU3F4-like gene expression was the highest in mantle edge. Subsequently, the dual-luciferase reporter result showed that the core regulatory region of CgB-aat1 gene was from -632 to -350 bp of promoter. In transient co-transfection assays, the strongest activity was activated only by CgPOU3F4-like, suggesting CgPOU3F4-like was a valid transcriptional activator of CgB-aat1 gene promoter. And the structural integrity of CgPOU3F4-like was essential for its activation function. In addition, site directed mutagenesis assay was applied to detect three key binding sites between CgPOU3F4-like and core region of CgB-aat1 gene promoter, and this interaction was verified by ChIP test. Furthermore, CgPOU3F4-like knockdown by RNA interference led to obvious decreases in CgB-aat1 and cystathionine beta-synthase (CgCbs) expressions at both mRNA and protein levels. Collectively, these results indicate that CgPOU3F4-like positively regulate CgB-aat1 gene expression and it may be a critical upstream transcriptional regulation factor in pheomelanin synthesis in C. gigas.

Isolation and characterization of blackish-brown BY2-melanin accumulated in cultured tobacco BY-2 cells

The tobacco BY-2 cell line is one of the most utilized plant cell lines. After long-term culture, the cells turn brown to black, but the causal pigment is unknown. We successfully isolated a blackish-brown pigment from BY-2 cells cultured for 3 weeks. Morphological and spectroscopic analyses indicated that the pigment had similar features to a melanin-like substance reported previously. Furthermore, physicochemical analyses revealed that this pigment possessed most of the properties of melanin-like pigments. In addition, the high nitrogen content suggested that it differed from common plant melanins classified as allomelanins, suggesting a novel eumelanin-like pigment: "BY2-melanin". This is the first example showing that eumelanin-like pigments are produced in the cultures of plant cells for which the accumulation of melanin has not been reported. This tobacco BY-2 cell culture technique may represent a customizable and sustainable alternative to conventional melanin production platforms, with significant potential for industrial and pharmacological applications.

Involvement of B-aat1 and Cbs in regulating mantle pigmentation in the Pacific oyster (Crassostrea gigas)

BACKGROUND

Shell color formation is an important physiological process in bivalves, the molecular genetic basis has potential application in bivalve aquaculture, but there is still remaining unclear about this issue. The cystine/glutamate transporter (Slc7a11) and cystathionine beta-synthase (Cbs) are integral genes in pheomelanin synthesis pathway, which is vital to skin pigmentation.

METHODS AND RESULTS

Here, the sequences of b (0, +) -type amino acid transporter 1 (B-aat1) and Cbs in Pacific oyster (Crassostrea gigas) (CgB-aat1, CgCbs) were characterized. Phylogenetically, the deduced amino acid sequences of CgB-aat1 and CgCbs both possessed conserved features. Genes were both ubiquitously expressed in six tested tissues with more abundant expression level in central mantle. Besides, the polyclonal antibodies of CgB-aat1, CgCbs, CgTyr, and CgTyrp2 were successfully prepared. Immunofluorescence analysis revealed that CgB-aat1 and CgCbs proteins were both expressed in gill rudiments of eyed-larvae and concentrated mainly in cytoplasm of epithelial cell and nerve axons in mantle. Additionally, after CgB-aat1 or CgCbs silencing, expressions at mRNA and protein levels of CgB-aat1 and CgCbs involved in pheomelanin synthesis were significantly suppressed, and CgTyr, CgTyrp1 and CgTyrp2 related to eumelanin synthesis were also down-regulated but no apparent differences, respectively. Moreover, micrographic examination found less brown-granules at mantle edge in CgB-aat1 interference group.

CONCLUSION

These results implied that pheomelanin synthesis was possible induced by CgB-aat1-CgTyr-CgCbs axis, and it played an essential role on mantle pigmentation in the oysters. These findings provide the useful genetic knowledge and enrich the physiological information for the shell color formation in bivalve aquaculture.

Iron and copper ions accelerate and modify dopamine oxidation to eumelanin: implications for neuromelanin genesis

Dopamine (DA) is a precursor of neuromelanin (NM) synthesized in the substantia nigra of the brain. NM is known to contain considerable levels of Fe and Cu. However, how Fe and Cu ions affect DA oxidation to DA-eumelanin (DA-EM) and modify its structure is poorly understood. EMs were prepared from 500 µM DA, dopaminechrome (DAC), or 5,6-dihydroxyindole (DHI). Autoxidation was carried out in the absence or presence of 50 µM Fe(II) or Cu(II) at pH 7.4 and 37 ℃. EMs were characterized by Soluene-350 solubilization analyzing absorbances at 500 nm (A500) and 650 nm (A650) and alkaline hydrogen peroxide oxidation (AHPO) yielding various pyrrole carboxylic acids. Pyrrole-2,3,4,5-tetracarboxylic acid (PTeCA) served as a molecular marker of cross-linked DHI units. Importantly, Fe and Cu accelerated DA oxidation to DA-EM and DHI oxidation to DHI-EM several-fold, whereas these metals only weakly affected the production of DAC-EM. The A500 values indicated that DA-EM contains considerable portions of uncyclized DA units. Analysis of the A650/A500 ratios suggests that Fe and Cu caused some degradation of DHI units of DA-EM during 72-h incubation. Results with AHPO were consistent with the A500 values and additionally revealed that (1) DA-EM is less cross-linked than DAC-EM and DHI-EM and (2) Fe and Cu promote cross-linking of DHI units. In conclusion, Fe and Cu not only accelerate the oxidation of DA to DA-EM but also promote cross-linking and degradation of DHI units. These results help to understand how Fe and Cu in the brain affect the production and properties of NM.

Differential gene expression patterns between the head and thorax of Gynaephora aureata are associated with high-altitude adaptation

Grassland caterpillars (Lepidoptera: Erebidae: Gynaephora) are important pests in alpine meadows of the Qinghai-Tibetan Plateau (QTP). These pests have morphological, behavioral, and genetic adaptations for survival in high-altitude environments. However, mechanisms underlying high-altitude adaptation in QTP Gynaephora species remain largely unknown. Here, we performed a comparative analysis of the head and thorax transcriptomes of G. aureata to explore the genetic basis of high-altitude adaptation. We detected 8,736 significantly differentially expressed genes (sDEGs) between the head and thorax, including genes related to carbohydrate metabolism, lipid metabolism, epidermal proteins, and detoxification. These sDEGs were significantly enriched in 312 Gene Ontology terms and 16 KEGG pathways. We identified 73 pigment-associated genes, including 8 rhodopsin-associated genes, 19 ommochrome-associated genes, 1 pteridine-associated gene, 37 melanin-associated genes, and 12 heme-associated genes. These pigment-associated genes were related to the formation of the red head and black thorax of G. aureata. A key gene, yellow-h, in the melanin pathway was significantly upregulated in the thorax, suggesting that it is related to the formation of the black body and contributed to the adaptation of G. aureata to low temperatures and high ultraviolet radiation in the QTP. Another key gene, cardinal, in the ommochrome pathway was significantly upregulated in the head and may be related to red warning color formation. We also identified 107 olfactory-related genes in G. aureata, including genes encoding 29 odorant-binding proteins, 16 chemosensory proteins, 22 odorant receptor proteins, 14 ionotropic receptors, 12 gustatory receptors, 12 odorant degrading enzymes, and 2 sensory neuron membrane proteins. Diversification of olfactory-related genes may be associated with the feeding habits of G. aureata, including larvae dispersal and searching for plant resources available in the QTP. These results provide new insights into high-altitude adaptation of Gynaephora in the QTP and may contribute to the development of new control strategies for these pests.

Effects of Arbuscular Mycorrhizal Fungi-Colonized Populus alba × P. berolinensis Seedlings on the Microbial and Metabolic Status of Gypsy Moth Larvae

Arbuscular mycorrhizal fungi (AMF) are considered as important biological factors that can affect insect resistance of plants. Herein, we used AMF-poplar seedlings that could either increase or decrease the resistance to gypsy moth larvae, to elucidate the mechanism of mycorrhizal-induced insect resistance/susceptibility at the larval microbial and metabolic levels. Our results found that larval plant consumption and growth were significantly inhibited in the Glomus mossae (GM)-colonized seedlings, whereas they were enhanced in the Glomus intraradices (GI)-colonized seedlings. GM inoculation reduced the beneficial bacteria abundance in the larval gut and inhibited the detoxification and metabolic functions of gut microbiota. However, GI inoculation improved the larval gut environment by decreasing the pathogenic bacteria and activating specific metabolic pathways. Furthermore, GM inoculation triggers a metabolic disorder in the larval fat body, accompanied by the suppression of detoxification and energy production pathways. The levels of differentially accumulated metabolites related to amino acid synthesis and metabolism and exogenous toxin metabolism pathways were significantly increased in the GI group. Taken together, the disadaptation of gypsy moth larvae to leaves of GM-colonized seedlings led to the GM-induced insect resistance in poplar, and to the GI-induced insect susceptibility involved in the improvement of larval gut environment and fat body energy metabolism.

Silencing tyrosine hydroxylase or dopa decarboxylase gene disrupts cuticle tanning during larva-pupa-adult transformation in Henosepilachna vigintioctopunctata

BACKGROUND

The 28-spotted potato ladybird, Henosepilachna vigintioctopunctata, is a notorious defoliator of many solanaceous and cucurbitaceous plants. Tyrosine hydroxylase (TH) and dopa decarboxylase (DDC) are responsible for cuticle tanning pathway in insects.

RESULTS

We identified HvTH and HvDDC in H. vigintioctopunctata, and found that high levels of them were accumulated just before or right after molting. Injection of dsHvTH or feeding 3-iodo-tyrosine (3-IT) at the third instar larval stage repressed tanning of the larval cuticle, reduced larval feeding, inhibited larval growth, and consequently caused 100% of larval mortality. Knockdown of HvDDC at the third instar larval stage hardly affected the coloration of larval head, and partially inhibited pigmentation of larval bodies and around 80% of the HvDDC RNAi larvae developed into albino pupae and adults. Moreover, depletion of HvTH or HvDDC at the fourth instar larval stage resulted in albino pupae and adults. The HvTH or HvDDC hypomorph adults fully or partially failed to remove the larval/pupal exuviae, possessed pale and abnormal wings, and poorly tanned heads and bodies, and eventually, struggled for several days without feeding on leaves before death.

CONCLUSION

These results show that TH and DDC play key roles in larval and adult cuticle tanning and development in H. vigintioctopunctata. Also, these findings suggest that dopa- and dopamine-originated pigments are essential for larval and adult feeding behavior and the molting process during emergence. © 2022 Society of Chemical Industry.

Exoskeleton ageing and its relation to longevity and fecundity in female Australian leaf insects (

Senescence is a decline in reproduction and survival rate with advancing age resulting from deterioration of somatic tissues and systems throughout the body. Age-related somatic changes (somatic ageing) have been studied extensively in vertebrates but are less well known in other animals, including insects. Since adult insects have very limited ability to repair their exoskeleton, somatic ageing could involve deterioration and discolouration of the cuticle. We investigated age-related changes in wing pigmentation and abdominal cuticle necrosis in females of the Australian leaf insect Phyllium monteithi. Adult females varied markedly in the extent and pattern of pigmentation on their bodies, and we found that pigment spots on the forewings increased in size with age in most individuals. As females aged, most individuals also exhibited increasing levels of abdominal cuticle necrosis, resulting in the loss of abdominal cuticle along the margin of the abdomen. Neither the extent of pigmentation nor cuticle loss were clearly associated with reduced fecundity or longevity in the protected laboratory environment, but it remains unknown whether these age-related changes have functional implications in the wild. Our results show that the P. monteithi exoskeleton undergoes complex changes with age, with potential implications for functional traits and fitness.

Integrated Analysis of Transcriptome and Proteome to Reveal Pupal Color Switch in Papilio xuthus Butterflies

Pupal color polyphenism in Papilio butterflies, including green, intermediate, or brown, is an excellent study system for understanding phenotypic plasticity. Previous studies suggested that development of brown pupae may be controlled by a hormone called pupal-cuticle-melanizing-hormone (PCMH) which is synthesized and secreted from brain-suboesophageal ganglion and prothoracic ganglion complexes (Br-SG-TG1) during the pre-pupa stage. However, detailed molecular mechanisms of neuroendocrine regulation in pupal color development remain unknown. In this study, we integrated the expression profiles of transcriptome and proteome at pre-pupa stages [2 h after gut purge (T1) and 3 h after forming the garter around the body (T2)] and pigmentation stages [10 h after ecdysis (T3) and 24 h after ecdysis (T4)] to identify important genes and pathways underlying the development of green and brown pupa in the swallowtail butterfly Papilio xuthus. Combined comparisons of each developmental stage and each tissue under green and brown conditions, a total of 1042 differentially expressed genes (DEGs) and 430 different abundance proteins (DAPs) were identified. Weighted gene co-expression network analysis (WGCNA) and enrichment analysis indicate that these DEGs were mainly related to oxidation-reduction, structural constituent of cuticle, and pigment binding. Soft clustering by Mfuzz and enrichment analysis indicate that these DAPs are mainly involved in tyrosine metabolism, insect hormone biosynthesis, and melanogenesis. By homologous alignment, we further identified those genes encoding neuropeptides (51), GPCRs (116), G-proteins (8), cuticular proteins (226), chitinases (16), and chitin deacetylases (8) in the whole genome of P. xuthus and analyzed their expression profiles. Although we identified no gene satisfying with hypothesized expression profile of PCMH, we found some genes in the neuropeptide cascade showed differentially expressed under two pupal color conditions. We also found that Toll signaling pathway genes, juvenile hormone (JH) related genes, and multiple cuticular proteins play important roles in the formation of selective pupal colors during the prepupal-pupal transition. Our data also suggest that both green and brown pupa include complex pigment system that is regulated by genes involved in black, blue, and yellow pigments. Our results provide important insights into the evolution of pupal protective colors among swallowtail butterflies.

Drosophila yellow-h encodes dopaminechrome tautomerase: A new enzyme in the eumelanin biosynthetic pathway

Melanin is a widely distributed phenolic pigment that is biosynthesized from tyrosine and its hydroxylated product, dopa, in all animals. However, recent studies reveal a significant deviation from this paradigm, as insects appear to use dopamine rather than dopa as the major precursor of melanin. This observation calls for a reconsideration of the insect melanogenic pathway. While phenoloxidases and laccases can oxidize dopamine for dopaminechrome production, the fate of dopaminechrome remains undetermined. Dopachrome decarboxylase/tautomerase, encoded by yellow-f/f2 of Drosophila melanogaster, can convert dopaminechrome into 5,6-dihydroxyindole, but the same enzyme from other organisms does not act on dopaminechrome, suggesting the existence of a specific dopaminechrome tautomerase (DPT). We now report the identification of this novel enzyme that biosynthesizes 5,6-dihydroxyindole from dopaminechrome in Drosophila. Dopaminechrome tautomerase acted on both dopaminechrome and N-methyl dopaminechrome but not on dopachrome or other aminochromes tested. Our biochemical and molecular studies reveal that this enzyme is encoded by the yellow-h gene, a member of the yellow gene family, and advance our understanding of the physiological functions of this gene family. Identification and characterization of DPT clarifies the precursor for melanin biosynthetic pathways and proves the existence of an independent melanogenic pathway in insects that utilizes dopamine as the primary precursor.

Pigmentation and color pattern diversity in Odonata

The order Odonata (dragonflies and damselflies) comprises diurnal insects with well-developed vision, showing diverse colors in adult wings and bodies. It is one of the most ancestral winged insect groups. Because Odonata species use visual cues to recognize each other, color patterns have been investigated from ecological and evolutionary viewpoints. Here we review the recent progress on molecular mechanisms of pigmentation, especially focused on light-blue coloration. Results from histology and pigment analysis showed that ommochrome pigments on the proximal layer and pteridine pigments on the distal layer of the epidermis are essential for light-blue coloration. We also summarize genes involved in the biosynthesis of three major insect pigments conserved across insects and discuss that gene-functional analysis deserves future studies.

Nonenzymatic Spontaneous Oxidative Transformation of 5,6-Dihydroxyindole

Melanin is an important phenolic skin pigment found throughout the animal kingdom. Tyrosine and its hydroxylated product dopa provide the starting material for melanin biosynthesis in all animals. Through a set of well-established reactions, they are converted to 5,6-dihydroxyindole (DHI) and DHI-2-carboxylic acid (DHICA). Oxidative polymerization of these two indoles produces the brown to black eumelanin pigment. The steps associated with these transformations are complicated by the extreme instability of the starting materials and the transient and highly reactive nature of the intermediates. We have used mass spectral studies to explore the nonenzymatic mechanism of oxidative transformation of DHI in water. Our results indicate the facile production of not only dimeric and trimeric products but also higher oligomeric forms of DHI upon exposure to air in solution, even under nonenzymatic conditions. Such instantaneous polymerization of DHI avoids toxicity to self-matter and ensures the much-needed deposition of melanin at (a) the wound site and (b) the infection site in arthropods. The rapid deposition of DHI melanin is advantageous for arthropods given their open circulatory system; the process limits blood loss during wounding and prevents the spread of parasites by encapsulating them in melanin, limiting the damage.

Molecular and Potential Regulatory Mechanisms of Melanin Synthesis in Harmonia axyridis

Melanization is a common phenomenon in insects, and melanin synthesis is a conserved physiological process that occurs in epidermal cells. Moreover, a comprehensive understanding of the mechanisms of melanin synthesis influencing insect pigmentation are well-suited for investigating phenotype variation. The Asian multi-colored (Harlequin) ladybird beetle, Harmonia axyridis, exhibits intraspecific polymorphism based on relative levels of melanization. However, the specific characteristics of melanin synthesis in H. axyridis remains elusive. In this study, we performed gene-silencing analysis of the pivotal inverting enzyme, tyrosine hydroxylase (TH), and DOPA decarboxylase (DDC) in the tyrosine metabolism pathway to investigate the molecular and regulatory mechanism of melanin synthesis in H. axyridis. Using RNAi of TH and DDC genes in fourth instar larvae, we demonstrated that dopamine melanin was the primary contributor to the overall body melanization of H. axyridis. Furthermore, our study provides the first conclusive evidence that dopamine serves as a melanin precursor for synthesis in the early pupal stage. According to transcription factor Pannier, which is essential for the formation of melanic color on the elytra in H. axyridis, we further demonstrated that suppression of HaPnr can significantly decrease expression levels of HaTH and HaDDC. These results in their entirety lead to the conclusion that transcription factor Pannier can regulate dopamine melanin synthesis in the dorsal elytral epidermis of H. axyridis.

DOPA decarboxylase is essential for cuticle tanning in Rhodnius prolixus (Hemiptera: Reduviidae), affecting ecdysis, survival and reproduction

Cuticle tanning occurs in insects immediately after hatching or molting. During this process, the cuticle becomes dark and rigid due to melanin deposition and protein crosslinking. In insects, different from mammals, melanin is synthesized mainly from dopamine, which is produced from DOPA by the enzyme DOPA decarboxylase. In this work, we report that the silencing of the RpAadc-2 gene, which encodes the putative Rhodnius prolixus DOPA decarboxylase enzyme, resulted in a reduction in nymph survival, with a high percentage of treated insects dying during the ecdysis process or in the expected ecdysis period. Those treated insects that could complete ecdysis presented a decrease in cuticle pigmentation and hardness after molting. In adult females, the knockdown of AADC-2 resulted in a reduction in the hatching of eggs; the nymphs that managed to hatch failed to tan the cuticle and were unable to feed. Despite the failure in cuticle tanning, knockdown of the AADC-2 did not increase the susceptibility to topically applied deltamethrin, a pyrethroid insecticide. Additionally, our results showed that the melanin synthesis pathway did not play a major role in the detoxification of the excess (potentially toxic) tyrosine from the diet, an essential trait for hematophagous arthropod survival after a blood meal.

Silencing cuticular pigmentation genes enables RNA FISH in intact insect appendages

Optical imaging of gene expression by fluorescence in situ hybridisation (FISH) in insects is often impeded by their pigmented cuticle. As most chemical bleaching agents are incompatible with FISH, we developed an RNA interference (RNAi)-based method for clearing cuticular pigmentation which enables the use of whole-mount body appendages for RNA FISH (termed RNA-i-FISH). Silencing laccase2 or tyrosine hydroxylase in two leaf beetles species (Chrysomela populi and Phaedon cochleariae) cleared their pigmented cuticle and decreased light absorbance. Subsequently, intact appendages (palps, antennae, legs) from RNAi-cleared individuals were used to image the expression and spatial distribution of antisense mRNA of two chemosensory genes encoding gustatory receptor and odorant-binding protein. Imaging did not work for RNAi controls because the pigmentation was retained, or for FISH controls (sense mRNA). Several bleaching agents were incompatible with FISH, because of degradation of RNA, lack of clearing efficacy or long incubation times. Overall, silencing pigmentation genes is a significant improvement over bleaching agents, enabling FISH in intact insect appendages.

Drosophila melanogaster has the enzymatic machinery to make the melanic component of neuromelanin

In Drosophila, the same set of genes that are used for cuticle pigmentation and sclerotization are present in the nervous system and are responsible for neurotransmitter recycling. In this study, we carried out biochemical analysis to determine whether insects have the enzymatic machinery to make melanic component of neuromelanin. We focused our attention on two key enzymes of melanogenesis, namely phenoloxidase and dopachrome decarboxylase/tautomerase. Activity staining of the proteins isolated from the Drosophila larval brain tissue, separated by native polyacrylamide gel electrophoresis, indicated the presence of these two enzymes. Mass spectral sequence analysis of the band also supported this finding. To best of our knowledge, this is the first report on the presence of the enzymatic machinery to make melanin part of neuromelanin in any insect brain.

On the Metal Cofactor in the Tyrosinase Family

The production of pigment in mammalian melanocytes requires the contribution of at least three melanogenic enzymes, tyrosinase and two other accessory enzymes called the tyrosinase-related proteins (Trp1 and Trp2), which regulate the type and amount of melanin. The last two proteins are paralogues to tyrosinase, and they appeared late in evolution by triplication of the tyrosinase gene. Tyrosinase is a copper-enzyme, and Trp2 is a zinc-enzyme. Trp1 has been more elusive, and the direct identification of its metal cofactor has never been achieved. However, due to its enzymatic activity and similarities with tyrosinase, it has been assumed as a copper-enzyme. Recently, recombinant human tyrosinase and Trp1 have been expressed in enough amounts to achieve for the first time their crystallization. Unexpectedly, it has been found that Trp1 contains a couple of Zn(II) at the active site. This review discusses data about the metal cofactor of tyrosinase and Trps. It points out differences in the studied models, and it proposes some possible points accounting for the apparent discrepancies currently appearing. Moreover, some proposals about the possible flexibility of the tyrosinase family to uptake copper or zinc are discussed.