Rule-breaking mimics: palatability of the butterflies Hypolimnas bolina and Hypolimnas misippus,a sister species pair

Sibling species are, by definition, closely related and Hypolimnas bolina and Hypolimnas misippus are good examples. It has been known for some years that H. misippus often breaks the rules of mimicry, being polymorphic in the female and retaining the mimetic patterns even where no models are present. Hypolimnas bolina is here shown to follow a similar course, with no models present in Madagascar. The toxicity of both butterflies seems usually to be largely dependent on the foodplant but this is irrelevant to mimicry in the absence of models. The evidence suggests that in both species mimicry at the present time is of little or no importance to the survival of these butterflies.     

Indian migrant butterflies displaced to Arabia by monsoon storm 'Aurora' in August 1983

Although many insect species are now thought to travel hundreds and even thousands of kilometres on the wind (Pedgley, 1982) only a few are butterflies. Three species for which there is reasonably convincing but circumstantial evidence are Danaus plexippus L. from the U.S.A. to the British Isles (Hurst, 1969), Hypolimnas bolina nerina (F.) from Australia to New Zealand (Tomlinson, 1973) and Nymphalis antiopa L. from northern Europe to the British Isles (Chalmers-Hunt, 1977). In this paper we describe the sudden appearance of Indian butterflies in Arabia and provide evidence that they were windborne across the Arabian Sea.     

Natural selection for rare and mimetic colour pattern combinations in wild populations of the diadem butterfly, Hypolimnas misippus L

Mark recapture and morph frequency data, gathered during a population irruption of Hypolimnas misippus in southern Ghana, provide evidence for apostatic and mimetic selection. During a period of low adult survival, both the recapture rate and the frequency of the commonest morph ( misippus ) were significantly reduced. Selection against this form increased phenotypic diversity and generated significant disequilibrium in the combinations of unlinked fore- and hindwing phenotypes. There was also evidence for selection against those forms (weak alcippoides ) which most closely resemble misippus . Other morphs, including both good mimics of Danaus chrysippus and rare non-mimics, showed no reductions in recapture rate during the period of low survival, but only the good mimics increased significantly in frequency. The results provide a predictive ecological model for density-dependent selection by predators which is consistent with field data from previous studies of H. misippus in Ghana and Tanzania. Their evolutionary implications are discussed, and it is suggested that anomalies in the mimicry of this species may be partly due to lack of predation when it is scarce.     

The genetics of the mimetic butterfly Hypolimnas bolina (L.).

Hypolimnas bolina is a Nymphalid butterfly having a west to east distribution from Madagascar to Easter Island, and a north to south one from Japan to Australasia. It is highly migratory in some areas. In much of the western part of its range the female is both monomorphic and a mimic of Euploea. Further east it is frequently polymorphic with the majority of the forms being non-mimetic. The polymorphism is sex-limited to the female and controlled by two unlinked loci, one with two allelomorphs, E and e, determining the extent of the dark pigmentation, the other with three allelomorphs, P, Pn and p, determining the presence and distribution of orange-brown. Only butterflies of the genotypes EEpp and to a lesser extent Eepp are satisfactory Batesian mimics of their Euploea models. The details of the mimetic pattern are under multifactorial control, following those of their local model, as is much of the variation within the non-mimetic forms, particularly with regard to the distribution of white and blue scaling.     

Wolbachia infection associated with all-female broods in Hypolimnas bolina (Lepidoptera: Nymphalidae): evidence for horizontal transmission of a butterfly male killer

Inherited bacteria that kill male hosts during embryogenesis infect a wide range of insect species. In order to ascertain if there are patterns to host infection, with particular male killing bacteria specialising on particular taxa, we investigated the male killing trait in the butterfly Hypolimnas bolina. All-female broods were first reported in this species in the 1920s. Investigation of this system in the Fiji Islands revealed the causal agent of sex ratio distortion in H. bolina to be a male killing Wolbachia bacterium. This bacterium is identical in wsp and ftsZ sequence to a male killer in the butterfly Acraea encedon in Tanzania, suggesting it has moved between host species, yet retained its phenotype. The prevalence of the Wolbachia was calculated for three different island groups of Fiji, and found to vary significantly across the country. Antibiotics failed to cure either the male killing trait or the Wolbachia infection. The implications of these results are discussed.     

The transient and steady state responses in oxygen consumption by tropical butterflies to temperature step transfer tests

A test is described which permits the determination of the respiratory rate of the insect to respond to, and recover from a short series of temperature changes. Both the transient and steady state respiratory responses were studied in the pupa, male and female of four species of tropical butterflies, Heliconius melpomene Linn., Papilio demoleus Wallace, Danaus chrysippus Linn., and Hypolimnas bolina Fabr., following a decrease and then an increase in environmental temperature.
Primary data consists of weight, oxygen consumption, and duration time of the transients; secondary data calculated from the above consisted of the Respiratory Change Ratio (RCR) and the % Recovery.
The RCR values were similar in pupa, male and female within a species, but showed significant differences between species, H. melpomene showing least change for a 10°C temperature change and H. bolina most. The ability to recover varied within and between species. In D. chrysippus there was no difference between pupa male and female; in H. bolina , pupa, male and female differed significantly in this. Heliconius melpomene showed very poor recuperative powers.
The most notable difference in the transients was the slow change following a drop in temperature compared with the instant increase to the steady state value following an increase in temperature. The duration of transients for body temperature were the same for both an increase and a decrease. The respiratory rate of the animal and its body temperature are clearly uncoupled during this period.
The transition of the respiratory rate associated with a decrease in temperature showed a smooth curve for the pupa but a momentary increase occurs in the adult.
A hypothesis is proposed to account for these results and their possible significance in the distribution and choice of a habitat by the butterflies discussed.     

Polymorphic mimicry, microhabitat use, and sex-specific behaviour

In order to assess the adaptive importance of microhabitat segregation for the maintenance of mimetic diversity, I explore how flight height varies between the sympatric forms of the polymorphic butterfly Heliconius numata and their respective models in the genus Melinaea. There is no evidence for vertical stratification of mimicry rings in these tiger-patterned butterflies, but males of H. numata tend to fly significantly higher than females and the Melinaea models. This difference in microhabitat preference likely results from females searching for host plants whereas males are patrolling for mates. I then present an extension of Muller's mimicry model for the case of partial behavioural or spatial segregation of sexes. The analysis suggests that sex-specific behaviours can make mimicry more beneficial, simply by reducing the effective population size participating in mimicry. The interaction between mimicry and sex-specific behaviours may therefore facilitate the evolution of polymorphism via enhanced, fine-scale local adaptation.     

Phenotypic plasticity in field populations of the tropical butterfly Hypolimnas bolina (L.) (Nymphalidae)

Phenotypic plasticity may enable organisms to maximize their fitness in seasonally variable environments. However, in butterflies, seasonal polyphenism is often striking but functionally obscure. This paper addresses the possible adaptive significance of phenotypic variation in the tropical butterfly Hypolimnas bolina (L.) (Nymphalidae). Plasticity in body size and wing coloration can be elicited in this species under laboratory conditions, however it is not known how this plasticity is expressed in the wild. Moreover, adult H. bolina spend the winter dry season in a reproductive diapause, which allows certain predictions regarding the occurrence of seasonal plasticity. Based on consideration of the requirements of diapausing and directly developing individuals, we predicted that if seasonal plasticity in phenotype were adaptive, then overwintering individuals should be larger and darker than their directly developing counterparts. This prediction was largely - although not entirely - fulfilled. Dry season butterflies were duller and darker than their wet season counterparts (this plasticity was superimposed on a genetic colour polymorphism), however size plasticity varied geographically. Dry season adults were consistently larger than wet season adults in the tropical north, but not in the south. We use these findings to discuss the possible adaptive significance of seasonal variation in the colour and size of this tropical butterfly.     

Adaptive introgression across species boundaries in Heliconius butterflies

It is widely documented that hybridisation occurs between many closely related species, but the importance of introgression in adaptive evolution remains unclear, especially in animals. Here, we have examined the role of introgressive hybridisation in transferring adaptations between mimetic Heliconius butterflies, taking advantage of the recent identification of a gene regulating red wing patterns in this genus. By sequencing regions both linked and unlinked to the red colour locus, we found a region that displays an almost perfect genotype by phenotype association across four species, H. melpomene, H. cydno, H. timareta, and H. heurippa. This particular segment is located 70 kb downstream of the red colour specification gene optix, and coalescent analysis indicates repeated introgression of adaptive alleles from H. melpomene into the H. cydno species clade. Our analytical methods complement recent genome scale data for the same region and suggest adaptive introgression has a crucial role in generating adaptive wing colour diversity in this group of butterflies.     

A set of microsatellite markers for Heliconius melpomene and closely related species

The butterflies in the genus Heliconius offer an exceptional opportunity for the study of the ecology and genetics of an adaptive radiation due to their extensive intra‐ and interspecific variation in wing colour patterns and mimetic associations. Here, we characterize 22 polymorphic microsatellite loci in Heliconius melpomene that have been shown to be useful for linkage mapping and population studies in this and other species. Levels of variation were high, although heterozygosity deficiencies were found in most loci, probably due to null alleles. The loci showed broad amplification success on six other species across the genus.     

Fighting back against male-killers

Male-killing endosymbionts create a genetic black hole into which host nuclear genes vanish. In a recent paper, Hornett et al. transferred male-killing Wolbachia between different strains of the butterfly Hypolimnas bolina through hybridization and backcrossing. Their results provide unambiguous evidence of genetic variation for resistance to male-killers. A possible consequence of such variation is that male-killing might appear and disappear quickly on an evolutionary timescale.     

Evolutionary responses by butterflies to patchy spatial distributions of resources in tropical environments

The greatest diversity of butterflies and their host plants occurs in tropical regions. Some groups of butterflies in the tropics exhibit monophagous feeding in the larval stage, exploiting only one family of plants; others are polyphagous, feeding on plants in two or more distinct families. The two major types of tropical habitats for butterflies, namely primary and secondary forests, offer very different evolutionary opportunities for the exploitation of plants as larval food. Butterflies are faced with the major logistical problem, as are many other herbivorous insects, of depositing eggs on the correct plant for successful larval feeding. This paper, using the concepts of phenotype set and spatial patchiness of resources, attemps to make some predictions as to the optimal phenotypic systems for monophagous and polyphagous feeding in tropical butterflies, as related to the spatial patchiness of larval host plants in primary and secondary forests. In addition to the secondary compound chemistry of larval host plants as playing a role in the evolution of monophagy and polyphagy, the assumption is made that the spatial patchiness of host plants within and among different families also acts as a major factor in determining optimal ranges of phenotypes for different patterns of larval feeding. Owing to the high spatial patchiness of primary forest species of canopy trees and vines, it is predicted that butterflies exploiting these will be mostly polyphagous, whereas secondary forests having stable formations of fewer plant species and larger patches of these plants, will have mostly monophagous species. Forest understories may have both monophagous and polyphagous species, depending upon the layer of forest and the general type of understory (i.e. palmaceous or dicotyledonous). Field data on some groups of butterflies from tropical America support these predictions. Polyphagous butterflies are predicted to possess a genetic system of mixed morphs with a population being polymorphic as a whole; monophagous butterflies are predicted to have individuals all more or less similar genetically, and with a high amount of genic variation within individuals. Other forms of monophagy may evolve in species that are essentially monomorphic but with various mechanisms (physiological, developmental, behavioral) of phenotypic flexibility at the individual level. Although the environment is essentially coarse-grained for larvae since most are sedentary and polymorphism is an optimal adaptive strategy, the oviposition strategy of the adult must also be considered and some situations (i.e. forest canopy) have resources (host plants) distributed in a fine-grained fashion. Other forms of limited polyphagy may result from monomorphic genetic systems in which there is considerable phenotypic flexibility.     

First chromosome scale genomes of ithomiine butterflies (Nymphalidae: Ithomiini): Comparative models for mimicry genetic studies

The ithomiine butterflies (Nymphalidae: Danainae) represent the largest known radiation of Müllerian mimetic butterflies. They dominate by number the mimetic butterfly communities, which include species such as the iconic neotropical Heliconius genus. Recent studies on the ecology and genetics of speciation in Ithomiini have suggested that sexual pheromones, colour pattern and perhaps hostplant could drive reproductive isolation. However, no reference genome was available for Ithomiini, which has hindered further exploration on the genetic architecture of these candidate traits, and more generally on the genomic patterns of divergence. Here, we generated high-quality, chromosome-scale genome assemblies for two Melinaea species, M. marsaeus and M. menophilus, and a draft genome of the species Ithomia salapia. We obtained genomes with a size ranging from 396 to 503 Mb across the three species and scaffold N50 of 40.5 and 23.2 Mb for the two chromosome-scale assemblies. Using collinearity analyses we identified massive rearrangements between the two closely related Melinaea species. An annotation of transposable elements and gene content was performed, as well as a specialist annotation to target chemosensory genes, which is crucial for host plant detection and mate recognition in mimetic species. A comparative genomic approach revealed independent gene expansions in ithomiines and particularly in gustatory receptor genes. These first three genomes of ithomiine mimetic butterflies constitute a valuable addition and a welcome comparison to existing biological models such as Heliconius, and will enable further understanding of the mechanisms of adaptation in butterflies.     

Dynamics of host plant use and species diversity in Polygonia butterflies (Nymphalidae)

The ability of insects to utilize different host plants has been suggested to be a dynamic and transient phase. During or after this phase, species can shift to novel host plants or respecialize on ancestral ones. Expanding the range of host plants might also be a factor leading to higher levels of net speciation rates. In this paper, we have studied the possible importance of host plant range for diversification in the genus Polygonia (Nymphalidae, Nymphalini). We have compared species richness between sistergroups in order to find out if there are any differences in number of species between clades including species that utilize only the ancestral host plants ('urticalean rosids') and their sisterclades with a broader (or in some cases potentially broader) host plant repertoire. Four comparisons could be made, and although these are not all phylogenetically or statistically independent, all showed clades including butterfly species using other or additional host plants than the urticalean rosids to be more species-rich than their sisterclade restricted to the ancestral host plants. These results are consistent with the theory that expansions in host plant range are involved in the process of diversification in butterflies and other phytophagous insects, in line with the general theory that plasticity may drive speciation.     

Discrimination of flying mimetic,passion-vine butterflies Heliconius

Wing-beat frequency and the degree of asymmetry in wing motion were more similar among mimics than among sister species of passion-vine butterflies in the genus Heliconius. Asymmetry in wing motion is not attributed to lift production, and serves as the first clear example of a mimetic behavioural signal for a flying organism. Because the similarities in wing motion are too subtle for humans to observe with the naked eye, they serve as a previously unexplored mimetic signal.     

Evolution of larval food plant associations in Delias Hübner butterflies (Lepidoptera: Pieridae)

A review of larval food plants of the genus Delias is presented. Larvae specialize primarily on aerial‐stem and root hemiparasites (“mistletoes”) in the order Santalales. Although butterfly food plant associations have been recorded for only a small proportion of the genus (28 species or ～11%, representing 12/24 species‐groups), available data suggest that the family Loranthaceae is used most frequently (77%), followed by the Santalaceae sensu stricto (14%) and Viscaceae (8%). With the possible exception of Euphorbiaceae (1%), almost all non‐mistletoe records are considered to be erroneous and, in most cases, probably represent the mistletoe host tree on which the larvae sometimes pupate. Of the eight major clades recognized in Delias, food plants have been recorded for six of these, although the majority of records (89%) are for three clades (hyparete, belladonna, nigrina). Optimization of the larval food plant data in the context of recent phylogenetic hypotheses for both butterflies and plants revealed little evidence of cospeciation at the higher systematic levels. The most parsimonious reconstruction was an origin of larval feeding on Loranthaceae, followed by at least six independent colonizations to Santalaceae + Viscaceae. In contrast to related pierids in the Aporiina associated with mistletoes in which further shifts from aerial‐stem mistletoes to distantly related plants (e.g. host trees parasitized by mistletoes) have facilitated differentiation at the generic level, there is no firm evidence to indicate that such secondary, monomorphic shifts have evolved in Delias. However, larvae of D. henningia (pasithoe group of belladonna clade) from Palawan and Luzon, the Philippines, appear to be polymorphic, feeding on both Loranthaceae and Euphorbiaceae.     

Once a Batesian mimic, not always a Batesian mimic: mimic reverts back to ancestral phenotype when the model is absent

Batesian mimics gain protection from predation through the evolution of physical similarities to a model species that possesses anti-predator defences. This protection should not be effective in the absence of the model since the predator does not identify the mimic as potentially dangerous and both the model and the mimic are highly conspicuous. Thus, Batesian mimics should probably encounter strong predation pressure outside the geographical range of the model species. There are several documented examples of Batesian mimics occurring in locations without their models, but the evolutionary responses remain largely unidentified. A mimetic species has four alternative evolutionary responses to the loss of model presence. If predation is weak, it could maintain its mimetic signal. If predation is intense, it is widely presumed the mimic will go extinct. However, the mimic could also evolve a new colour pattern to mimic another model species or it could revert back to its ancestral, less conspicuous phenotype. We used molecular phylogenetic approaches to reconstruct and test the evolution of mimicry in the North American admiral butterflies (Limenitis: Nymphalidae). We confirmed that the more cryptic white-banded form is the ancestral phenotype of North American admiral butterflies. However, one species, Limenitis arthemis, evolved the black pipevine swallowtail mimetic form but later reverted to the white-banded more cryptic ancestral form. This character reversion is strongly correlated with the geographical absence of the model species and its host plant, but not the host plant distribution of L. arthemis. Our results support the prediction that a Batesian mimic does not persist in locations without its model, but it does not go extinct either. The mimic can revert back to its ancestral, less conspicuous form and persist.     

All-female broods in Hypolimnas bolina (L.) A re-survey of West Fiji after 60 years

In the Xymphalid butterfly Hypolimnas bolina (L.) H. W. Simmonds in 1921 demonstrated, by breeding experiments, the occurrence on the islands of West Fiji of all-female broods, and he compared their frequency with that of normal bisexual broods in the same localities. In two areas he found only unisexual families (14 of them in Suva and one on Vanua Levuj, in one island (Ovalau) only a single bisexual brood and in the other two islands, Taveuni and Kandavu, one bisexual and one of each type respectively. Nearly 60 years, and probably at least 150 generations later, we found that all-female broods were still present in four of the five islands re-investigated, and breeding showed that in these four (which included Suva) the proportion of the two types of female was approximately equal. In the fifth island only a bisexual family was bred. The only marked change between 1921 and 1980 was in the Suva area, where Simmonds found far more unisexual females than we did.
By carrying out spermatophore counts in many of our wild females we found that almost all of them had been mated, the majority of them on one occasion only. Though the data are scanty, they suggest that there is no wastage of females and that the population in 1980 was stable for the two types of brood.
Reasons for our findings are discussed, particularly in the light of the Heuch model, but we conclude that no entirely satisfactory explanation has yet been given for the persistence of all-female broods. Again, there is so far no explanation of the mechanism though we favour a cytoplasmic factor.
We feel that the status of H, bolina as a mimic of species of Euploea is less assured as the result of our survey.     

Adaptive radiation and convergence in subdivisions of the butterfly genus Heliconius (Lepidoptera: Nymphalidae)

The process of adaptive radiation and convergence, usually regarded as a feature of macro-evolution, can be seen in the mimetic colour patterns of the butterflies within the confines of the South American genus Heliconius. This can be shown by dividing the genus into subgroups on the basis of adult, pupal and larval morphology: the theory that the mimicry between species results solely from close systematic relationships is thereby refuted, as members of the same morphological group can display widely divergent mimetic patterns, and conversely mutual mimics may belong to several different morphological groups. Various forms of parallel and convergent evolution are thought to account for the present pattern of mimicry, the process is known to start even before full speciation has taken place. A new subgenus (Neruda) is created to contain three atypical members of the genus.     

Cryptic differences in colour among Müllerian mimics: how can the visual capacities of predators and prey shape the evolution of wing colours?

Antagonistic interactions between predators and prey often lead to co‐evolution. In the case of toxic prey, aposematic colours act as warning signals for predators and play a protective role. Evolutionary convergence in colour patterns among toxic prey evolves due to positive density‐dependent selection and the benefits of mutual resemblance in spreading the mortality cost of educating predators over a larger prey assemblage. Comimetic species evolve highly similar colour patterns, but such convergence may interfere with intraspecific signalling and recognition in the prey community, especially for species involved in polymorphic mimicry. Using spectrophotometry measures, we investigated the variation in wing coloration among comimetic butterflies from distantly related lineages. We focused on seven morphs of the polymorphic species Heliconius numata and the seven corresponding comimetic species from the genus Melinaea. Significant differences in the yellow, orange and black patches of the wing were detected between genera. Perceptions of these cryptic differences by bird and butterfly observers were then estimated using models of animal vision based on physiological data. Our results showed that the most strikingly perceived differences were obtained for the contrast of yellow against a black background. The capacity to discriminate between comimetic genera based on this colour contrast was also evaluated to be higher for butterflies than for birds, suggesting that this variation in colour, likely undetectable to birds, might be used by butterflies for distinguishing mating partners without losing the benefits of mimicry. The evolution of wing colour in mimetic butterflies might thus be shaped by the opposite selective pressures exerted by predation and species recognition.