Functional analysis of genes involved in color pattern formation in Lepidoptera

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.

aristaless1 has a dual role in appendage formation and wing color specification during butterfly development

BACKGROUND

Highly diverse butterfly wing patterns have emerged as a powerful system for understanding the genetic basis of phenotypic variation. While the genetic basis of this pattern variation is being clarified, the precise developmental pathways linking genotype to phenotype are not well understood. The gene aristaless, which plays a role in appendage patterning and extension, has been duplicated in Lepidoptera. One copy, aristaless1, has been shown to control a white/yellow color switch in the butterfly Heliconius cydno, suggesting a novel function associated with color patterning and pigmentation. Here we investigate the developmental basis of al1 in embryos, larvae, and pupae using new antibodies, CRISPR/Cas9, RNAi, qPCR assays of downstream targets, and pharmacological manipulation of an upstream activator.

RESULTS

We find that Al1 is expressed at the distal tips of developing embryonic appendages consistent with its ancestral role. In developing wings, we observe Al1 accumulation within developing scale cells of white H. cydno during early pupation while yellow scale cells exhibit little Al1 at this time point. Reduced Al1 expression is also associated with yellow scale development in al1 knockouts and knockdowns. We propose that Al1 expression in future white scales might be related to an observed downregulation of the enzyme Cinnabar and other genes that synthesize and transport the yellow pigment, 3-hydroxykynurenine (3-OHK). Finally, we provide evidence that Al1 activation is under the control of Wnt signaling.

CONCLUSIONS

We propose a model in which high levels of Al1 during early pupation, which are mediated by Wnt, are important for melanic pigmentation and specifying white portions of the wing while reduced levels of Al1 during early pupation promote upregulation of proteins needed to move and synthesize 3-OHK, promoting yellow pigmentation. In addition, we discuss how the ancestral role of aristaless in appendage extension may be relevant in understanding the cellular mechanism behind color patterning in the context of the heterochrony hypothesis.

Using DNA barcodes to test the association of sexes and morphs in Calodesma spp. (Lepidoptera, Erebidae, Arctiinae, Arctiini, Pericopina) of Trinidad, West Indies, with an overview of the genus, taxonomic changes and a new species

Phalaena militta Stoll, [1781], currently in the combination Thyrgis militta, is transferred to the new combination Calodesma militta. Phalaena militta is the type species of Thyrgis Walker, 1854, and so Thyrgis is a junior synonym of Calodesma Hübner, [1820]. The reinstated genus Seileria Dognin, 1923 is the next available name for the genus previously known as Thyrgis, and the remaining eight species and their subspecies currently in Thyrgis are transferred to new combinations as species of Seileria: S. angustifascia (Hering, 1925), S. basipunctata (Hering, 1926), S. constrictifascia (Dognin, 1919), S. flavonigra (Dognin, 1910), S. investigatorum (Toulgoët, 1988), S. marginata (Butler, 1875), S. meres (Druce, 1911), S. phlegon (Druce, 1885), S. phlegon ruscia (Druce, 1895), S. tenuifascia (Hering, 1930) and S. tenuifascia daguana (Hering, 1930). Eucyanoides Toulgoët, 1988, currently a synonym of Thyrgis, is made a new subjective synonym of Seileria. Based on DNA barcodes, we recognise three very similar, sexually dimorphic and in two cases polymorphic South American species of Calodesma with some phenotypes in common but very similar male genitalia: C. militta (BOLD:AAK1660), C. sp. cf. collaris (BOLD:ABZ2392) and C. pseudocollaris Cock new species (BOLD:AEI2170). Calodesma militta is widespread in South America, with two male morphs (collaris and dioptis) and two female morphs with variable markings (white and orange morphs). Centronia plorator Kaye, [1923] and Thyrgis lacryma Dognin, 1919 are variants of the white female morph and are new synonyms of Calodesma militta. A third female morph with red markings was not sequenced and could not be allocated to a species. Calodesma sp. cf. collaris (BOLD:ABZ2392) occurs in southern South America with both male morphs but only a white female morph. Calodesma pseudocollaris new species (BOLD:AEI2170) is only known from Trinidad, with one male morph (collaris) and the white female morph. Although more than ten morphs relating to this complex have been described as species, they cannot be synonymised without more data on distribution of the different species or DNA barcodes from the type specimens. Collated life history information indicates species of this group are split between Malpighiaceae feeders and Bromeliaceae feeders, but more work is needed to define these differences. The morphism patterns observed are discussed in terms of Müllerian mimicry and mimicry rings, and we suggest that in Trinidad (and elsewhere) there is a loose mimicry ring of diurnal black species with white spots or transparent patches on the wings which are most conspicuous and frequently observed when feeding on white Asteraceae flowers.

Functional unit of supergene in female-limited Batesian mimicry of Papilio polytes

Supergenes are sets of genes and genetic elements that are inherited like a single gene and control complex adaptive traits, but their functional roles and units are poorly understood. In Papilio polytes, female-limited Batesian mimicry is thought to be regulated by a ∼130 kb inversion region (highly diversified region: HDR) containing 3 genes, UXT, U3X, and doublesex (dsx) which switches non-mimetic and mimetic types. To determine the functional unit, we here performed electroporation-mediated RNAi analyses (and further Crispr/Cas9 for UXT) of genes within and flanking the HDR in pupal hindwings. We first clarified that non-mimetic dsx-h had a function to form the non-mimetic trait in female and only dsx-H isoform 3 had an important function in the formation of mimetic traits. Next, we found that UXT was involved in making mimetic-type pale-yellow spots and adjacent gene sir2 in making red spots in hindwings, both of which refine more elaborate mimicry. Furthermore, downstream gene networks of dsx, U3X, and UXT screened by RNA sequencing showed that U3X upregulated dsx-H expression and repressed UXT expression. These findings demonstrate that a set of multiple genes, not only inside but also flanking HDR, can function as supergene members, which extends the definition of supergene unit than we considered before. Also, our results indicate that dsx functions as the switching gene and some other genes such as UXT and sir2 within the supergene unit work as the modifier gene.

Functional analysis of ABCG2 gene in pigment transport of Neocaridina denticulata sinensis

Many strains of Neocaridina denticulata sinensis (cherry shrimp) possess vivid body colors.However, the molecular underpinnings of these various body colors are scarcely understood. To study the role of the ABCG2 gene in the pigmentation of cherry shrimp, four strains (red, yellow, blue, and wild strains) were sampled. The sequence and expression pattern of ABCG2in tissues and embryos were analyzed, and the distribution of ABCG2 was also explored via WFISH (whole mount flurescence in situ hybridization). And further, RNA interference (RNAi) was used to explore the role of ABCG2 in body color deposition. The results showed that the ABCG2 sequence contained the conserved motif of Walker A, Walker B, Q-loop, d-loop, and H-loop. In tissues, ABCG2 was highly expressed in the epidermis of the four strains. During development stages, it was first expressed at the cleavage stage, then decreased at the gastrula stage, with the lowest expression at the pre-nauplius stage. From the metanauplius stage, its expression level was significantly upregulated until it reached the highest level at the membrane-zoea stage. WFISH showed that ABCG2 was first expressed at the cleavage stage in all four strains, and its distribution was similar from the cleavage stage to the before-zoea stage. The strongest positive signals were observed at the membrane-zoea stage, which was consistent with the qPCR results. Moreover, at the membrane-zoea stage, the positive signals of the four strains were mainly distributed in the compound eye and appendages. RNAi knockdown of ABCG2 encumbered the development of compound eye pigment cells (CEPCs) and erythrophores. It had effects on the expressions of other genes related to body color. These results suggest that ABCG2 is involved in the development of compound eye and the proliferation of erythrophores. This study provides new ideas for the cultivation of novel body colors in cherry shrimp at the molecular level.

Genomic architecture and functional unit of mimicry supergene in female limited Batesian mimic Papilio butterflies

It has long been suggested that dimorphic female-limited Batesian mimicry of two closely related Papilio butterflies, Papilio memnon and Papilio polytes, is controlled by supergenes. Whole-genome sequencing, genome-wide association studies and functional analyses have recently identified mimicry supergenes, including the doublesex (dsx) gene. Although supergenes of both the species are composed of highly divergent regions between mimetic and non-mimetic alleles and are located at the same chromosomal locus, they show critical differences in genomic architecture, particularly with or without an inversion: P. polytes has an inversion, but P. memnon does not. This review introduces and compares the detailed genomic structure of mimicry supergenes in two Papilio species, including gene composition, repetitive sequence composition, breakpoint/boundary site structure, chromosomal inversion and linkage disequilibrium. Expression patterns and functional analyses of the respective genes within or flanking the supergene suggest that dsx and other genes are involved in mimetic traits. In addition, structural comparison of the corresponding region for the mimicry supergene among further Papilio species suggests three scenarios for the evolution of the mimicry supergene between the two Papilio species. The structural features revealed in the Papilio mimicry supergene provide insight into the formation, maintenance and evolution of supergenes. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.

doublesex Controls Both Hindwing and Abdominal Mimicry Traits in the Female-Limited Batesian Mimicry of Papilio memnon

Papilio butterflies are known to possess female-limited Batesian mimicry polymorphisms. In Papilio memnon, females have mimetic and non-mimetic forms, whereas males are monomorphic and non-mimetic. Mimetic females are characterized by color patterns and tails in the hindwing and yellow abdomens. Recently, an analysis of whole-genome sequences has shown that an approximately 160 kb region of chromosome 25 is responsible for mimicry and has high diversity between mimetic (A) and non-mimetic (a) alleles (highly diversified region: HDR). The HDR includes three genes, UXT, doublesex (dsx), and Nach-like, but the functions of these genes are unknown. Here, we investigated the function of dsx, a gene involved in sexual differentiation, which is expected to be functionally important for hindwing and abdominal mimetic traits in P. memnon. Expression analysis by reverse transcription quantitative PCR (RT-qPCR) and RNA sequencing showed that mimetic dsx (dsx-A) was highly expressed in the hindwings in the early pupal stage. In the abdomen, both dsx-A and dsx-a were highly expressed during the early pupal stage. When dsx was knocked down using small interfering RNAs (siRNAs) designed in the common region of dsx-A and dsx-a, a male-like pattern appeared on the hindwings of mimetic and non-mimetic females. Similarly, when dsx was knocked down in the abdomen, the yellow scales characteristic of mimetic females changed to black. Furthermore, when dsx-a was specifically knocked down, the color pattern of the hindwings changed, as in the case of dsx knockdown in non-mimetic females but not mimetic females. These results suggest that dsx-a is involved in color pattern formation on the hindwings of non-mimetic females, whereas dsx-A is involved in hindwing and abdominal mimetic traits. dsx was involved in abdominal and hindwing mimetic traits, but dsx expression patterns in the hindwing and abdomen were different, suggesting that different regulatory mechanisms may exist. Our study is the first to show that the same gene (dsx) regulates both the hindwing and abdominal mimetic traits. This is the first functional analysis of abdominal mimicry in butterflies.

Heme-Peroxidase 2 Modulated by POU2F1 and SOX5 is Involved in Pigmentation in Pacific Oyster (Crassostrea gigas)

Color polymorphism is frequently observed in molluscan shellfish, while the molecular regulation of shell pigmentation is not well understood. Peroxidase is a key enzyme involved in melanogenesis. Here, we identified a heme-peroxidase 2 gene (CgHPX2), and characterized the expression patterns and transcriptional regulation of CgHPX2 in the Pacific oyster Crassostrea gigas. Tissues expression analysis showed that CgHPX2 was a mantle-specific gene and primarily expressed in the edge mantle in black shell color oyster compared with white shell oyster. In situ hybridization showed that strong signals for CgHPX2 were detected in the both inner and outer surface of the outer fold of mantle in the black shell color oyster, whereas positive signals in white shell oyster were mainly localized in the outer surface of the outer fold of mantle. In the embryos and larvae, a high expression level of CgHPX2 was detected in the trochophore stage in both black and white shell color oysters. The temporal localization of CgHPX2 was mainly detected in the shell gland and edge mantle of trochophore and calcified shell larvae, respectively. In addition, a 2227 bp of 5' flanking region sequence of CgHPX2 was cloned, which contained a presumed core promoter region and many potential transcription factor binding sites. Further luciferase assay experiment confirmed that POU domain, class 2, transcription factor 1 (POU2F1), and SRY-box transcription factor 5 (SOX5) were involved in transcriptional regulation of CgHPX2 gene through binding to its specific promoter region. After CgPOU2F1 and CgSOX5 RNA interference, the CgHPX2 gene expression was significantly decreased. These results suggested that CgPOU2F1 and CgSOX5 might be two important transcription factors that positively regulated the expression of CgHPX2 gene, improving our understanding of the transcriptional regulation of molluscan shell pigmentation.

The evolution and genetics of lepidopteran egg and caterpillar coloration

Insect colors and color patterns have fascinated biologists for centuries. While extensive research has focused on the adult colors of Drosophila and butterflies, our understanding of how colors are generated and diversified in embryonic and larval stages remains limited, especially, the genetics behind the protective coloration of the immobile embryonic and larval stages. Lepidoptera, one of the most widespread and species-rich insect orders, are extremely helpful uncovering those mechanisms due to their remarkable diverse colors in eggs and caterpillars within or among species, and these colors usually are variable in different developmental stages or in response to different environments. Here we review the recent progress on coloration of lepidopteran eggs and caterpillars, focusing on the genetic basis, developmental mechanisms, ecology, and evolution underlying the remarkable color diversity.

Electroporation-mediated nucleic acid delivery during non-embryonic stages for gene-function analysis in Anopheles sinensis

The delivery of exogenous nucleic acids to eggs or non-embryonic individuals by microinjection is a vital reverse genetics technique used to determine gene function in mosquitoes. However, DNA delivery to eggs is complex and time-consuming, and conventional, non-embryonic-injection techniques may result in unobvious phenotypes caused by insufficient absorption of nucleic acid fragments by cells at target body parts or tissues. In this study, we developed a set of electroporation-mediated non-embryonic microinjections for the delivery of exogenous nucleic acids in Anopheles sinensis. Gene silencing using this method led to down-regulation of target gene expression (AsCPR128) by 77% in targeted body parts, compared with only 10% in non-targeted body parts, thus increasing the defect-phenotype rate in the target area by 5.3-fold, compared with non-shock injected controls. Electroporation-mediated somatic transgenesis resulted in stable phenotypic characteristics of the reporter gene at the shocked body parts during the pupal-adult stages in about 69% of individuals. Furthermore, injecting plasmid DNA near the ovaries of female mosquitoes after a blood meal followed by electric shock produced three germline G₁ transgenic lines, with a transformation rate of about 11.1% (calculated from ovulatory G₀ females). Among the positive G₁ lines, 42%, 40%, and 31% of individuals emitted red fluorescence in the larval stage. When the red fluorescent larvae developed into adults, green fluorescence was emitted from the ovaries of the females upon feeding. These results suggest that electroporation-mediated non-embryonic microinjection can be an efficient, rapid, and simple technique for analyzing gene function in non-model mosquitoes or other small insects.

Imaginal disc growth factor maintains cuticle structure and controls melanization in the spot pattern formation of Bombyx mori

The complex stripes and patterns of insects play key roles in behavior and ecology. However, the fine-scale regulation mechanisms underlying pigment formation and morphological divergence remain largely unelucidated. Here we demonstrated that imaginal disc growth factor (IDGF) maintains cuticle structure and controls melanization in spot pattern formation of Bombyx mori. Moreover, our knockout experiments showed that IDGF is suggested to impact the expression levels of the ecdysone inducible transcription factor E75A and pleiotropic factors apt-like and Toll8/spz3, to further control the melanin metabolism. Furthermore, the untargeted metabolomics analyses revealed that BmIDGF significantly affected critical metabolites involved in phenylalanine, beta-alanine, purine, and tyrosine metabolism pathways. Our findings highlighted not only the universal function of IDGF to the maintenance of normal cuticle structure but also an underexplored space in the gene function affecting melanin formation. Therefore, this study furthers our understanding of insect pigment metabolism and melanin pattern polymorphisms.

The diverse stripe patterns of animals are usually used for warning or camouflage. However, the actual mechanisms underlying diverse stripe pattern formation remains largely unknown. This study provides direct evidence that imaginal disc growth factor (IDGF) maintains cuticle structure and controls melanization in the spot pattern formation. Our exhaustive knockout experiments reveal that BmIDGF is involved in the melanin pigmentation of Bombyx mori. We demonstrate that IDGF impacts the expression levels of the 20E-inducible transcription factor E75A and pleiotropic factors apt-like and Toll8/spz3, to further affect the melanin metabolism. Furthermore, the metabolome of BmIDGF gene deletion connects metabolism to gene function. Thus, this study shed light on not only the unique function of IDGF but also the molecular mechanism of spot pattern formation.

Evaluating responses to temperature during pre-metamorphosis and carry-over effects at post-metamorphosis in the wood tiger moth (Arctia plantaginis)

Insect metamorphosis is one of the most recognized processes delimiting transitions between phenotypes. It has been traditionally postulated as an adaptive process decoupling traits between life stages, allowing evolutionary independence of pre- and post-metamorphic phenotypes. However, the degree of autonomy between these life stages varies depending on the species and has not been studied in detail over multiple traits simultaneously. Here, we reared full-sib larvae of the warningly coloured wood tiger moth (Arctia plantaginis) in different temperatures and examined their responses for phenotypic (melanization change, number of moults), gene expression (RNA-seq and qPCR of candidate genes for melanization and flight performance) and life-histories traits (pupal weight, and larval and pupal ages). In the emerging adults, we examined their phenotypes (melanization and size) and compared them at three condition proxies: heat absorption (ability to engage flight), flight metabolism (ability to sustain flight) and overall flight performance. We found that some larval responses, as evidenced by gene expression and change in melanization, did not have an effect on the adult (i.e. size and wing melanization), whereas other adult traits such as heat absorption, body melanization and flight performance were found to be impacted by rearing temperature. Adults reared at high temperature showed higher resting metabolic rate, lower body melanization, faster heating rate, lower body temperature at take-off and inferior flight performance than cold-reared adults. Thus, our results did not unambiguously support the environment-matching hypothesis. Our results illustrate the importance of assessing multiple traits across life stages as these may only be partly decoupled by metamorphosis. This article is part of the theme issue 'The evolution of complete metamorphosis'.

The mimetic wing pattern of Papilio polytes butterflies is regulated by a doublesex-orchestrated gene network

The swallowtail butterfly Papilio polytes is sexually dimorphic and exhibits female-limited Batesian mimicry. This species also has two female forms, a non-mimetic form with male-like wing patterns, and a mimetic form resembling an unpalatable model, Pachliopta aristolochiae. The mimicry locus H constitutes a dimorphic Mendelian 'supergene', including a transcription factor gene doublesex (dsx). However, how the mimetic-type dsx (dsx-H) orchestrates the downstream gene network and causes the mimetic traits remains unclear. Here we performed RNA-seq-based gene screening and found that Wnt1 and Wnt6 are up-regulated by dsx-H during the early pupal stage and are involved in the red/white pigmentation and patterning of mimetic female wings. In contrast, a homeobox gene abdominal-A is repressed by dsx-H and involved in the non-mimetic colouration pattern. These findings suggest that dual regulation by dsx-H, induction of mimetic gene networks and repression of non-mimetic gene networks, is essential for the switch from non-mimetic to mimetic pattern in mimetic female wings.

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.

Gypsy moth genome provides insights into flight capability and virus-host interactions

Since its accidental introduction to Massachusetts in the late 1800s, the European gypsy moth (EGM; Lymantria dispar dispar) has become a major defoliator in North American forests. However, in part because females are flightless, the spread of the EGM across the United States and Canada has been relatively slow over the past 150 years. In contrast, females of the Asian gypsy moth (AGM; Lymantria dispar asiatica) subspecies have fully developed wings and can fly, thereby posing a serious economic threat if populations are established in North America. To explore the genetic determinants of these phenotypic differences, we sequenced and annotated a draft genome of L. dispar and used it to identify genetic variation between EGM and AGM populations. The 865-Mb gypsy moth genome is the largest Lepidoptera genome sequenced to date and encodes ∼13,300 proteins. Gene ontology analyses of EGM and AGM samples revealed divergence between these populations in genes enriched for several gene ontology categories related to muscle adaptation, chemosensory communication, detoxification of food plant foliage, and immunity. These genetic differences likely contribute to variations in flight ability, chemical sensing, and pathogen interactions among EGM and AGM populations. Finally, we use our new genomic and transcriptomic tools to provide insights into genome-wide gene-expression changes of the gypsy moth after viral infection. Characterizing the immunological response of gypsy moths to virus infection may aid in the improvement of virus-based bioinsecticides currently used to control larval populations.

Lepidoptera genomes: current knowledge, gaps and future directions

Butterflies and moths (Lepidoptera) are one of the most ecologically diverse and speciose insect orders. With recent advances in genomics, new Lepidoptera genomes are regularly being sequenced, and many of them are playing principal roles in genomics studies, particularly in the fields of phylo-genomics and functional genomics. Thus far, assembled genomes are only available for <10 of the 43 Lepidoptera superfamilies. Nearly all are model species, found in the speciose clade Ditrysia. Community support for Lepidoptera genomics is growing with successful management and dissemination of data and analytical tools in centralized databases. With genomic studies quickly becoming integrated with ecological and evolutionary research, the Lepidoptera community will unquestionably benefit from new high-quality reference genomes that are more evenly distributed throughout the order.

Toll ligand Spätzle3 controls melanization in the stripe pattern formation in caterpillars

A stripe pattern is an aposematic or camouflage coloration often observed among various caterpillars. However, how this ecologically important pattern is formed is largely unknown. The silkworm dominant mutant Zebra (Ze) has a black stripe in the anterior margin of each dorsal segment. Here, fine linkage mapping of 3,135 larvae revealed a 63-kbp region responsible for the Ze locus, which contained three candidate genes, including the Toll ligand gene spätzle3 (spz-3). Both electroporation-mediated ectopic expression and RNAi analyses showed that, among candidate genes, only processed spz-3 induced melanin pigmentation and that Toll-8 was the candidate receptor gene of spz-3 This Toll ligand/receptor set is also involved in melanization of other mutant Striped (p^S ), which has broader stripes. Additional knockdown of 5 other spz family and 10 Toll-related genes caused no drastic change in the pigmentation of either mutant, suggesting that only spz-3/Toll-8 is mainly involved in the melanization process rather than pattern formation. The downstream pigmentation gene yellow was specifically up-regulated in the striped region of the Ze mutant, but spz-3 showed no such region-specific expression. Toll signaling pathways are known to be involved in innate immunity, dorsoventral axis formation, and neurotrophic functions. This study provides direct evidence that a Toll signaling pathway is co-opted to control the melanization process and adaptive striped pattern formation in caterpillars.

Developmental and evolutionary mechanisms shaping butterfly eyespots

Butterfly eyespots are visually compelling models to study the reciprocal interactions between evolutionary and developmental processes that shape phenotypic variation. They are evolutionarily diversified, ecologically relevant, and developmentally tractable, and have made key contributions to linking genotype, development, phenotype and fitness. Advances in the availability of analytical tools (e.g. gene editing and visualization techniques) and resources (e.g. genomic and transcriptomic data) are boosting the detailed dissection of the mechanisms underlying eyespot development and evolution. Here, we review current knowledge on the ecology, development, and evolution of butterfly eyespots, with focus on recent advances. We also highlight a number of unsolved mysteries in our understanding of the patterns and processes underlying the diversification of these structures.