The molecular basis of the plumage colour polymorphism in the Tahiti reed‐warbler Acrocephalus caffer

The endemic Tahiti reed‐warbler Acrocephalus caffer occurs in two distinct morphs, a typical or ‘yellow’ morph and a melanic or ‘dark’ morph, which are found together in the valleys of the eastern and central parts of the island of Tahiti (Society Islands, French Polynesia). We investigated the molecular basis of the plumage colour polymorphism in this species using sequences of the melanocortin‐1 receptor (MC1R), a gene often found associated to melanism in birds. We found that the MC1R genotype was perfectly associated with plumage colour in the Tahiti reed‐warbler, with the same nonsynonymous substitution that showed a correlation with phenotype in the Caribbean bananaquit Coereba flaveola. An heterozygous reed‐warbler at this site presented a melanic phenotype, suggesting that the melanic allele is dominant. All other Polynesian reed‐warbler species, which do not have a melanic morph, shared the ‘yellow’ nucleotide at this position. These results suggested that the same mutation point was linked to a melanic polymorphism in two unrelated passerine birds.     

Melanism in a Chinese Population of Harmonia axyridis (Coleoptera: Coccinellidae): A Criterion for Male Investment with Pleiotropic Effects on Behavior and Fertility

In Beijing, China, females of Harmonia axyridis are promiscuous but prefer typical (succinea form) males to melanic ones in the spring generation, ostensibly due to the thermal disadvantages of melanism during summer. We used laboratory observations to test whether males invested differentially in females according to their elytral color, and whether male behavior was phenotype-dependent. Video-recording was used to monitor no-choice mating tests between virgin adults in all phenotype combinations and females were isolated post-copula to observe their egg retention times and reproduction over 5 days. Females tended to wait longer before using the sperm of melanic males, and melanic females delayed longer than succinic females. Melanic males spent longer in copula with succinic than melanic females and the latter received fewer bouts of male abdominal shaking that correlate with sperm transfer, regardless of the phenotype of their mate. Although melanic males abandoned melanic females faster than did succinic males, they remained in copula with females of both phenotypes for a longer period after shaking, suggesting a larger investment in mate guarding by the less-preferred male phenotype. Although female fecundity did not vary among phenotype combinations, egg fertility was lower for females mated to melanic males, suggesting a pleiotropic effect of melanism on male fertility in addition to its effects on male mating behavior.     

Effects of polymorphic melanism and larval diet on life history traits of Malacosoma disstria moths

In this study we investigated the presence and possible genetic basis of polymorphic melanism in the forest tent caterpillar (Malacosoma disstria) moth. Adult moths were classified into pattern-based phenotypes and wing darkness was measured to quantify the degree of melanization. We found that two distinct phenotypes, melanic and simple, are present in these moths. Although the full melanic phenotype is sex-limited to males, it is partially expressed in females. We also provide support for the theory that the melanic allele is autosomal and dominant. The effects of larval diet quality on the survival, development and wing melanization of each phenotype were studied by rearing larvae on the foliage of either a primary or secondary host. Diet quality did not differentially affect the two phenotypes; however, melanic males were found to be smaller than simple males regardless of larval diet. Such inherent developmental differences between the two phenotypes could have important consequences for the frequencies of the two morphs.     

Industrial melanism in the spittlebug Philaenus spumarius (L) (Homoptera: Aphrophoridae)*

The visible polymorphism of the spittlebug, Philaenus spumarius, has been investigated in the vicinity of a smokeless fuel factory in the Cynon Valley of south Wales. The factory is a significant source of local particulate air pollution. A striking relationship exists between the combined frequencies of the eight dark (melanic) morphs and proximity to the factory. Maximum melanic phenotype frequencies of over 95“, occur in both sexes immediately adjacent to it and decline to levels normal for south Wales 1.5–6 km away, depending on direction. This relationship is largely confined to sites within the valley; samples from adjacent localities outside it have melanic frequencies within normal limits for the south Wales area. Maximum melanic phenotype frequencies in the Cynon Valley are far higher than any known from elsewhere in the species range in Europe, Asia and North America. No consistent difference is apparent in total melanic frequency between males and females at any of the sites in this study. However, marked differences exist between the sexes in the relative contributions of the eight melanic phenotypes to the overall association with the factory. For females the industrial melanism is entirely attributable to the group flanicollis + gibbus + leucocephalus (mainly leucocephalus) whereas in males both this group and the group quadnmaculatus + albomaculatus + leucopthalmus contribute to the relationship. It is suggested that this relationship is due to the selective effects of the local air pollution from the factory. The exact nature of the selection involved is as yet uncertain; it would appear to be strong since the local adaptation involved has developed in a maximum of 40 generations since the factory was opened. Finally, comparison is made with two other insects, a ladybird and a moth, in which high frequencies of melanic forms are also associated with this pollution source.     

The Genetic,Morphological, and Physiological Characterization of a Dark Larval Cuticle Mutation in the Butterfly,Bicyclus anynana

Studies on insect melanism have greatly contributed to our understanding of natural selection and the ultimate factors influencing the evolution of darkly pigmented phenotypes. Research on several species of melanic lepidopteran larvae have found that low levels of circulating juvenile hormone (JH) titers are associated with a melanic phenotype, suggesting that genetic changes in the JH biosynthetic pathway give rise to increased deposition of melanin granules in the cuticle in this group. But does melanism arise through different molecular mechanisms in different species? The present study reports on a Bicyclus anynana (Lepidoptera: Nymphalidae) dark larvae single locus mutation, in which larvae exhibit a darker cuticle relative to wild type. Unlike other lepidopteran melanic larvae mutations, this one is autosomal recessive and does not appear to involve a deficiency in JH titers. Unlike JH deficiency mutants, dark larvae mutants display similar growth rates and sexual behaviors as wild type, and topical application of a JH analogue failed to rescue the wild type cuticular coloration. Finally, transmission electron microscopy showed that sclerotization or deposition of diffuse melanin, rather than deposition of melanin granules, produces the dark coloration found in the cuticle of this species. We conclude that different molecular mechanisms underlie larval melanism in different species of Lepidoptera.     

Recent history of melanism in American peppered moths

Industrial melanism in peppered moths has been studied most intensively in Britain. The first melanic phenotype (effectively solid black) was recorded near Manchester in 1848. By 1895 about 98% of the specimens near Manchester were melanic, and this once rare phenotype had spread across regions of the country blackened by industrial soot. In rural, unpolluted regions, well away from industrial centers, the pale phenotype (peppered with white and black scales) remained the predominant form. During the latter half of the 20th century, following legislation designed to improve air quality, melanics began to decline in frequency and are now rare where once they had been common. Similar evolutionary changes have occurred elsewhere, but records from outside Britain are fragmentary. We have extended previous surveys of American peppered moth populations and present a composite picture of the recent decline in melanism in northern industrial states-Michigan and Pennsylvania-where melanic phenotypes decreased from more than 90% in 1959 to 6% by 2001. We contrast these changes to the near absence of melanism in a southern state-Virginia-during that same period. As in Britain, the decline in melanism in American peppered moths followed clean air legislation.     

Parallel evolution of the melanic form of the California legless lizard, Anniella pulchra, inferred from mitochondrial DNA sequence variation

Abstract.— The phylogenetic relationships among populations of the fossorial California legless lizard, Anniella pulchra , were examined by sequencing a 990-bp region of the mitochondrial cytochrome b gene. The mitochondrial DNA gene tree was then compared with the geographic distributions of two currently recognized subspecies: A. p. nigra , a melanic form restricted to two disjunct coastal populations, and A. p. pulchra , a more widely distributed, silvery form. We tested the null hypothesis that all A. p. nigra form a clade that is monophyletic with respect to A. p. pulchra . Our results strongly reject the monophyletic origin of the melanic forms and suggest that the two populations of the nominal subspecies A. p. nigra may have arisen independently from different ancestral populations in a parallel evolutionary response to selection in cool, coastal habitats.     

The costs of colour: plasticity of melanin pigmentation in an outbreaking polymorphic forest moth

Outside the context of industrial melanism, little is known about the physiological and ecological importance of genetic melanic polymorphisms in moths. Melanin pigments are synthesized from amino acid precursors and should therefore be costly to produce in nitrogen‐limited insects. A genetic melanic polymorphism is present in adult Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), a widespread forest moth with outbreaking population dynamics. We test the hypotheses that melanin‐based colouration is physiologically costly in M. disstria, that expression of melanin‐based colouration is a plastic trait which varies with population density and nutrition, and that the genetically based melanic phenotype is disadvantaged under nutritionally poor conditions. Two experiments were used to test these hypotheses. A field study compared pigmentation and phenotypic frequencies in moths collected from high‐ and low‐density populations. A laboratory experiment investigated the effects of larval nitrogen availability on adult pigmentation and phenotypic frequencies. High population density and nitrogen limitation reduced pigmentation and size of all moths, but phenotypic frequencies were not affected in either experiment. The effects of diet on both pigmentation and size were stronger for melanic moths than for typical moths. Our results show that adult melanism in M. disstria is physiologically costly, that colour expression is plastic despite its genetic component, and that the melanic phenotype may be disadvantaged under poor conditions but favoured under good conditions. We suggest that temporal variation in selection and trait plasticity help maintain polymorphism stability.     

Localized industrial melanism in the spittlebug Philaenus spumarius (L.) (Homoptera: Aphrophoridae) in Cardiff docks, south Wales

Twenty-four large adult samples of the meadow spittlebug, Philaenus spumarius , were collected from within a 4 times 3 km area containing Cardiff docks, south Wales. All samples are highly polymorphic for colour and pattern and show marked geographic variation in the combined frequency of the eight melanic morphs (from <15% to > 75%). Maximum melanic phenotype frequencies in both sexes coincide with dock wharfs currently containing large accumulations of pulverized coal and these samples are heavily contaminated with small coal particles. However, interpretation of this association is complicated by the existence, until 1976, of a smokeless fuel factory causing local air pollution immediately adjacent to the area of highest melanic frequency. It is suggested that the association of high levels of melanism in P. spumarius with areas heavily contaminated with particulate pollutants is due to direct selective effects of small particles favouring melanies.     

Divergent mechanisms for water conservation in Drosophila species

The role of melanization and cuticular lipids in water conservation has been studied in many Drosophila species (Diptera: Drosophilidae). Nevertheless, a comparative approach to larval and adult stages of ecologically diverse, wild Drosophila species is still required. Based upon abdominal cuticular melanization patterns, wild‐caught Drosophila species were categorized as (1) melanic, (2) fixed‐melanic, or (3) non‐melanic. At the interspecific level, the ecological significance of melanization and cuticular lipids was determined by the inverse association of melanization and cuticular water loss in melanic species, and of cuticular lipids and cuticular water loss in fixed‐melanic and non‐melanic species. Interestingly, higher amounts of cuticular lipids were also evident in fixed as well as non‐melanic species, as compared to melanic species at larval stages, which is consistent with their differences in reduced water loss rates. Moreover, fixed‐melanic and non‐melanic species exhibited comparatively higher (ca. 1.8–2.0 fold) desiccation resistance. Thus, cuticular lipids provide a better waterproofing mechanism than melanization. Furthermore, acclimation to dehydration stress in adults improved desiccation resistance in melanic species, whereas such effects were lacking in fixed‐melanic and non‐melanic species. However, there were no changes in cuticular components as a consequence of desiccation acclimation. Thus, our results indicate that melanic, fixed‐melanic, and non‐melanic Drosophila species differ in the evolved physiological mechanisms of water conservation to adapt to dry conditions.     

Different genes underlie adaptive melanism in different populations of rock pocket mice

Identifying the genes responsible for adaptation has been an elusive goal in evolutionary biology. Rock pocket mice (Chaetodipus intermedius) provide a useful system for studying the genetics of adaptation: most C. intermedius are light-coloured and live on light-coloured rocks, but in several different geographical regions, C. intermedius are melanic and live on dark-coloured basalt lava, presumably as an adaptation for crypsis. Previous work demonstrated that mutations at the melanocortin-1 receptor gene (Mc1r) are responsible for the dark/light difference in mice from one population in Arizona. Here, we investigate whether melanism has evolved independently in populations of dark C. intermedius from New Mexico, and whether the same or different genes underlie the dark phenotype in mice from these populations compared with the dark mice from Arizona. Seventy-six mice were collected from pairs of dark and light localities representing four different lava flows and adjacent light-coloured rocks; lava flows were separated by 70-750 km. Spectrophotometric analysis of mouse pelage and of rock samples revealed a strong positive association between coat colour and substrate colour. No significant differences were observed in the colour of rocks among the four lava flows, suggesting that mice in these separate populations have experienced similar selection for crypsis. Despite this similarity in environment, melanic mice from the three New Mexico populations were slightly, but significantly, darker than melanic mice from Arizona. The entire Mc1r gene was sequenced in all mice. The previously identified mutations responsible for the light/dark difference in mice from Arizona were absent in all melanic mice from three different populations in New Mexico. Five new Mc1r polymorphisms were observed among mice from New Mexico, but none showed any association with coat colour. These results indicate that adaptive melanism has arisen at least twice in C. intermedius and that these similar phenotypic changes have a different genetic basis.     

Seasonal cycles of assortative mating and reproductive behaviour in polymorphic populations of Harmonia axyridis in China

Abstract 1. We observed native populations of Harmonia axyridis (Pallas) around Beijing, China, over 2 years and performed choice and no‐choice mating tests between melanic and succinic (non‐melanic) beetles in the laboratory. 2. Succinic phenotypes outnumbered melanics by 5:1 in autumn, but melanics became equally abundant in spring, supporting previous inferences that melanism is advantageous in winter, but costly in summer. 3. Female H. axyridis expressed mate preference overtly, by rejecting less‐preferred phenotypes, and cryptically, by retaining their eggs for longer periods after matings with less‐preferred males, ostensibly to replace their sperm. 4. Succinic pairs formed more quickly in the spring generation, and melanic pairs in the autumn, and the time to copula was affected by both male and female phenotype. The strength of mate preference was contingent on female phenotype, suggesting melanic alleles had pleiotropic effects. 5. Whereas pair formation was under female control, the duration of copula was under male control and lasted longer in the autumn generation than in the spring. Copulations in the choice test tended to be shorter between similar phenotypes, suggesting that males invested more in dissimilar females when alternative mates were available. 6. Although spring and autumn generations were raised under identical conditions, significant contrasts were observed in their reproductive behaviour. 7. Two alternative hypotheses are advanced to explain why gender‐specific reproductive behaviours might vary between generations: maternally‐mediated epigenetic factors that influence the expression of genes in progeny as a function of maternal environment, and linkage disequilibria among alleles that cycle in frequency seasonally as a function of assortative mating.     

Predation and the persistence of melanic male mosquitofish (Gambusia holbrooki)

The empirical reasons for the persistent rarity of a genotype are typically complex and tedious to identify, particularly in nature. Yet rare morphs occur in a substantial fraction of phenotypic polymorphisms. A colour polymorphism has persisted for decades in the eastern mosquitofish, yet why this is so remains obscure. Here, I report the results of (1) intensive sampling at 45 natural sites to obtain the frequency distribution of the melanic (black) mosquitofish morph in Florida, (2) predation trials, conducted independently in mesocosms, with three different predatory species and (3) two mark-recapture studies, conducted in nature. This work (1) documents the rarity of melanic mosquitofish in nature, (2) demonstrates that melanic males experience a selective advantage over silver males in the presence of predators, (3) indicates no difference in the colour morphs' survival at a natural site essentially devoid of these predators, although suggesting a higher rate of recapture for melanic males at a site rife with predators. Overall, selective predation appears to contribute to the persistence of the melanic morph, despite its rarity in nature.     

Paternal effects associated with melanism in Harmonia axyridis (Coleoptera: Coccinellidae): mating sequence asymmetries and interactions with age‐specific maternal effects

1. For this study a cohort of melanic Harmonia axyridis males homozygous for the spectabilis allele was produced. These were used to produce four kinds of twice‐mated females, comprising all four permutations of melanic and non‐melanic (succinic) males. A series of 12 larvae were then reared from the first and 10th clutches of each female to compare progeny developmental phenotypes. 2. There were no effects of mating treatment on overall female reproductive performance (preoviposition period, 20‐day fecundity, or egg fertility). 3. Age‐specific maternal effects were evident in progeny developmental phenotypes; larvae of 10th clutches developed more slowly, pupation was shorter, and adults emerged at heavier weights. 4. Paternal effects were superimposed on maternal effects and affected progeny independent of their paternity; melanic males induced slower larval development and slower pupation, but only in 10th clutches and only when they mated second. 5. There was a significant three‐way interaction between male mating treatment, clutch number and progeny phenotype, indicating that progeny developmental responses to (mixed) paternal effects varied depending on their own phenotype and the time elapsed since their mother's last mating. 6. Melanic males mating second obtained a P2 advantage over succinic males, which increased from first to 10th clutch, but the reverse was not true when succinic males mated second. Thus, polyandry in H. axyridis facilitates both genetic and epigenetic competition among males while simultaneously enabling the sharing of predominant paternal effects among the progeny of different fathers.     

Genetic drift and patterns of diversity among colour-polymorphic populations of the homopteran Philaenus spumarius in an island archipelago

Populations of the sedentary spittlebug Philaenus spumarius were samled on 26 islands in the Isles of Scilly archipelago. The islands vary in size from 0.2 to 662 ha. Patterns of phenotypic and genic diversity at a melanic colour-polymorphic locus are examined. The indices of diversity were characterized by a comet-shaped distribution when plotted against island area with increasing uniformity among larger islands. A similar pattern occurred in a plot against the first principal component from analysis of nine independent island variables, including area. The greater diversity among smaller islands was associated with the absence of melanics (and thus of rare alleles) on some of them and a generally higher variance in allele and phenotype frequencies. There was no overall loss of heterozygosity. One melanic allele (I) occurred at a substantial frequency on one of the smallest islands even though it was not detected anywhere else. Patterns of population density, island vegetation and height suggest that small-island populations are likely to experience bottlenecks or extinction-recolonization cycles. The observations are consistent with a major role of intermittent drift and founder effects among the smaller islands. The results are compared with those of Halkka and his co-workers for islands in the Gulf of Finland.     

The rise and fall of the Carbonaria form of the peppered moth

The evidence for change in frequency of the melanic carbonaria morph in the peppered moth Biston betularia (L.) (Lepidoptera: Geometridae) in England and Wales is reviewed. At mid-20th century a steep cline of melanic phenotype frequency running from the north of Wales to the southern coast of England separated a region of 5% or less to west from 90% or more to northeast. By the 1980s the plateau of 90% frequency had contracted to northern England. The frequency has since continued to drop so that the maximum is now less than 50% and in most places below 10%. There have been similar declines in Europe and North America. Evidence from surveys and from two-point records shows the change to require 5% to 20% selection against the melanic. The melanic is more disadvantageous in regions where its frequency was initially high than in regions where it was low. Experiments to investigate predation by birds show a net advantage to carbonaria morphs in regions where typical frequencies were low at the time of the experiment, and a disadvantage where typical frequencies were high. This would be expected if environment and frequency were associated, and selective predation played a part in generating the association. The cryptic advantage of carbonaria was large in areas of heavy pollution where typical frequencies were 20% or less. The moth usually has a low density but is relatively highly mobile. The ability of present information to explain the patterns has been tested in simulations. They indicate a system under strong selection that has always been in a dynamic state without equilibria.     

Ecological genetics of adaptive color polymorphism in pocket mice: geographic variation in selected and neutral genes

Patterns of geographic variation in phenotype or genotype may provide evidence for natural selection. Here, we compare phenotypic variation in color, allele frequencies of a pigmentation gene (the melanocortin-1 receptor, Mc1r), and patterns of neutral mitochondrial DNA (mtDNA) variation in rock pocket mice (Chaetodipus intermedius) across a habitat gradient in southern Arizona. Pocket mice inhabiting volcanic lava have dark coats with unbanded, uniformly melanic hairs, whereas mice from nearby light-colored granitic rocks have light coats with banded hairs. This color polymorphism is a presumed adaptation to avoid predation. Previous work has demonstrated that two Mc1r alleles, D and d, differ by four amino acids, and are responsible for the color polymorphism: DD and Dd genotypes are melanic whereas dd genotypes are light colored. To determine the frequency of the two Mc1r allelic classes across the dark-colored lava and neighboring light-colored granite, we sequenced the Mc1r gene in 175 individuals from a 35-km transect in the Pinacate lava region. We also sequenced two neutral mtDNA genes, COIII and ND3, in the same individuals. We found a strong correlation between Mc1r allele frequency and habitat color and no correlation between mtDNA markers and habitat color. Using estimates of migration from mtDNA haplotypes between dark- and light-colored sampling sites and Mc1r allele frequencies at each site, we estimated selection coefficients against mismatched Mc1r alleles, assuming a simple model of migration-selection balance. Habitat-dependent selection appears strong but asymmetric: selection is stronger against light mice on dark rock than against melanic mice on light rock. Together these results suggest that natural selection acts to match pocket mouse coat color to substrate color, despite high levels of gene flow between light and melanic populations.     

黑化粘虫的形态特征及其遗传模式

本文首次研究报道了黑化粘虫Mythimna separata的形态特征及其遗传模式。所获的结果表明,黑化型与正常型粘虫的外部形态特征差异主要表现在成虫期。与正常型粘虫相比,黑化粘虫的成虫体色几乎全部变黑,正常型成虫所具有的黑色特征在黑化粘虫中已完全看不见,其他的颜色特征也由于黑化而产生了变异。卵、幼虫和蛹的颜色与正常型的没有明显的区别,只是黑化型的蛹在将羽化时的体色比正常型的深。黑化粘虫的遗传遵循孟德尔的隐性遗传规律,由位于常染色体上的单基因控制。这些结果表明,黑化型与正常型粘虫虽然仍属于同一个种,但黑化粘虫已经演变成了不同的基因型。最后,对黑化粘虫产生的原因及其在进化上的适应意义进行了讨论。     

The genetic basis of the plumage polymorphism in red-footed boobies (Sula sula): a melanocortin-1 receptor (MC1R) analysis

The red-footed booby (Sula sula) is considered one of the most polymorphic seabirds, with 3 recognized major adult plumage types: 1) white, 2) white-tailed brown, and 3) brown and several degrees of intermediates. Here we show that the white/melanic polymorphism observed in this species is perfectly associated with 2 point substitutions, Val85Met and His207Arg, at the melanocortin-1 receptor (MC1R) gene. Among the melanic plumage variants, we also found a strong association between the degree of melanism and the number of copies of variant MC1R alleles. Furthermore, the Val85Met point substitution has been previously shown to be associated with melanic phenotypes in the lesser snow goose (Anser c. caerulescens), suggesting parallel evolution of the melanic allele, and hence, melanism, between these 2 distantly related species. We also compared the MC1R locus in red-footed boobies with a nonpolymorphic congener, the Nazca booby (Sula granti), in which all adults are white. We found that Nazca boobies present the same genotype at sites 85 and 207 as white morph red-footed boobies.     

The understanding of industrial melanism in the peppered moth (Biston betularia) (Lepidoptera: Geometridae)

Melanic polymorphism in B. betularia has been extensively studied. Correlations between high melanic frequency and high levels of air pollution have been demonstrated. Kettlewell and others have shown that differential bird predation has an important effect on the maintenance of the polymorphism, and coefficients of visual selection have been obtained on the assumption that the moth habitually rests on tree trunks. Computer models based on these selective coefficients show that they are not sufficient accurately to explain observed melanic frequencies. Other non-visual selective factors and weak frequency-dependent selection have been invoked to improve fits. Analysis of the resting positions of moths recorded in the wild demonstrates that B. betularia does not usually rest in exposed positions on tree trunks, but rather rests on the underside of branches, on trunks in shaded positions just below major branch joints or on foliate twigs. The results of a pilot selection experiment, while agreeing qualitatively with Kettlewell's results, suggest that fitness estimates that assume trunk-resting are quantitively incorrect. The error is greatest for melanic moths in rural areas. It is suggested that visual selective coefficients based on a true assessment of the resting behaviour of the moths may considerably improve the fit between computer predictions and observed phenotype frequency distributions.