Batesian mimicry protects animals from predators through resemblance with distasteful models in shape, color pattern, or behavior. To elucidate the wing coloration mechanisms involved in the mimicry, we investigated chemical composition and gene expression of the pale yellow and red pigments of a swallowtail butterfly, Papilio polytes, whose females mimic the unpalatable butterfly Pachliopta aristolochiae. Using LC/MS, we showed that the pale yellow wing regions in non-mimetic females consist of kynurenine and N-β-alanyldopamine (NBAD). Moreover, qRT-PCR showed that kynurenine/NBAD biosynthetic genes were upregulated in these regions in non-mimetic females. However, these pigments were absent in mimetic females. RNA-sequencing showed that kynurenine/NBAD synthesis and Toll signaling genes were upregulated in the red spots specific to mimetic female wings. These results demonstrated that drastic changes in gene networks in the red and pale yellow regions can switch wing color patterns between non-mimetic and mimetic females of P. polytes.
This paper presents the first evidence of tetrachromacy among invertebrates. The Japanese yellow swallowtail butterfly, Papilio xuthus, uses colour vision when foraging. The retina of Papilio is furnished with eight varieties of spectral receptors of six classes that are the ultraviolet (UV), violet, blue (narrow-band and wide-band), green (single-peaked and double-peaked), red and broad-band classes. We investigated whether all of the spectral receptors are involved in colour vision by measuring the wavelength discrimination ability of foraging Papilio. We trained Papilio to take nectar while seeing a certain monochromatic light. We then let the trained Papilio choose between two lights of different wavelengths and determined the minimum discriminable wavelength difference Δλ. The Δλ function of Papilio has three minima at approximately 430, 480 and 560 nm, where the Δλ values approximately 1 nm. This is the smallest value found for wavelength discrimination so far, including that of humans. The profile of the Δλ function of Papilio can be best reproduced by postulating that the UV, blue (narrow-band and wide-band), green (double-peaked) and red classes are involved in foraging. Papilio colour vision is therefore tetrachromatic.
colour vision; wavelength discrimination; spectral receptor; compound eye; ommatidium
Wing pattern evolution in Heliconius butterflies provides some of the most striking examples of adaptation by natural selection. The genes controlling pattern variation are classic examples of Mendelian loci of large effect, where allelic variation causes large and discrete phenotypic changes and is responsible for both convergent and highly divergent wing pattern evolution across the genus. We characterize nucleotide variation, genotype-by-phenotype associations, linkage disequilibrium (LD), and candidate gene expression patterns across two unlinked genomic intervals that control yellow and red wing pattern variation among mimetic forms of Heliconius erato. Despite very strong natural selection on color pattern, we see neither a strong reduction in genetic diversity nor evidence for extended LD across either patterning interval. This observation highlights the extent that recombination can erase the signature of selection in natural populations and is consistent with the hypothesis that either the adaptive radiation or the alleles controlling it are quite old. However, across both patterning intervals we identified SNPs clustered in several coding regions that were strongly associated with color pattern phenotype. Interestingly, coding regions with associated SNPs were widely separated, suggesting that color pattern alleles may be composed of multiple functional sites, conforming to previous descriptions of these loci as “supergenes.” Examination of gene expression levels of genes flanking these regions in both H. erato and its co-mimic, H. melpomene, implicate a gene with high sequence similarity to a kinesin as playing a key role in modulating pattern and provides convincing evidence for parallel changes in gene regulation across co-mimetic lineages. The complex genetic architecture at these color pattern loci stands in marked contrast to the single casual mutations often identified in genetic studies of adaptation, but may be more indicative of the type of genetic changes responsible for much of the adaptive variation found in natural populations.
Identifying the genetic changes responsible for beneficial variation is essential for understanding how organisms adapt. Here, we use a combination of mapping, population genetic analysis, and gene expression studies to identify the genomic regions responsible for phenotypic evolution in the Neotropical butterfly Heliconius erato. H. erato, together with its co-mimic H. melpomene, have undergone parallel and concordant radiations in their warningly colored wing patterns across Central and South America. The “genes” underlying the H. erato color pattern radiation are classic examples of Mendelian loci of large effect and are under strong natural selection. Nonetheless, we do not see a clear molecular signal of recent natural selection, suggesting that the H. erato color pattern radiation, or the alleles that underlie it, may be quite old. Moreover, rather than being single locus, the genetic patterns suggest that multiple, widely dispersed loci may underlie pattern variation in H. erato. One of these loci, a kinesin gene, shows parallel expression differences between races during wing pattern formation in both H. erato and H. melpomene, suggesting that it plays an important role in pattern variation. High rates of recombination within naturally occurring H. erato hybrid zones mean that finer genetic dissection will allow us to localize causative sites and better understand the history and molecular basis of this extraordinary adaptive radiation.
This study focuses on the sense of brightness in the foraging Japanese yellow swallowtail butterfly, Papilio xuthus. We presented two red discs of different intensity on a grey background to butterflies, and trained them to select one of the discs. They were successfully trained to select either a high intensity or a low intensity disc. The trained butterflies were tested on their ability to perceive brightness in two different protocols: (i) two orange discs of different intensity presented on the same intensity grey background and (ii) two orange discs of the same intensity separately presented on a grey background that was either higher or lower in intensity than the training background. The butterflies trained to high intensity red selected the orange disc of high intensity in protocol 1, and the disc on the background of low intensity grey in protocol 2. We obtained similar results in another set of experiments with purple discs instead of orange discs. The choices of the butterflies trained to low intensity red were opposite to those just described. Taken together, we conclude that Papilio has the ability to learn brightness and darkness of targets independent of colour, and that they have the so-called simultaneous brightness contrast.
vision; insect; compound eye; neuroethology
The insect cuticle is composed of various proteins and formed during the molt under hormonal regulation, although its precise composition and formation mechanism are largely unknown. The exhaustive catalogue of genes expressed in epidermis at the molt constitutes a massive amount of information from which to draw a complete picture of the molt and cuticle formation in insects. Therefore, we have catalogued a library of full-length cDNAs (designated epM) from epidermal cells during the last larval molt of Bombyx mori.
Of the 10,368 sequences in the library, we isolated 6,653 usable expressed sequence tags (ESTs), which were categorized into 1,451 nonredundant gene clusters. Seventy-one clusters were considered to be isoforms or premature forms of other clusters. Therefore, we have identified 1,380 putative genes. Of the 6,653 expressed sequences, 48% were derived from 92 cuticular protein genes (RR-1, 24; RR-2, 17; glycine-rich, 29; other classes, 22). A comparison of epM with another epidermal EST data set, epV3 (feeding stage: fifth instar, day 3), showed marked differences in cuticular protein gene. Various types of cuticular proteins are expressed in epM but virtually only RR-1 proteins were expressed in epV3. Cuticular protein genes expressed specifically in epidermis, with several types of expression patterns during the molt, suggest different types of responses to the ecdysteroid pulse. Compared with other Bombyx EST libraries, 13 genes were preferentially included in epM data set. We isolated 290 genes for proteins other than cuticular proteins, whose amino acid sequences retain putative signal peptides, suggesting that they play some role in cuticle formation or in other molting events. Several gene groups were also included in this data set: hormone metabolism, P450, modifier of cuticular protein structure, small-ligand-binding protein, transcription factor, and pigmentation genes.
We have identified 1,380 genes in epM data set and 13 preferentially expressed genes in epidermis at the molt. The comparison of the epM and other EST libraries clarified the totally different gene expression patterns in epidermis between the molting and feeding stages and many novel tissue- and stage-specifically expressed epidermal genes. These data should further our understanding of cuticle formation and the insect molt.
Hybrid speciation appears to be rare in animals, yet characterization of possible examples offers to shed light on the genomic consequences of this unique phenomenon, as well as more general processes such as the role of adaptation in speciation. Here, we first generate transcriptome assemblies for a putative hybrid butterfly species, Papilio appalachiensis, its parental species, P. glaucus and P. canadensis, and an outgroup, P. polytes. Then, we use these data to infer genome-wide patterns of introgression and genomic mosaicism using both phylogenetic and population genetic approaches. Our results reveal that there is little genetic divergence among all three of the focal species, but the subset of gene trees that strongly support a specific tree topology suggest widespread sharing of genetic variation between P. appalachiensis and both parental species, likely as a result of hybrid speciation. We also find evidence for substantial shared genetic variation between P. glaucus and P. canadensis, which may be due to gene flow or ancestral variation. Consistent with previous work, we show that P. applachiensis is more similar to P. canadensis at Z-linked genes and more similar to P. glaucus at mitochondrial genes. We also identify a variety of targets of adaptive evolution, which appear to be enriched for traits that are likely to be important in the evolution of this butterfly system, such as pigmentation, hormone sensitivity, developmental processes, and cuticle formation. Overall, our results provide a genome-wide portrait of divergence and introgression associated with adaptation and speciation in an iconic butterfly radiation.
transcriptome; adaptation; hybrid speciation; introgression; Papilio
Several recent studies have demonstrated the use of Roche 454 sequencing technology for de novo transcriptome analysis. Low error rates and high coverage also allow for effective SNP discovery and genetic diversity estimates. However, genetically diverse datasets, such as those sourced from natural populations, pose challenges for assembly programs and subsequent analysis. Further, estimating the effectiveness of transcript discovery using Roche 454 transcriptome data is still a difficult task.
Using the Roche 454 FLX Titanium platform, we sequenced and assembled larval transcriptomes for two butterfly species: the Propertius duskywing, Erynnis propertius (Lepidoptera: Hesperiidae) and the Anise swallowtail, Papilio zelicaon (Lepidoptera: Papilionidae). The Expressed Sequence Tags (ESTs) generated represent a diverse sample drawn from multiple populations, developmental stages, and stress treatments.
Despite this diversity, > 95% of the ESTs assembled into long (> 714 bp on average) and highly covered (> 9.6× on average) contigs. To estimate the effectiveness of transcript discovery, we compared the number of bases in the hit region of unigenes (contigs and singletons) to the length of the best match silkworm (Bombyx mori) protein--this "ortholog hit ratio" gives a close estimate on the amount of the transcript discovered relative to a model lepidopteran genome. For each species, we tested two assembly programs and two parameter sets; although CAP3 is commonly used for such data, the assemblies produced by Celera Assembler with modified parameters were chosen over those produced by CAP3 based on contig and singleton counts as well as ortholog hit ratio analysis. In the final assemblies, 1,413 E. propertius and 1,940 P. zelicaon unigenes had a ratio > 0.8; 2,866 E. propertius and 4,015 P. zelicaon unigenes had a ratio > 0.5.
Ultimately, these assemblies and SNP data will be used to generate microarrays for ecoinformatics examining climate change tolerance of different natural populations. These studies will benefit from high quality assemblies with few singletons (less than 26% of bases for each assembled transcriptome are present in unassembled singleton ESTs) and effective transcript discovery (over 6,500 of our putative orthologs cover at least 50% of the corresponding model silkworm gene).
The origin and modification of novel traits are important aspects of biological diversification. Studies combining concepts and approaches of developmental genetics and evolutionary biology have uncovered many examples of the recruitment, or co-option, of genes conserved across lineages for the formation of novel, lineage-restricted traits. However, little is known about the evolutionary history of the recruitment of those genes, and of the relationship between them -for example, whether the co-option involves whole or parts of existing networks, or whether it occurs by redeployment of individual genes with de novo rewiring. We use a model novel trait, color pattern elements on butterfly wings called eyespots, to explore these questions. Eyespots have greatly diversified under natural and sexual selection, and their formation involves genetic circuitries shared across insects.
We investigated the evolutionary history of the recruitment and co-recruitment of four conserved transcription regulators to the larval wing disc region where circular pattern elements develop. The co-localization of Antennapedia, Notch, Distal-less, and Spalt with presumptive (eye)spot organizers was examined in 13 butterfly species, providing the largest comparative dataset available for the system. We found variation between families, between subfamilies, and between tribes. Phylogenetic reconstructions by parsimony and maximum likelihood methods revealed an unambiguous evolutionary history only for Antennapedia, with a resolved single origin of eyespot-associated expression, and many homoplastic events for Notch, Distal-less, and Spalt. The flexibility in the (co-)recruitment of the targeted genes includes cases where different gene combinations are associated with morphologically similar eyespots, as well as cases where identical protein combinations are associated with very different phenotypes.
The evolutionary history of gene (co-)recruitment is consistent with both divergence from a recruited putative ancestral network, and with independent co-option of individual genes. The diversity in the combinations of genes expressed in association with eyespot formation does not parallel diversity in characteristics of the adult phenotype. We discuss these results in the context of inferring homology. Our study underscores the importance of widening the representation of phylogenetic, morphological, and genetic diversity in order to establish general principles about the mechanisms behind the evolution of novel traits.
The swallowtail butterfly, Papilio dardanus, is an iconic example of a polymorphic Batesian mimic. The expression of various female-limited colour forms is thought to be controlled by a single autosomal locus, termed H, whose function in determining the wing pattern remains elusive. As a step towards the physical mapping of H, we established a set of 272 polymorphic amplified fragment length polymorphism (AFLP) markers (EcoRI-MseI). Segregation patterns in a ‘female-informative’ brood (exploiting the absence of crossing over in female Lepidoptera) mapped these AFLPs to 30 linkage groups (putative chromosomes). The difference between the hippocoon and cenea female forms segregating in this family resides on a single one of these linkage groups, defined by 14 AFLPs. In a ‘male-informative’ cross (markers segregating within a linkage group), a pair of AFLPs co-segregated closely with the two female forms, except in four recombinants out of 19 female offspring. Linkage with these AFLP markers using four further female-informative families demonstrated that the genetic factor determining other morphs (poultoni, lamborni and trimeni) also maps to this same linkage group. The candidate gene invected, obtained in a screen for co-segregation of developmental genes with the colour forms, resides in a 13.9 cM interval flanked by the two AFLP markers. In the male-informative family invected co-segregated perfectly with the hippocoon/cenea factor, despite the four crossovers with the AFLPs. These findings make invected, and possibly its closely linked paralogue engrailed, strong candidates for H. This is supported by their known role in eyespot specification in nymphalid butterfly wings.
evolutionary genetics; mimicry; phenotype–genotype association; engrailed; candidate genes; amplified fragment length polymorphism
Butterfly and moth eyespots can share a similar appearance, involving multiple concentric rings of colored scales, but usually occuring in non-homologous positions on the wing. Within the butterflies, on the other hand, spots that share the same homologous position may not share the concentric ring structure; and, in butterfly species that have eyespots with concentric rings, ectopic eyespots with a similar ring structure can be induced by means of a simple epidermal wound. The extent to which all these eyespots, natural or induced, share similar genes and developmental mechanisms is investigated here by means of protein in-situ localizations in selected butterfly and moth species. In addition to looking at some of the transcription factors previously identified as being involved in eyespot formation, we also tested the involvement of candidate genes from the Wingless and TGF-β signaling pathways as putative morphogens for eyespot development.
Saturniid moth and nymphalid butterfly eyespots with concentric rings of color express at least two transcription factors, Distal-less and Engrailed, in the center of the future pattern. Nymphalid eyespots centers also express the ligand Wingless and an activated signal transducer, a phosphorylated Smad protein, but neither these proteins nor the previous two proteins are found in pierid spot centers, which consist of a single patch of color. Both butterfly wing patterns, however, express a third transcription factor, Spalt, a portion of whose expression domain maps to the black scales on the adult wing. Wounding a nymphalid wing, on the other hand, leads to upregulation of Distal-less, engrailed and spalt in subsets of cells around the wounding site, mimicking concentric eyespot development.
Wingless and TGF-β ligands are both candidate morphogens involved in nymphalid butterfly eyespot formation. These eyespots, as well as saturniid moth eyespots with concentric circles, share two genes that are associated with the differentiation of the signaling cells in nymphalid eyespots. This commonality suggests that they may be produced via the same developmental mechanism despite their non-homologous location. By contrast, pierid butterfly spots of a single color share some of the same genes but appear to be produced by a different mechanism. Eyespots with concentric rings may have co-opted a wound healing genetic network during their evolution.
Detoxification of host plant defensive compounds by larval Lepidoptera is mediated by cytochrome P450 monooxygenases (P450s) such as CYP6B1, which is expressed in Papilio polyxenes (black swallowtail) larvae in response to xanthotoxin, a linear furanocoumarin. Baculovirus-mediated expression of two cloned CYP6B1 cDNAs in lepidopteran cell lines has demonstrated that CYP6B1 isozymes primarily metabolize the linear furanocoumarins, xanthotoxin and bergapten, and not angular furanocoumarins. To characterize the regulatory features of the CYP6B1 transcription unit, we have isolated the first full-length CYP6B1v3 genomic DNA clone from P. polyxenes. The open reading frame of this gene is interrupted by a single intron and is virtually identical to the previously characterized CYP6B1 cDNAs. Primer extension and ribonuclease protection analyses have localized the transcription initiation site to a point 28 nucleotides upstream from the AUG initiation codon. RNase protection analyses on RNA from larvae induced by linear and angular furanocoumarins indicate that transcription of the CYP6B1 gene is induced in insects significantly in response to xanthotoxin and only slightly in response to bergapten. Angular furanocoumarins, such as angelicin, which are not appreciably metabolized by the CYP6B1 gene product, do not significantly induce transcription of this gene. We conclude that this P450 gene is transcriptionally regulated in vivo by at least one of the substrates which the encoded protein metabolizes. Transient expression of CAT fusion constructs in transfected Sf9 lepidopteran cells demonstrates that nucleotides -1 to -838 upstream from the CYP6B1v3 transcription initiation site retain basal and xanthotoxin-inducible transcriptional activities in this heterologous cell line. These data clearly indicate that P. polyxenes has adapted to the presence of furanocoumarins in its host plants by evolving P450 isozymes and regulatory cascades which respond to specific toxins.
Theoretical and empirical observations generally support Darwin's view that sexual dimorphism evolves due to sexual selection on, and deviation in, exaggerated male traits. Wallace presented a radical alternative, which is largely untested, that sexual dimorphism results from naturally selected deviation in protective female coloration. This leads to the prediction that deviation in female rather than male phenotype causes sexual dimorphism. Here I test Wallace's model of sexual dimorphism by tracing the evolutionary history of Batesian mimicry—an example of naturally selected protective coloration—on a molecular phylogeny of Papilio butterflies. I show that sexual dimorphism in Papilio is significantly correlated with both female-limited Batesian mimicry, where females are mimetic and males are non-mimetic, and with the deviation of female wing colour patterns from the ancestral patterns conserved in males. Thus, Wallace's model largely explains sexual dimorphism in Papilio. This finding, along with indirect support from recent studies on birds and lizards, suggests that Wallace's model may be more widely useful in explaining sexual dimorphism. These results also highlight the contribution of naturally selected female traits in driving phenotypic divergence between species, instead of merely facilitating the divergence in male sexual traits as described by Darwin's model.
Batesian mimicry; polymorphism; female-limited mimicry; directional selection; stabilizing sexual selection; convergence
Melanism, the overall darkening of the body, is a widespread form of animal adaptation to particular environments, and includes bookcase examples of evolution by natural selection, such as industrial melanism in the peppered moth. The major components of the melanin biosynthesis pathway have been characterized in model insects, but little is known about the genetic basis of life-stage specific melanism such as cases described in some lepidopteran species. Here, we investigate two melanic mutations of Bicyclus anynana butterflies, called Chocolate and melanine, that exclusively affect pigmentation of the larval and adult stages, respectively. Our analysis of Mendelian segregation patterns reveals that the larval and adult melanic phenotypes are due to alleles at different, independently segregating loci. Our linkage mapping analysis excludes the pigmentation candidate gene black as the melanine locus, and implicates a gene encoding a putative pyridoxal phosphate-dependant cysteine sulfinic acid decarboxylase as the Chocolate locus. We show variation in coding sequence and in expression levels for this candidate larval melanism locus. This is the first study that suggests a biological function for this gene in insects. Our findings open up exciting opportunities to study the role of this locus in the evolution of adaptive variation in pigmentation, and the uncoupling of regulation of pigment biosynthesis across developmental stages with different ecologies and pressures on body coloration.
evo-devo; pigmentation; lepidoptera; melanogenesis; PLP-dependent enzyme; black
Many animals reduce the risk of being attacked by a predator through crypsis, masquerade or, alternatively, by advertising unprofitability by means of aposematic signalling. Behavioural attributes in prey employed after discovery, however, signify the importance of also having an effective secondary defence if a predator uncovers, or is immune to, the prey’s primary defence. In butterflies, as in most animals, secondary defence generally consists of escape flights. However, some butterfly species have evolved other means of secondary defence such as deimatic displays/startle displays. The European swallowtail, Papilio machaon, employs what appears to be a startle display by exposing its brightly coloured dorsal wing surface upon disturbance and, if the disturbance continues, by intermittently protracting and relaxing its wing muscles generating a jerky motion of the wings. This display appears directed towards predators but whether it is effective in intimidating predators so that they refrain from attacks has never been tested experimentally.
In this study we staged encounters between a passerine predator, the great tit, Parus major, and live and dead swallowtail butterflies in a two-choice experiment. Results showed that the dead butterfly was virtually always attacked before the live butterfly, and that it took four times longer before a bird attacked the live butterfly. When the live butterfly was approached by a bird this generally elicited the butterfly’s startle display, which usually caused the approaching bird to flee. We also performed a palatability test of the butterflies and results show that the great tits seemed to find them palatable.
We conclude that the swallowtail’s startle display of conspicuous coloration and jerky movements is an efficient secondary defence against small passerines. We also discuss under what conditions predator-prey systems are likely to aid the evolution of deimatic behaviours in harmless and palatable prey.
Phenotypic plasticity in pupal colour occurs in three families of butterflies (the Nymphalidae, Papilionidae and Pieridae), typically in species whose pupation sites vary unpredictably in colour. In all species studied to date, larvae ready for pupation respond to environmental cues associated with the colour of their pupation sites and moult into cryptic light (yellow–green) or dark (brown–black) pupae. In nymphalids and pierids, pupal colour is controlled by a neuroendocrine factor, pupal melanization-reducing factor (PMRF), the release of which inhibits the melanization of the pupal cuticle resulting in light pupae. In contrast, the neuroendocrine factor controlling pupal colour in papilionid butterflies results in the production of brown pupae. PMRF was extracted from the ventral nerve chains of the peacock butterfly Inachis io (Nymphalidae) and black swallowtail butterfly Papilio polyxenes (Papilionidae). When injected into pre-pupae, the extracts resulted in yellow pupae in I. io but brown pupae in P. polyxenes. These results suggest that the same neuroendocrine factor controls the plasticity in pupal colour, but that plasticity in pupal colour in these species has evolved independently (convergently).
Heliconius butterfly wing pattern diversity offers a unique opportunity to investigate how natural genetic variation can drive the evolution of complex adaptive phenotypes. Positional cloning and candidate gene studies have identified a handful of regulatory and pigmentation genes implicated in Heliconius wing pattern variation, but little is known about the greater developmental networks within which these genes interact to pattern a wing. Here we took a large-scale transcriptomic approach to identify the network of genes involved in Heliconius wing pattern development and variation. This included applying over 140 transcriptome microarrays to assay gene expression in dissected wing pattern elements across a range of developmental stages and wing pattern morphs of Heliconius erato.
We identified a number of putative early prepattern genes with color-pattern related expression domains. We also identified 51 genes differentially expressed in association with natural color pattern variation. Of these, the previously identified color pattern “switch gene” optix was recovered as the first transcript to show color-specific differential expression. Most differentially expressed genes were transcribed late in pupal development and have roles in cuticle formation or pigment synthesis. These include previously undescribed transporter genes associated with ommochrome pigmentation. Furthermore, we observed upregulation of melanin-repressing genes such as ebony and Dat1 in non-melanic patterns.
This study identifies many new genes implicated in butterfly wing pattern development and provides a glimpse into the number and types of genes affected by variation in genes that drive color pattern evolution.
Heliconius; Adaptation; Pigmentation; Ommochrome; Melanin; Genomics; Evo-devo
Little is known about the genetic basis of ecologically important morphological variation such as the diverse color patterns of mammals. Here we identify genetic changes contributing to an adaptive difference in color pattern between two subspecies of oldfield mice (Peromyscus polionotus). One mainland subspecies has a cryptic dark brown dorsal coat, while a younger beach-dwelling subspecies has a lighter coat produced by natural selection for camouflage on pale coastal sand dunes. Using genome-wide linkage mapping, we identified three chromosomal regions (two of major and one of minor effect) associated with differences in pigmentation traits. Two candidate genes, the melanocortin-1 receptor (Mc1r) and its antagonist, the Agouti signaling protein (Agouti), map to independent regions that together are responsible for most of the difference in pigmentation between subspecies. A derived mutation in the coding region of Mc1r, rather than change in its expression level, contributes to light pigmentation. Conversely, beach mice have a derived increase in Agouti mRNA expression but no changes in protein sequence. These two genes also interact epistatically: the phenotypic effects of Mc1r are visible only in genetic backgrounds containing the derived Agouti allele. These results demonstrate that cryptic coloration can be based largely on a few interacting genes of major effect.
The tremendous amount of variation in color patterns among organisms helps individuals survive and reproduce in the wild, yet we know surprisingly little about the genes that produce these adaptive patterns. Here we used a genomic analysis to uncover the molecular basis of a pale color pattern that camouflages beach mice inhabiting the sandy dunes of Florida's coast from predators. We identified two pigmentation genes, the melanocortin-1 receptor (Mc1r) and its ligand, the agouti signaling protein (Agouti), which together produce a light color pattern. We show that this light pigmentation results partly from a single amino acid mutation in Mc1r, which reduces the activity of the receptor but does not affect the gene's expression level, and partly from the derived Agouti allele, which shows no change in protein sequence but does exhibit an increase in mRNA expression. We also show that these two genes do not act additively to produce pale color; rather, the derived Agouti allele must be present to see any effect of Mc1r on pigmentation. Thus, the light color pattern of beach mice largely results from the physical interaction between a structural change in a receptor (reducing Mc1r activity) and a regulatory change in the receptor's antagonist (increasing Agouti expression).
Species of oldfield mice have coat colors adapted for their environment. By using genome-wide linkage mapping, the authors show that three chromosomal regions are associated with differences in pigmentation traits.
The African Mocker Swallowtail, Papilio dardanus, is a textbook example in evolutionary genetics. Classical breeding experiments have shown that wing pattern variation in this polymorphic Batesian mimic is determined by the polyallelic H locus that controls a set of distinct mimetic phenotypes. Using bacterial artificial chromosome (BAC) sequencing, recombination analyses and comparative genomics, we show that H co-segregates with an interval of less than 500 kb that is collinear with two other Lepidoptera genomes and contains 24 genes, including the transcription factor genes engrailed (en) and invected (inv). H is located in a region of conserved gene order, which argues against any role for genomic translocations in the evolution of a hypothesized multi-gene mimicry locus. Natural populations of P. dardanus show significant associations of specific morphs with single nucleotide polymorphisms (SNPs), centred on en. In addition, SNP variation in the H region reveals evidence of non-neutral molecular evolution in the en gene alone. We find evidence for a duplication potentially driving physical constraints on recombination in the lamborni morph. Absence of perfect linkage disequilibrium between different genes in the other morphs suggests that H is limited to nucleotide positions in the regulatory and coding regions of en. Our results therefore support the hypothesis that a single gene underlies wing pattern variation in P. dardanus.
Batesian mimicry; polymorphism; Lepidoptera; supergene; genotype–phenotype associations
The butterfly Papilio dardanus is well known for the spectacular phenotypic polymorphism in the female of the species. We show that numerical simulations of a reaction diffusion model on a geometrically accurate wing domain produce spatial patterns that are consistent with many of those observed on the butterfly. Our results suggest that the wing coloration is due to a simple underlying stripe-like pattern of some pigment-inducing morphogen. We focus on the effect of key factors such as parameter values for mode selection, threshold values which determine colour, wing shape and boundary conditions. The generality of our approach should allow us to investigate other butterfly species. The relationship between these key factors and gene activities is discussed in the context of recent biological advances.
Lepidopterans (butterflies and moths) are a rich and diverse order of insects, which, despite their economic impact and unusual biological properties, are relatively underrepresented in terms of genomic resources. The genome of the silkworm Bombyx mori has been fully sequenced, but comparative lepidopteran genomics has been hampered by the scarcity of information for other species. This is especially striking for butterflies, even though they have diverse and derived phenotypes (such as color vision and wing color patterns) and are considered prime models for the evolutionary and developmental analysis of ecologically relevant, complex traits. We focus on Bicyclus anynana butterflies, a laboratory system for studying the diversification of novelties and serially repeated traits. With a panel of 12 small families and a biphasic mapping approach, we first assigned 508 expressed genes to segregation groups and then ordered 297 of them within individual linkage groups. We also coarsely mapped seven color pattern loci. This is the richest gene-based map available for any butterfly species and allowed for a broad-coverage analysis of synteny with the lepidopteran reference genome. Based on 462 pairs of mapped orthologous markers in Bi. anynana and Bo. mori, we observed strong conservation of gene assignment to chromosomes, but also evidence for numerous large- and small-scale chromosomal rearrangements. With gene collections growing for a variety of target organisms, the ability to place those genes in their proper genomic context is paramount. Methods to map expressed genes and to compare maps with relevant model systems are crucial to extend genomic-level analysis outside classical model species. Maps with gene-based markers are useful for comparative genomics and to resolve mapped genomic regions to a tractable number of candidate genes, especially if there is synteny with related model species. This is discussed in relation to the identification of the loci contributing to color pattern evolution in butterflies.
Butterflies and moths (called the Lepidoptera) are a large and diverse group of insects that has long captured the attention of biologists and laymen. The colorful patterns on the wings of butterflies, in particular, offer an ideal system to investigate which genes and developmental mechanisms contribute to evolutionary diversification. Genetic analyses that try to find the position of genes along chromosomes are invaluable for such efforts, also because they allow researchers to compare chromosome content between species. Here, we report on a study which built a gene-based map for the chromosomes of a butterfly “lab rat” and identified chromosomes carrying color pattern genes. We compare our map to that of the reference lepidopteran species, the silkworm. Despite these species having diverged some 100 million years ago, there is much conservation in terms of which genes are found together in chromosomes and even how genes are ordered within chromosomes. However, because we were able to compare positioning of many more genes than had ever been reported before for this group, we also found evidence of several large- and small-scale chromosomal rearrangements. We discuss the advantages of gene-based maps in understanding the genetic basis of color pattern evolution.
Butterfly wing color patterns are a key model for integrating evolutionary developmental biology and the study of adaptive morphological evolution. Yet, despite the biological, economical and educational value of butterflies they are still relatively under-represented in terms of available genomic resources. Here, we describe an Expression Sequence Tag (EST) project for Bicyclus anynana that has identified the largest available collection to date of expressed genes for any butterfly.
By targeting cDNAs from developing wings at the stages when pattern is specified, we biased gene discovery towards genes potentially involved in pattern formation. Assembly of 9,903 ESTs from a subtracted library allowed us to identify 4,251 genes of which 2,461 were annotated based on BLAST analyses against relevant gene collections. Gene prediction software identified 2,202 peptides, of which 215 longer than 100 amino acids had no homology to any known proteins and, thus, potentially represent novel or highly diverged butterfly genes. We combined gene and Single Nucleotide Polymorphism (SNP) identification by constructing cDNA libraries from pools of outbred individuals, and by sequencing clones from the 3' end to maximize alignment depth. Alignments of multi-member contigs allowed us to identify over 14,000 putative SNPs, with 316 genes having at least one high confidence double-hit SNP. We furthermore identified 320 microsatellites in transcribed genes that can potentially be used as genetic markers.
Our project was designed to combine gene and sequence polymorphism discovery and has generated the largest gene collection available for any butterfly and many potential markers in expressed genes. These resources will be invaluable for exploring the potential of B. anynana in particular, and butterflies in general, as models in ecological, evolutionary, and developmental genetics.
Geographical variation in the mimetic wing patterns of the butterfly Heliconius erato is a textbook example of adaptive polymorphism; however, little is known about how this variation is controlled developmentally. Using microarrays and qPCR, we identified and compared expression of candidate genes potentially involved with a red/yellow forewing band polymorphism in H. erato. We found that transcripts encoding the pigment synthesis enzymes cinnabar and vermilion showed pattern- and polymorphism-related expression patterns, respectively. cinnabar expression was associated with the forewing band regardless of pigment colour, providing the first gene expression pattern known to be correlated with a major Heliconius colour pattern. In contrast, vermilion expression changed spatially over time in red-banded butterflies, but was not expressed at detectable levels in yellow-banded butterflies, suggesting that regulation of this gene may be involved with the red/yellow polymorphism. Furthermore, we found that the yellow pigment, 3-hydroxykynurenine, is incorporated into wing scales from the haemolymph rather than being synthesized in situ. We propose that some aspects of Heliconius colour patterns are determined by spatio-temporal overlap of pigment gene transcription prepatterns and speculate that evolutionary changes in vermilion regulation may in part underlie an adaptive colour pattern polymorphism.
heliconius; evo-devo; cinnabar; vermillion; pigmentation; colour patterns
Hybrid speciation, or the formation of a daughter species due to interbreeding between two parental species, is a potentially important means of diversification, because it generates new forms from existing variation. However, factors responsible for the origin and maintenance of hybrid species are largely unknown. Here we show that the North American butterfly Papilio appalachiensis is a hybrid species, with genomic admixture from Papilio glaucus and Papilio canadensis. Papilio appalachiensis has a mosaic phenotype, which is hypothesized to be the result of combining sex-linked traits from P. glaucus and P. canadensis. We show that P. appalachiensis' Z-linked genes associated with a cooler thermal habitat were inherited from P. canadensis, whereas its W-linked mimicry and mitochondrial DNA were inherited from P. glaucus. Furthermore, genome-wide AFLP markers showed nearly equal contributions from each parental species in the origin of P. appalachiensis, indicating that it formed from a burst of hybridization between the parental species, with little subsequent backcrossing. However, analyses of genetic differentiation, clustering, and polymorphism based on molecular data also showed that P. appalachiensis is genetically distinct from both parental species. Population genetic simulations revealed P. appalachiensis to be much younger than the parental species, with unidirectional gene flow from P. glaucus and P. canadensis into P. appalachiensis. Finally, phylogenetic analyses, combined with ancestral state reconstruction, showed that the two traits that define P. appalachiensis' mosaic phenotype, obligatory pupal diapause and mimicry, evolved uniquely in P. canadensis and P. glaucus, respectively, and were then recombined through hybridization to form P. appalachiensis. These results suggest that natural selection and sex-linked traits may have played an important role in the origin and maintenance of P. appalachiensis as a hybrid species. In particular, ecological barriers associated with a steep thermal cline appear to maintain the distinct, mosaic genome of P. appalachiensis despite contact and occasional hybridization with both parental species.
Hybridization between two species may sometimes lead to the formation of a third species. While this phenomenon is of interest because it can generate new species from existing variation, it is rare in animals. Therefore, the factors responsible for the origin and maintenance of hybrid species are largely unknown. Here we show that the butterfly Papilio appalachiensis appears to be a hybrid species, which received nearly equal contributions to its genome from P. glaucus and P. canadensis. Two sets of traits define P. appalachiensis' hybrid phenotype: like P. canadensis it inhabits a cold habitat and has a single generation every year, and like P. glaucus it mimics a toxic butterfly and its females are dimorphic. The genes responsible for these traits are on two different sex chromosomes. Our genetic data show that P. appalachiensis inherited the sex chromosome associated with the cold habitat from P. canadensis, whereas it inherited the sex chromosome associated with mimicry and dimorphism from P. glaucus. Historical climate change may have facilitated the origin of P. appalachiensis, which our data suggest coincided with the last interglacial period in North America. This example shows the importance of sex-linked traits and a potential role for natural selection in the origin and maintenance of hybrid species.
The idea that an aposematic prey combines crypsis at a distance with conspicuousness close up was tested in an experiment using human subjects. We estimated detectability of the aposematic larva of the swallowtail butterfly, Papilio machaon, in two habitats, by presenting, on a touch screen, photographs taken at four different distances and measuring the time elapsed to discovery. The detectability of larvae in these images was compared with images that were manipulated, using existing colours either to increase or decrease conspicuousness. Detection time increased with distance for all colourations. However, at the closest distance, detection time was longer for the larvae manipulated to be more cryptic than for the natural and more conspicuous forms. This indicates that the natural colouration is not maximally cryptic at a short distance. Further, smaller increments in distance were needed to increase detection time for the natural than for the conspicuous larva. This indicates that the natural colouration is not maximally conspicuous at longer distances. Taken together, we present the first empirical support for the idea that some colour patterns may combine warning colouration at a close range with crypsis at a longer range. The implications of this result for the evolution of aposematism are discussed.
aposematism; crypsis; conspicuousness; predation; distance; background
Candidate genes for color pattern formation in butterfly wings have been known based on gene expression patterns since the 1990s, but their functions remain elusive due to a lack of a functional assay. Several methods of transferring and expressing a foreign gene in butterfly wings have been reported, but they have suffered from low success rates or low expression levels. Here, we developed a simple, practical method to efficiently deliver and express a foreign gene using baculovirus-mediated gene transfer in butterfly wings in vivo.
A recombinant baculovirus containing a gene for green fluorescent protein (GFP) was injected into pupae of the blue pansy butterfly Junonia orithya (Nymphalidae). GFP fluorescence was detected in the pupal wings and other body parts of the injected individuals three to five days post-injection at various degrees of fluorescence. We obtained a high GFP expression rate at relatively high virus titers, but it was associated with pupal death before color pattern formation in wings. To reduce the high mortality rate caused by the baculovirus treatment, we administered an anti-gp64 antibody, which was raised against baculovirus coat protein gp64, to infected pupae after the baculovirus injection. This treatment greatly reduced the mortality rate of the infected pupae. GFP fluorescence was observed in pupal and adult wings and other body parts of the antibody-treated individuals at various degrees of fluorescence. Importantly, we obtained completely developed wings with a normal color pattern, in which fluorescent signals originated directly from scales or the basal membrane after the removal of scales. GFP fluorescence in wing tissues spatially coincided with anti-GFP antibody staining, confirming that the fluorescent signals originated from the expressed GFP molecules.
Our baculovirus-mediated gene transfer system with an anti-gp64 antibody is reasonably efficient, and it can be an invaluable tool to transfer, express, and functionally examine foreign genes in butterfly wings and also in other non-model insect systems.
Butterfly wing; Gene transfer; Baculovirus vector; Green fluorescent protein; gp64; Virus infection; Antibody injection; Immunohistochemistry