Melanin-based Pigmentation Research Articles

Whole genome resequencing reveals the genetic basis of albino phenotype in an ornamental fish, Channa asiatica

Body color is one of the most noticeable phenotypic and economic characteristics in fish. Fish exhibit a diverse array of body color mutations influenced by factors such as artificial selection and natural mutations. Elucidating the genetic mechanisms of these color variants in fish is crucial for developing new breeds with higher ornamental value. In this study, we identified a nonsense mutation in csf1ra (c. 1249 C>T) with whole genome resequencing technology, and which may potentially contribute to the albino phenotype in Channa asiatica. The albino individuals exhibited a substantial decrease in the expression level of csf1ra compared to that of the wild individuals. To unravel the underlying mechanism of csf1ra, we delved into comparative transcriptomic and gene expression analyses. Our findings suggest that csf1ra may impact tyrosine kinase activity. Furthermore, csf1ra may not only regulate the expression levels of genes (sox9, ephb6, and wnt9) involved in melanin-based pigmentation but also influence genes (ampd3, ak1, and bco1) associated with non-melanin-based pigmentation. Our study investigated the genetic basis of the albino mutant of C. asiatica and provided new insights into the function of csf1ra in fish pigmentation.

Genomic basis of melanin-associated phenotypes suggests colour-specific environmental adaptations in tawny owls.

Feathers comprise a series of evolutionary innovations but also harbour colour, a key biological trait known to co-vary with life history or complex traits. Those relationships are particularly true in melanin-based pigmentation species due to known pleiotropic effects of the melanocortin pathway - originating from melanin-associated phenotypes. Here, we explore the molecular basis of melanin colouration and expected co-variation at the molecular level in the melanin-based, colour polymorphic system of the tawny owl (Strix aluco). An extensive body of literature has revealed that grey and brown tawny owl colour morphs differ in a series of life history and behavioural traits. Thus, it is plausible to expect co-variation also at molecular level between colour morphs. To investigate this possibility, we assembled the first draft genome of the species against which we mapped ddRADseq reads from 220 grey and 150 brown morphs - representing 10 years of pedigree data from a population in Southern Finland - and explored genome-wide associations with colour phenotype. Our results revealed putative molecular signatures of cold adaptation strongly associated with the grey phenotype, namely, a non-synonymous substitution in MCHR1, plus 2 substitutions in non-coding regions of FTCD and FAM135A whose genotype combinations obtained a predictive power of up to 100% (predicting grey colour). These suggest a molecular basis of cold environment adaptations predicted to be grey-morph specific. Our results potentially reveal part of the molecular machinery of melanin-associated phenotypes and provide novel insights towards understanding the functional genomics of colour polymorphism in melanin-based pigmented species.

Background matching through fast and reversible melanin-based pigmentation plasticity in tadpoles comes with morphological and antioxidant changes

Facultative colour change is widespread in the animal kingdom, and has been documented in many distantly related amphibians. However, experimental data testing the extent of facultative colour change, and associated physiological and morphological implications are comparatively scarce. Background matching in the face of spatial and temporal environmental variation is thought to be an important proximate function of colour change in aquatic amphibian larvae. This is particularly relevant for species with long larval periods such as the western spadefoot toad, Pelobates cultripes, whose tadpoles spend up to six months developing in temporary waterbodies with temporally variable vegetation. By rearing tadpoles on different coloured backgrounds, we show that P. cultripes larvae can regulate pigmentation to track fine-grained differences in background brightness, but not hue or saturation. We found that colour change is rapid, reversible, and primarily achieved through changes in the quantity of eumelanin in the skin. We show that this increased eumelanin production and/or maintenance is also correlated with changes in morphology and oxidative stress, with more pigmented tadpoles growing larger tail fins and having an improved redox status.

Ecogeography of Plumage Pigmentation in Great Horned Owls

ABSTRACT Plumage pigmentation is fundamental to a bird's phenotype, with pigment deposition causing relative crypsis or conspicuousness, depending on the environmental context. Geographic variation in plumage melanin tends to be predictable, suggesting that aspects of climate cause local matching of plumage to environment via genetic adaptation. Ecogeographic rules describe this predictability: Gloger's rule predicts that populations in wetter and warmer environments will be more pigmented; Bogert's Rule predicts more pigmentation in cold environments. The Great Horned Owl (Bubo virginianus) exhibits extensive geographic variation in the degree of melanin-based pigmentation. We examined fine-scale spatial variation in owl plumage melanism along environmental gradients in southwestern North America. We tested whether variation is explained by either of two non-mutually exclusive hypotheses: (1) a history of allopatric divergence between subspecies or (2) in situ local adaptation consistent with ecogeographic rules. The allopatric divergence hypothesis predicts a bimodal distribution of plumage melanism, with a geographic cline across a zone of secondary contact, whereas the local adaptation hypothesis predicts that climate explains variation independently of geography. Using a colorimeter, we measured coloration in 101 museum specimens of breeding-season Great Horned Owls that had been obtained from variable environments and elevations. Specimens previously identified as separate subspecies were distinguishable by colorimetry. Plumage lightness, however, was continuously distributed, rather than bimodal. While accounting for males having reduced pigmentation relative to females, linear models revealed that lighter plumage was associated with low latitude, low elevation, high temperature, and low precipitation. These findings suggest that variation in Great Horned Owl plumage pigmentation is best understood as continuous ecogeographic variation, consistent with ecogeographic predictions, and currently maintained in situ along multiple environmental gradients that characterize the “sky island” topography of the southwestern USA.

Functional Domains and Evolutionary History of the PMEL and GPNMB Family Proteins.

The ancient paralogs premelanosome protein (PMEL) and glycoprotein nonmetastatic melanoma protein B (GPNMB) have independently emerged as intriguing disease loci in recent years. Both proteins possess common functional domains and variants that cause a shared spectrum of overlapping phenotypes and disease associations: melanin-based pigmentation, cancer, neurodegenerative disease and glaucoma. Surprisingly, these proteins have yet to be shown to physically or genetically interact within the same cellular pathway. This juxtaposition inspired us to compare and contrast this family across a breadth of species to better understand the divergent evolutionary trajectories of two related, but distinct, genes. In this study, we investigated the evolutionary history of PMEL and GPNMB in clade-representative species and identified TMEM130 as the most ancient paralog of the family. By curating the functional domains in each paralog, we identified many commonalities dating back to the emergence of the gene family in basal metazoans. PMEL and GPNMB have gained functional domains since their divergence from TMEM130, including the core amyloid fragment (CAF) that is critical for the amyloid potential of PMEL. Additionally, the PMEL gene has acquired the enigmatic repeat domain (RPT), composed of a variable number of imperfect tandem repeats; this domain acts in an accessory role to control amyloid formation. Our analyses revealed the vast variability in sequence, length and repeat number in homologous RPT domains between craniates, even within the same taxonomic class. We hope that these analyses inspire further investigation into a gene family that is remarkable from the evolutionary, pathological and cell biology perspectives.

Shed Light in the DaRk LineagES of the Fungal Tree of Life-STRES.

The polyphyletic group of black fungi within the Ascomycota (Arthoniomycetes, Dothideomycetes, and Eurotiomycetes) is ubiquitous in natural and anthropogenic habitats. Partly because of their dark, melanin-based pigmentation, black fungi are resistant to stresses including UV- and ionizing-radiation, heat and desiccation, toxic metals, and organic pollutants. Consequently, they are amongst the most stunning extremophiles and poly-extreme-tolerant organisms on Earth. Even though ca. 60 black fungal genomes have been sequenced to date, [mostly in the family Herpotrichiellaceae (Eurotiomycetes)], the class Dothideomycetes that hosts the largest majority of extremophiles has only been sparsely sampled. By sequencing up to 92 species that will become reference genomes, the “Shed light in The daRk lineagES of the fungal tree of life” (STRES) project will cover a broad collection of black fungal diversity spread throughout the Fungal Tree of Life. Interestingly, the STRES project will focus on mostly unsampled genera that display different ecologies and life-styles (e.g., ant- and lichen-associated fungi, rock-inhabiting fungi, etc.). With a resequencing strategy of 10- to 15-fold depth coverage of up to ~550 strains, numerous new reference genomes will be established. To identify metabolites and functional processes, these new genomic resources will be enriched with metabolomics analyses coupled with transcriptomics experiments on selected species under various stress conditions (salinity, dryness, UV radiation, oligotrophy). The data acquired will serve as a reference and foundation for establishing an encyclopedic database for fungal metagenomics as well as the biology, evolution, and ecology of the fungi in extreme environments.

Impairment of mixed melanin-based pigmentation in parrots.

Parrots and allies (Order Psittaciformes) have evolved an exclusive capacity to synthesize polyene pigments called psittacofulvins at feather follicles, which allows them to produce a striking diversity of pigmentation phenotypes. Melanins are polymers constituting the most abundant pigments in animals, and the sulphurated form (pheomelanin) produces colors that are similar to those produced by psittacofulvins. However, the differential contribution of these pigments to psittaciform phenotypic diversity has not been investigated. Given the color redundancy, and physiological limitations associated to pheomelanin synthesis, we hypothesized that the latter would be avoided by psittaciform birds. Here we test this by using Raman spectroscopy to identify pigments in feathers exhibiting colors suspicious of being produced by pheomelanin (i.e., dull red, yellow and grey- and green-brownish) in 26 species from the three main lineages of Psittaciformes. We detected the non-sulphurated melanin form (eumelanin) in black, grey and brown plumage patches, and psittacofulvins in red, yellow and green patches, but no evidence of pheomelanin. As natural melanins are assumed to be composed of eumelanin and pheomelanin in varying ratios, our results represent the first report of impairment of mixed melanin-based pigmentation in animals. Given that psittaciforms also avoid the uptake of circulating carotenoid pigments, these birds seem to have evolved a capacity to avoid functional redundancy between pigments, likely by regulating follicular gene expression. Ours study provides the first vibrational characterization of different psittacofulvin-based colors and thus helps to determine the relative polyene chain length in these pigments, which is related to their antireductant protection activity.

Carotenoid- but not melanin-based plumage coloration is negatively related to metal exposure and proximity to the road in an urban songbird

Rapid urbanization is a global phenomenon that is increasingly exposing organisms to novel stressors. These novel stressors can affect diverse aspects of organismal function, including development of condition-dependent ornaments, which play critical roles in social and sexual selection. We investigated the relationship between metal pollution, proximity to roads, and carotenoid- and melanin-based plumage coloration in a common songbird, the great tit (Parus major). We studied populations located across a well-characterized metal pollution gradient and surrounded by roadway networks. Metal exposure and road-associated pollution could reduce carotenoid-based pigmentation by inducing oxidative stress or affecting habitat quality, but metals could also enhance melanin-based pigmentation, through effects on melanogenesis and testosterone concentrations. Using a large sample size (N > 500), we found that birds residing close to a point source for metals had reduced ultraviolet chroma, a component of carotenoid-based pigmentation. Moreover, birds with high feather metal concentrations had lower carotenoid chroma, hue, and ultraviolet chroma, with effects modified by age class. Birds residing closer to roads also had lower carotenoid chroma and hue. Melanin-based pigmentation showed high between-year repeatability, and no association with anthropogenic pollution. Results suggest that carotenoid-, but not melanin-, based pigmentation is negatively affected by multiple anthropogenic stressors. We are the first to demonstrate a negative association between roads and a plumage-based signaling trait, which could have important implications for sexual signaling dynamics in urban landscapes.

Pheomelanin molecular vibration is associated with mitochondrial ROS production in melanocytes and systemic oxidative stress and damage.

Vibrations in covalent bonds affect electron delocalization within molecules, as reported in polymers. If synthesized by living cells, the electron delocalization of polymers affects the stabilization of cellular free radicals, but biomolecular vibration has never been considered a potential source of cytotoxicity. Here we show that the vibrational characteristics of natural pheomelanin and eumelanin contribute to feather color expression in four poultry breeds with different melanin-based pigmentation patterns, but only the vibrational characteristics of pheomelanin are related to the production of reactive oxygen species (ROS) in the mitochondria of melanocytes and to systemic levels of cellular oxidative stress and damage. This association may be explained by the close physical contact existing between mitochondria and melanosomes, and reveals an unprecedented factor affecting the viability of organisms through their pigmentation. These findings open a new avenue for understanding the mechanism linking pheomelanin synthesis to human melanoma risk.

Genetic colour polymorphism is associated with avian malarial infections.

Individual genetic diversity is predicted to influence host-parasite interactions. Together with the genes directly associated with immune responses, variation in genes regulating vertebrate melanin-based pigmentation may play an important role in these interactions, mainly through the pleiotropic effects that affect colour-specific physiology, behaviour and immunity. Here, we test the hypothesis that the prevalence of avian malarial parasites differs between phenotypes in a raptor species in which the genetic basis of colour polymorphism and its pleiotropic effects over immune functions are known. We found that dark morphs had a higher prevalence of Plasmodium parasites than pale ones but detected no such association for Haemoproteus This pattern may be associated with unequal exposure to vectors or, as suggested by our circumstantial evidence, to a differential ability to mount an immune response against blood parasites.

A novel role for pigment genes in the stress response in rainbow trout (Oncorhynchus mykiss).

In many vertebrate species visible melanin-based pigmentation patterns correlate with high stress- and disease-resistance, but proximate mechanisms for this trait association remain enigmatic. Here we show that a missense mutation in a classical pigmentation gene, melanocyte stimulating hormone receptor (MC1R), is strongly associated with distinct differences in steroidogenic melanocortin 2 receptor (MC2R) mRNA expression between high- (HR) and low-responsive (LR) rainbow trout (Oncorhynchus mykiss). We also show experimentally that cortisol implants increase the expression of agouti signaling protein (ASIP) mRNA in skin, likely explaining the association between HR-traits and reduced skin melanin patterning. Molecular dynamics simulations predict that melanocortin 2 receptor accessory protein (MRAP), needed for MC2R function, binds differently to the two MC1R variants. Considering that mRNA for MC2R and the MC1R variants are present in head kidney cells, we hypothesized that MC2R activity is modulated in part by different binding affinities of the MC1R variants for MRAP. Experiments in mammalian cells confirmed that trout MRAP interacts with the two trout MC1R variants and MC2R, but failed to detect regulation of MC2R signaling, possibly due to high constitutive MC1R activity.

Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone.

BackgroundEvolutionary processes, including selection and differential fitness, shape the introgression of genetic material across a hybrid zone, resulting in the exchange of some genes but not others. Differential introgression of molecular or phenotypic markers can thus provide insight into factors contributing to reproductive isolation. We characterized patterns of genetic variation across a hybrid zone between two tidal marsh birds, Saltmarsh (Ammodramus caudacutus) and Nelson’s (A. nelsoni) sparrows (n = 286), and compared patterns of introgression among multiple genetic markers and phenotypic traits.ResultsGeographic and genomic cline analyses revealed variable patterns of introgression among marker types. Most markers exhibited gradual clines and indicated that introgression exceeds the spatial extent of the previously documented hybrid zone. We found steeper clines, indicating strong selection for loci associated with traits related to tidal marsh adaptations, including for a marker linked to a gene region associated with metabolic functions, including an osmotic regulatory pathway, as well as for a marker related to melanin-based pigmentation, supporting an adaptive role of darker plumage (salt marsh melanism) in tidal marshes. Narrow clines at mitochondrial and sex-linked markers also offer support for Haldane’s rule. We detected patterns of asymmetrical introgression toward A. caudacutus, which may be driven by differences in mating strategy or differences in population density between the two species.ConclusionsOur findings offer insight into the dynamics of a hybrid zone traversing a unique environmental gradient and provide evidence for a role of ecological divergence in the maintenance of pure species boundaries despite ongoing gene flow.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0635-y) contains supplementary material, which is available to authorized users.

Candidate Gene Analysis Suggests Untapped Genetic Complexity in Melanin-Based Pigmentation in Birds.

Studies on melanin-based color variation in a context of natural selection have provided a wealth of information on the link between phenotypic and genetic variation. Here, we evaluated associations between melanic plumage patterns and genetic polymorphism in the Réunion grey white-eye (Zosterops borbonicus), a species in which mutations on MC1R do not seem to play any role in explaining melanic variation. This species exhibits 5 plumage color variants that can be grouped into 3 color forms which occupy discrete geographic regions in the lowlands of Réunion, and a fourth high-elevation form which comprises 2 color morphs (grey and brown) and represents a true color polymorphism. We conducted a comprehensive survey of sequence variation in 96 individuals at a series of 7 candidate genes other than MC1R that have been previously shown to influence melanin-based color patterns in vertebrates, including genes that have rarely been studied in a wild bird species before: POMC, Agouti, TYR, TYRP1, DCT, Corin, and SLC24A5 Of these 7 genes, 2 (Corin and TYRP1) displayed an interesting shift in allele frequencies between lowland and highland forms and a departure from mutation-drift equilibrium consistent with balancing selection in the polymorphic highland form only. Sequence variation at Agouti, a gene frequently involved in melanin-based pigmentation patterning, was not associated with color forms or morphs. Thus, we suggest that functionally important changes in loci other than those classically studied are involved in the color polymorphism exhibited by the Réunion grey white-eye and possibly many other nonmodel species.

Trace metals, melanin-based pigmentation and their interaction influence immune parameters in feral pigeons (Columba livia).

Understanding the effects of trace metals emitted by anthropogenic activities on wildlife is of great concern in urban ecology; yet, information on how they affect individuals, populations, communities and ecosystems remains scarce. In particular, trace metals may impact survival by altering the immune system response to parasites. Plumage melanin is assumed to influence the effects of trace metals on immunity owing to its ability to bind metal ions in feathers and its synthesis being coded by a pleiotropic gene. We thus hypothesized that trace metal exposure would interact with plumage colouration in shaping immune response. We experimentally investigated the interactive effect between exposure to an environmentally relevant range of zinc and/or lead and melanin-based plumage colouration on components of the immune system in feral pigeons (Columba livia). We found that zinc increased anti-keyhole limpet hemocyanin (KLH) IgY primary response maintenance, buffered the negative effect of lead on anti-KLH IgY secondary response maintenance and tended to increase T-cell mediated phytohaemagglutinin (PHA) skin response. Lead decreased the peak of the anti-KLH IgY secondary response. In addition, pheomelanic pigeons exhibited a higher secondary anti-KLH IgY response than did eumelanic ones. Finally, T-cell mediated PHA skin response decreased with increasing plumage eumelanin level of birds exposed to lead. Neither treatments nor plumage colouration correlated with endoparasite intensity. Overall, our study points out the effects of trace metals on some parameters of birds' immunity, independently from other confounding urbanization factors, and underlines the need to investigate their impacts on other life history traits and their consequences in the ecology and evolution of host-parasite interactions.

Brightness of melanin-based plumage coloration is a cue to oxidative stress in Himalayan Black Bulbuls (Hypsipetes leucocephalus nigerrimus)

Melanin-based coloration is often found to be associated with variations in physiological and behavioral traits that are essential in avian communication. However, the function of melanin-based traits to their bearers has been investigated less than that of carotenoid-based ones. It is commonly believed that expression of melanin-based pigmentation is genetically controlled. However, whether it could be modulated by physical condition has remained controversial. In this study, we demonstrate that the level of melanin-based plumage pigmentation could be negatively correlated with an individual’s oxidative stress level. In this study, we used two groups of Black Bulbuls (Hypsipetes leucocephalus nigerrimus) obtained from a pet-shop in 2010 and 2011 to examine the correlation between the melanin-based plumage coloration and the ratio of lymphocytes to heterocytes, which is an indicator of the oxidative stress experienced by an individual. We found a negative correlation between oxidative stress levels and expression of melanin-based pigmentation on the breast and scapular feathers, but the correlation only appeared in the 2011 group, individuals of which had higher oxidative stress level and brighter plumage (i.e. more melanins) than those of the 2010 group. Our data suggest that melanin-based plumage could reflect an individual’s physical condition in certain situations. Therefore, melanin-based plumage coloration could have a function in Himalayan Black Bulbuls’ communication.

Social stress effects on pigmentation and monoamines in Arctic charr

Pigmentation often signals status and in general melanin-based pigmentation is indicative of aggression and stress resilience in vertebrates. This is evident in the salmonids Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) where more melanin spotted individuals are more stress resilient. However, in the salmonid Arctic charr (Salvelinus alpinus) it seems as if it is carotenoid-based pigmentation that signals aggression and stress resilience. In our study, social stress effects on carotenoid-based spots, and behavioural and physiological stress responses were investigated. Socially stressed individuals have more spots, and behavioural stress responses were associated with spots. Some of the results concerning physiological stress responses, such as plasma cortisol levels and monoaminergic activity, are associated with spottiness. Further, the earlier proposed lateralization of spots, with left side connected to stress responsiveness and right side to aggression, is to some extent validated although not conclusively. In conclusion, this study provides further evidence that more stressed charr have more carotenoid spots, and for the first time monoaminergic activity is shown to be connected with carotenoid pigmentation.

Morph-specific genetic and environmental variation in innate and acquired immune response in a color polymorphic raptor.

Genetic color polymorphism is widespread in nature. There is an increasing interest in understanding the adaptive value of heritable color variation and trade-off resolution by differently colored individuals. Melanin-based pigmentation is often associated with variation in many different life history traits. These associations have recently been suggested to be the outcome of pleiotropic effects of the melanocortin system. Although pharmacological research supports that MC1R, a gene with a major role in vertebrate pigmentation, has important immunomodulatory effects, evidence regarding pleiotropy at MC1R in natural populations is still under debate. We experimentally assessed whether MC1R-based pigmentation covaries with both inflammatory and humoral immune responses in the color polymorphic Eleonora's falcon. By means of a cross-fostering experiment, we disentangled potential genetic effects from environmental effects on the covariation between coloration and immunity. Variation in both immune responses was primarily due to genetic factors via the nestlings' MC1R-related color genotype/phenotype, although environmental effects via the color morph of the foster father also had an influence. Overall, dark nestlings had lower immune responses than pale ones. The effect of the color morph of the foster father was also high, but in the opposite direction, and nestlings raised by dark eumelanic foster fathers had higher immune responses than those raised by pale foster fathers. Although we cannot completely discard alternative explanations, our results suggest that MC1R might influence immunity in this species. Morph-specific variation in immunity as well as pathogen pressure may therefore contribute to the long-term maintenance of genetic color polymorphism in natural populations.

The effect of anesthetics on carotenoid pigmentation and behavior in Arctic char (Salvelinus alpinus)

Abstract Status of individual animals is often shown in pigmentation. For instance, stress responsiveness is associated with melanin-based pigmentation in vertebrates in general. This pattern is evident in Atlantic salmon ( Salmo salar ) and rainbow trout ( Oncorhynchus mykiss ), with melanin-based spots indicating stress coping style. Arctic char ( Salvelinus alpinus ) differ from other salmonids in pigmentation but have an association between carotenoid-based pigmentation and stress coping style. These pigmentation differences could be used as a fast and simple tool for improving breeding programs. However, the procedure for photographing individuals is stressful. Therefore, we wanted to investigate if 3 common inhalation anesthetics could be used to reduce the stress involved in the procedure compared to a no-anesthetics group. Behavior was also monitored during the anesthetization. All 3 anesthetics (Aquacalm, Benzocaine, and MS-222) differed in pigmentation compared to the no-anesthetics group by having a higher number of spots. In the anesthetic treatments as well as the no-anesthetics group, the fish had elevated plasma cortisol levels. In fact, the no-anesthetics group had higher cortisol than treated fish. This was probably because of procedural differences such as time affecting the stress response. Furthermore, in a long-term experiment, all fish survived and had similar condition factors 1 week after photographing as before. Therefore, the method for photographing Arctic char is deemed safe and could be used as a tool for estimating stress coping style, but careful planning is needed for experiments using it.

Multiple sexual pigments, assortative social pairing, and genetic paternity in the yellow warbler (Setophaga petechia)

In socially monogamous species, extra-pair paternity may increase the reproductive success of highly ornamented males, mediating the evolution of sexual ornaments. However, ornaments may also attract social mates, and a tradeoff between extra-pair paternity (EPP) and within-pair paternity (WPP) may complicate mating strategies. Further, in many socially monogamous species, females are also ornamented, and the relationship between female ornamentation and patterns of EPP has been neglected. We investigated the patterns of genetic paternity with respect to carotenoid- and melanin-based pigmentation in yellow warblers (Setophaga petechia) of both sexes. We asked whether males face tradeoffs between EPP and WPP, how paternity patterns relate to carotenoid- versus melanin-based pigmentation, and whether less EPP occurs in broods when males and females are assortatively paired. Males faced a tradeoff between EPP and WPP. Moreover, non-additive relationships existed between paternity patterns and the two pigment types in both sexes. Males with high melanin coverage but dull carotenoid pigmentation achieved EPP but lost WPP, whereas males with high levels of both pigment types had high WPP but gained little EPP. A parallel pattern occurred in females. Warblers paired assortatively by pigmentation and EPP was less common in broods when the sexes were assortatively paired by carotenoid pigmentation. Results suggest that the most colorful birds obtain high quality social mates and advance reproductive success through WPP, show that correlations can arise between female ornamentation and patterns of EPP, and also uniquely suggest that social pairing patterns may influence extra-pair mating strategies.

The size of a melanin-based plumage ornament correlates with glucocorticoid receptor concentrations in the skin of that ornament

Glucocorticoid hormones such as corticosterone (CORT) play crucial roles in many physiological processes. CORT's actions are primarily mediated via binding to two receptors (glucocorticoid receptors (GRs) and mineralocorticoid receptors (MRs)) in different target tissues. CORT receptors can be independently regulated from circulating hormone titres, from tissue to tissue and even within different regions of the same tissue type. Increasing evidence has shown relationships between circulating CORT and melanin-based pigmentation in skin and feathers, yet to our knowledge, there have been no studies of CORT receptors in the skin of melanized ornaments. Male house sparrows (Passer domesticus) have a black, melanized bib, and evidence suggests that bib size is an important intraspecific signal. We examined the relationship between bib area and tissue sensitivity to CORT by quantifying GR and MR in bib skin and in adjacent paler-feathered belly skin (as a control tissue) at different life-history stages using radioligand binding assays. Males with larger bibs relative to their life-history stage had less GR in bib skin, but not belly skin, than males with smaller bibs. These results suggest a connection between the size of a melanin-based ornament and the underlying tissue's responsiveness to CORT.