Testosterone regulates CYP2J19-linked carotenoid signal expression in male red-backed fairywrens (Malurus melanocephalus)

Captivity affects mitochondrial aerobic respiration and carotenoid metabolism in the house finch (Haemorhous mexicanus)

In many species of animals, red carotenoid-based coloration is produced by metabolizing yellow dietary pigments, and this red ornamentation can be an honest signal of individual quality. However, the physiological basis for associations between organism function and the metabolism of red ornamental carotenoids from yellow dietary carotenoids remains uncertain. A recent hypothesis posits that carotenoid metabolism depends on mitochondrial performance, with diminished red coloration resulting from altered mitochondrial aerobic respiration. To test for an association between mitochondrial respiration and red carotenoids, we held wild-caught, molting male house finches in either small bird cages or large flight cages to create environmental challenges during the period when red ornamental coloration is produced. We predicted that small cages would present a less favorable environment than large flight cages and that captivity itself would decrease both mitochondrial performance and the abundance of red carotenoids compared with free-living birds. We found that captive-held birds circulated fewer red carotenoids, showed increased mitochondrial respiratory rates, and had lower complex II respiratory control ratios - a metric associated with mitochondrial efficiency - compared with free-living birds, though we did not detect a difference in the effects of small cages versus large cages. Among captive individuals, the birds that circulated the highest concentrations of red carotenoids had the highest mitochondrial respiratory control ratio for complex II substrate. These data support the hypothesis that the metabolism of red carotenoid pigments is linked to mitochondrial aerobic respiration in the house finch, but the mechanisms for this association remain to be established.

The Ancestral Modulation Hypothesis: Predicting Mechanistic Control of Sexually Heteromorphic Traits Using Evolutionary History

AbstractAcross the animal kingdom there are myriad forms within a sex across, and even within, species, rendering concepts of universal sex traits moot. The mechanisms that regulate the development of these trait differences are varied, although in vertebrates, common pathways involve gonadal steroid hormones. Gonadal steroids are often associated with heteromorphic trait development, where the steroid found at higher circulating levels is the one involved in trait development for that sex. Occasionally, there are situations in which a gonadal steroid associated with heteromorphic trait development in one sex is involved in heteromorphic or monomorphic trait development in another sex. We propose a verbal hypothesis, the ancestral modulation hypothesis (AMH), that uses the evolutionary history of the trait-particularly which sex ancestrally possessed higher trait values-to predict the regulatory pathway that governs trait expression. The AMH predicts that the genomic architecture appears first to resolve sexual conflict in an initially monomorphic trait. This architecture takes advantage of existing sex-biased signals, the gonadal steroid pathway, to generate trait heteromorphism. In cases where the other sex experiences evolutionary pressure for the new phenotype, that sex will co-opt the existing architecture by altering its signal to match that of the original high-trait-value sex. We describe the integrated levels needed to produce this pattern and what the expected outcomes will be given the evolutionary history of the trait. We present this framework as a testable hypothesis for the scientific community to investigate and to create further engagement and analysis of both ultimate and proximate approaches to sexual heteromorphism.

Revisiting the Baltimore-Bullock's Oriole hybrid zone reveals changing plumage colour in Bullock's Orioles

Hybrid zones are dynamic areas where populations of two or more interbreeding species may change through an influx of novel genetic material resulting from hybridization or selection on standing genetic variation. Documenting changes in populations through time, however, is challenging because repeated samples are often missing or because long-term storage can affect trait morphologies, especially colour traits that may fade through time. We document a change in carotenoid-based orange breast feathers of Bullock's Orioles (Icterus bullockii) from the Great Plains hybrid zone, USA. Contemporary Bullock's Orioles are more orange than historic individuals from the same location sampled approximately 60 years ago. Spectrophotometry revealed that contemporary Bullock's Orioles resemble orange colour profiles of Baltimore Orioles (I. galbula), the species with which they hybridize. Fading or changes in diet hypotheses do not appear to explain the shift in colour we report for Bullock's Orioles. We propose that these changes in colour are facilitated through introgression with Baltimore Orioles, and favoured by females that choose brighter, more orange males. Our study highlights the long memory of natural history collections and how they offer new insights to the dynamic roll of hybrid zones in trait evolution between interacting species.

Relocation to avoid costs: A hypothesis on red carotenoid-based signals based on recent CYP2J19 gene expression data

In many vertebrates, the enzymatic oxidation of dietary yellow carotenoids generates red keto-carotenoids giving color to ornaments. The oxidase CYP2J19 is here a key effector. Its purported intracellular location suggests a shared biochemical pathway between trait expression and cell functioning. This might guarantee the reliability of red colorations as individual quality signals independent of production costs. We hypothesize that the ornament type (feathers vs. bare parts) and production costs (probably CYP2J19 activity compromising vital functions) could have promoted tissue-specific gene relocation. We review current avian tissue-specific CYP2J19 expression data. Among the ten red-billed species showing CYP2J19 bill expression, only one showed strong hepatic expression. Moreover, a phylogenetically-controlled analysis of 25 red-colored species shows that those producing red bare parts are less likely to have strong hepatic CYP2J19 expression than species with only red plumages. Thus, both production costs and shared pathways might have contributed to the evolution of red signals.

Seasonal but not sex-biased gene expression of the carotenoid ketolase, CYP2J19, in the sexually dichromatic southern red bishop (Euplectes orix)

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids. A coloration function of CYP2J19 is supported by a seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, CYP2J19 expression was similarly high in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative roles of hepatic CYP2J19 or its products, such as detoxification or antioxidant functions. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, probably of adrenal origin and compatible with luteinizing hormone-induced and (in females) oestrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signalling of male quality, CYP2J19 expression was not related to plumage redness or male body condition.

Colours of stress in male Indian rock agamas predict testosterone levels but not performance

Rapid physiological colour change offers dynamic signalling opportunities that can reveal distinct information to receivers in different contexts. Information content in dynamic colours, however, is largely unexplored. In males of the Indian rock agama (Psammophilus dorsalis), stressful events, including male-male agonistic interactions, induce a colour change, wherein the dorsal band turns yellow and the lateral bands turn orange. We aimed to determine whether these pigment-based dynamic colours convey information about individual quality. Using an agamid-specific visual model, we first quantified the chromatic and achromatic contrasts of each colour component displayed by males during handling stress, which induces the maximal response of aggression-typical colours. We then measured baseline testosterone levels, morphology (body mass and size), and performance measures (bite force and sprint speed) of these lizards. Chromatic contrasts of the dorsal yellow and lateral orange bands, individually and relative to each other (internal pair), were negatively correlated with testosterone levels, while the chromatic contrast of the internal pair was positively correlated with body condition. The lack of an association between colour contrasts and both bite force and sprint speed indicate that the conspicuousness of colours expressed during stressful events, such as agonistic interactions, do not reveal male performance ability. Despite our expectations of a positive relationship with testosterone, morphology (body condition), and performance (bite force, sprint speed), we find that for P. dorsalis, the conspicuousness of stress-induced colours provide only some information about individual quality. We speculate that the dynamicity of physiological colours may influence their function as content-containing signals in social interactions.

Seasonal but not sex-biased gene expression of the carotenoid ketolase, CYP2J19, in the sexually dichromatic southern red bishop (Euplectes orix)

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids. A coloration function of CYP2J19 is supported by a seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, CYP2J19 expression was similarly high in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative roles of hepatic CYP2J19 or its products, such as detoxification or antioxidant functions. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, probably of adrenal origin and compatible with luteinizing hormone-induced and (in females) oestrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signalling of male quality, CYP2J19 expression was not related to plumage redness or male body condition.

Seasonal but not sex-biased gene expression of the carotenoid ketolase, CYP2J19, in the sexually dichromatic southern red bishop (Euplectes orix)

Intense red colours in birds are often owing to ketocarotenoids (KCs). In many land birds, KCs are oxidized from dietary yellow precursors, presumably by the avian carotenoid ketolase CYP2J19, the regulation and constraints of which have important implications for condition-dependence and honest signalling of carotenoid colour displays. We investigated hepatic CYP2J19 gene expression in the seasonally and sexually dichromatic southern red bishop (Euplectes orix) in relation to season, sex, progression of the prenuptial moult, testis size, body condition, redness and circulating sex steroids. A coloration function of CYP2J19 is supported by a seasonal upregulation prior to and during the carotenoid-depositing stage of the male prenuptial moult. However, CYP2J19 expression was similarly high in females (which do not moult prenuptially), and remained high in males after moult, suggesting additional or alternative roles of hepatic CYP2J19 or its products, such as detoxification or antioxidant functions. In males, the CYP2J19 upregulation preceded and was unrelated to the rise in plasma testosterone, but was correlated with androstenedione, probably of adrenal origin and compatible with luteinizing hormone-induced and (in females) oestrogen-suppressed moult. Finally, contrary to ideas that carotenoid ketolation rate mediates honest signalling of male quality, CYP2J19 expression was not related to plumage redness or male body condition.

The evolutionary history and mechanistic basis of female ornamentation in a tropical songbird

Ornamentation, such as the showy plumage of birds, is widespread among female vertebrates, yet the evolutionary pressures shaping female ornamentation remain uncertain. In part this is due to a poor understanding of the mechanistic route to ornamentation in females. To address this issue, we evaluated the evolutionary history of ornament expression in a tropical passerine bird, the White-shouldered Fairywren, whose females, but not males, strongly vary between populations in occurrence of ornamented black-and-white plumage. We first use phylogenomic analysis to demonstrate that female ornamentation is derived and that female ornamentation evolves independently of changes in male plumage. We then use exogenous testosterone in a field experiment to induce partial ornamentation in naturally unornamented females. By sequencing the transcriptome of experimentally induced ornamented and natural feathers, we identify genes expressed during ornament production and evaluate the degree to which female ornamentation in this system is associated with elevated testosterone, as is common in males. We reveal that some ornamentation in females is linked to testosterone and that sexes differ in ornament-linked gene expression. Lastly, using genomic outlier analysis we identify a candidate melanogenesis gene that lies in a region of high genomic divergence among populations that is also differentially expressed in feather follicles of different female plumages. Taken together, these findings are consistent with sex-specific selection favoring the evolution of female ornaments and demonstrate a key role for testosterone in generating population divergence in female ornamentation through gene regulation. More broadly, our work highlights similarities and differences in how ornamentation evolves in the sexes.

Environmental conditions and male quality traits simultaneously explain variation of multiple colour signals in male lizards

Male lizards often display multiple pigment‐based and structural colour signals which may reflect various quality traits (e.g. performance, parasitism), with testosterone (T) often mediating these relationships. Furthermore, environmental conditions can explain colour signal variation by affecting processes such as signal efficacy, thermoregulation and camouflage. The relationships between colour signals, male quality traits and environmental factors have often been analysed in isolation, but simultaneous analyses are rare. Thus, the response of multiple colour signals to variation in all these factors in an integrative analysis remains to be investigated.

Here, we investigated how multiple colour signals relate to their information content, examined the role of T as a potential mediator of these relationships and how environmental factors explain colour signal variation.

We performed an integrative study to examine the covariation between three colour signals (melanin‐based black, carotenoid‐based yellow–orange and structural UV), physiological performance, parasitism, T levels and environmental factors (microclimate, forest cover) in male common lizards Zootoca vivipara from 13 populations.

We found that the three colour signals conveyed information on different aspects of male condition, supporting a multiple message hypothesis. T influenced only parasitism, suggesting that T does not directly mediate the relationships between colour signals and their information content. Moreover, colour signals became more saturated in forested habitats, suggesting an adaptation to degraded light conditions, and became generally brighter in mesic conditions, in contradiction with the thermal melanism hypothesis.

We show that distinct individual quality traits and environmental factors simultaneously explain variations of multiple colour signals with different production modes. Our study therefore highlights the complexity of colour signal evolution, involving various sets of selective pressures acting at the same time, but in different ways depending on colour production mechanism.

Drivers of phenotypic divergence in a Mesoamerican highland bird

Animals derive their coloration from a variety of pigments as well as non-pigmentary structural features. One of the most widespread types of pigments are carotenoids, which are used by all invertebrate taxa and most vertebrate orders to generate red, pink, orange and yellow coloration. Despite their widespread use by diverse animal groups, animals obligately obtain carotenoid pigments from diet. Carotenoid-based coloration is therefore modulated by evolutionary and ecological processes that affect the acquisition and deposition of these pigments into tegumentary structures. The Flame-colored Tanager (Piranga bidentata) is a highland songbird in the cardinal family (Cardinalidae) that is distributed from Mexican sierras through Central America up to western Panama. While female plumage throughout its entire range is predominantly yellow, males exhibit a noticeable split in ventral plumage color, which is bright orange on the West slope and the Tres Marias Islands and blood red in Eastern Mexico and Central America. We used Multiple Regression on Matrices (MRM) to evaluate the relative contributions of geographic distance, climate and genetic distance on color divergence and body differences between geographically disjunct populations. We found that differentiation in carotenoid plumage coloration was mainly explained by rainfall differences between disjunct populations, whereas body size differences was best explained by variation in the annual mean temperature and temperature of coldest quarter. These results indicate that climate is a strong driver of phenotypic divergence in Piranga bidentata.

Testosterone, signal coloration, and signal color perception in male zebra finch contests

Many animals use assessment signals to resolve contests over limited resources while minimizing the costs of those contests. The carotenoid-based orange to red bills of male zebra finches (Taeniopygia guttata) are thought to function as assessment signals in male-male contests, but behavioral analyses relating contest behaviors and outcomes to bill coloration have yielded mixed results. We examined the relationship between bill color and contests while incorporating measurements of color perception and testosterone (T) production, for an integrative view of aggressive signal behavior, production, and perception. We assayed the T production capabilities of 12 males in response to a gonadotropin-releasing hormone (GnRH) challenge. We then quantified the initiation, escalation, and outcome of over 400 contests in the group, and measured bill color using calibrated photography. Finally, because signal perception can influence signal function, we tested how males perceive variation in bill coloration, asking if males exhibit categorical perception of bill color, as has been shown recently in female zebra finches. The data suggest that males with greater T production capabilities than their rivals were more likely to initiate contests against those rivals, while males with redder bills than their rivals were more likely to win contests. Males exhibited categorical color perception, but individual variation in the effect of categorical perception on color discrimination abilities did not predict any aspects of contest behavior or outcomes. Our results are consistent with the hypotheses that T plays a role in zebra finch contests and that bill coloration functions as an aggressive signal. We suggest future approaches, based on animal contest theory, for how links among signals, perception, and assessment can be tested.

Androgen Elevation Accelerates Reproductive Senescence in Three-Spined Stickleback

Costs of reproduction shape the life-history evolution of investment in current and future reproduction and thereby aging. Androgens have been proposed to regulate the physiology governing these investments. Furthermore, androgens are hypothesized to play a central role in carotenoid-dependent sexual signaling, regulating how much carotenoids are diverted to ornamentation and away from somatic maintenance, increasing oxidative stress, and accelerating aging. We investigated these relationships in male three-spined stickleback in which we elevated 11-ketotestosterone and supplied vitamin E, an antioxidant, in a 2 × 2 design. Androgen elevation shortened the time stickleback maintained reproductive activities. We suspect that this effect is caused by 11-ketotestosterone stimulating investment in current reproduction, but we detected no evidence for this in our measurements of reproductive effort: nest building, body composition, and breeding coloration. Carotenoid-dependent coloration was even slightly decreased by 11-ketotestosterone elevation and was left unaffected by vitamin E. Red coloration correlated with life expectancy and reproductive capacity in a quadratic manner, suggesting overinvestment of the individuals exhibiting the reddest bellies. In contrast, blue iris color showed a negative relationship with survival, suggesting physiological costs of producing this aspect of nuptial coloration. In conclusion, our results support the hypothesis that androgens regulate investment in current versus future reproduction, yet the precise mechanisms remain elusive. The quadratic relationships between sexual signal expression and aspects of quality have wider consequences for how we view sexual selection on ornamentation and its relationship with aging.

Information out of the blue: phenotypic correlates of abdominal color patches in Sceloporus lizards

Colorful ornaments are important visual signals for animal communication that can provide critical information about the quality of the signaler. In this study, we focused on different color characteristics of the abdominal patches of males of six lizard species from the genus Sceloporus. We addressed three main objectives. First, we examined if size, brightness, saturation, and conspicuousness of these ornaments are indicative of body size, condition, immune function, or levels of testosterone and corticosterone. Second, we evaluated if the distinct components of these abdominal patches (blue or green patches and black stripes) transmit similar information about the signaler, which would support the redundant signal hypothesis, or if these components are related to different phenotypic traits, which would support the multiple message hypothesis. Third, we compared the phenotypic correlates of these ornaments among our six species to understand the degree of conservatism in the signaling patterns or to find species-specific signals. Using data collected from males in natural conditions and a multi-model inference framework, we found that in most species the area of the patches and the brightness of the blue component are positively related to body size. Thus, these color characteristics are presumably indicative of the physical strength and competitive ability of males and these shared signals were likely inherited from a common ancestor. In half of the species, males in good body condition also exhibit relatively larger blue and black areas, suggesting that the expression of these ornaments is condition-dependent. Abdominal patches also provide information about immunocompetence of the males as indicated by different correlations between certain color characteristics and ectoparasite load, counts of heterophils, and the heterophil:lymphocyte ratio. Our findings reveal that area and brightness of the abdominal patches signal the size and body condition of males, whereas blue saturation and conspicuousness with respect to the surrounding substrate are indicative of immune condition, thus supporting the multiple message hypothesis. However, some of these correlations were not shared by all species and, hence, point to intriguing species-specific signals.

Development and genetics of red coloration in the zebrafish relative Danio albolineatus

Animal pigment patterns play important roles in behavior and, in many species, red coloration serves as an honest signal of individual quality in mate choice. Among Danio fishes, some species develop erythrophores, pigment cells that contain red ketocarotenoids, whereas other species, like zebrafish (D. rerio) only have yellow xanthophores. Here, we use pearl danio (D. albolineatus) to assess the developmental origin of erythrophores and their mechanisms of differentiation. We show that erythrophores in the fin of D. albolineatus share a common progenitor with xanthophores and maintain plasticity in cell fate even after differentiation. We further identify the predominant ketocarotenoids that confer red coloration to erythrophores and use reverse genetics to pinpoint genes required for the differentiation and maintenance of these cells. Our analyses are a first step toward defining the mechanisms underlying the development of erythrophore-mediated red coloration in Danio and reveal striking parallels with the mechanism of red coloration in birds.

Sexual selection and introgression in avian hybrid zones: Spotlight on Manacus

Hybrid zones offer a window into the processes and outcomes of evolution, from species formation or fusion to genomic underpinnings of specific traits and isolating mechanisms. Sexual selection is believed to be an important factor in speciation processes, and hybrid zones present special opportunities to probe its impact. The manakins (Aves, Pipridae) are a promising group in which to study the interplay of sexual selection and natural hybridization: they show substantial variation across the family in the strength of sexual selection they experience, they readily hybridize within and between genera, and they appear to have formed hybrid species, a rare event in birds. A hybrid zone between two manakins in the genus Manacus is unusual in that plumage and behavioral traits of one species have introgressed asymmetrically into populations of the second species through positive sexual selection, then apparently stalled at a river barrier. This is one of a handful of documented examples of asymmetric sexual trait introgression with a known selective mechanism. It offers opportunities to examine reproductive isolation, introgression, plumage color evolution, and natural factors enhancing or constraining the effects of sexual selection in real time. Here, we review previous work in this system, propose new hypotheses for observed patterns, and recommend approaches to test them.

A review and assessment of the Shared-Pathway Hypothesis for the maintenance of signal honesty in red ketocarotenoid-based coloration

For decades, scientists have noted connections between individual condition and carotenoid-based coloration in terrestrial and aquatic animals. Organisms that produce more vibrant carotenoid-based coloration tend to have better physiological performance and behavioral displays compared to less colorful members of the same species. Traditional explanations for this association between ornamental coloration and performance invoked the need for color displays to be costly, but evidence for such hypothesized costs is equivocal. An alternative explanation for the condition-dependence of carotenoid-based coloration, the Shared-Pathway Hypothesis, was developed in response. This hypothesis proposes that red ketocarotenoid-based coloration is tied to core cellular processes involving a shared pathway with mitochondrial energy metabolism, making the concentration of carotenoids an index of mitochondrial function. Since the presentation of this hypothesis, empirical tests of the mechanisms proposed therein have been conducted in many species. In this manuscript, we review the Shared-Pathway Hypothesis and the growing number of studies that have investigated a connection between carotenoid-based coloration and mitochondrial function. We also discuss future strategies for assessing the Shared-Pathway Hypothesis to more effectively disentangle evidence that may simultaneously support evidence of carotenoid-resource tradeoffs.

Extensive hybridization reveals multiple coloration genes underlying a complex plumage phenotype

Coloration is an important target of both natural and sexual selection. Discovering the genetic basis of colour differences can help us to understand how this visually striking phenotype evolves. Hybridizing taxa with both clear colour differences and shallow genomic divergences are unusually tractable for associating coloration phenotypes with their causal genotypes. Here, we leverage the extensive admixture between two common North American woodpeckers-yellow-shafted and red-shafted flickers-to identify the genomic bases of six distinct plumage patches involving both melanin and carotenoid pigments. Comparisons between flickers across approximately 7.25 million genome-wide SNPs show that these two forms differ at only a small proportion of the genome (mean F_ST = 0.008). Within the few highly differentiated genomic regions, we identify 368 SNPs significantly associated with four of the six plumage patches. These SNPs are linked to multiple genes known to be involved in melanin and carotenoid pigmentation. For example, a gene (CYP2J19) known to cause yellow to red colour transitions in other birds is strongly associated with the yellow versus red differences in the wing and tail feathers of these flickers. Additionally, our analyses suggest novel links between known melanin genes and carotenoid coloration. Our finding of patch-specific control of plumage coloration adds to the growing body of literature suggesting colour diversity in animals could be created through selection acting on novel combinations of coloration genes.

Testosterone regulates CYP2J19-linked carotenoid signal expression in male red-backed fairywrens (Malurus melanocephalus)

Carotenoid pigments produce most red, orange and yellow colours in vertebrates. This coloration can serve as an honest signal of quality that mediates social and mating interactions, but our understanding of the underlying mechanisms that control carotenoid signal production, including how different physiological pathways interact to shape and maintain these signals, remains incomplete. We investigated the role of testosterone in mediating gene expression associated with a red plumage sexual signal in red-backed fairywrens (Malurus melanocephalus). In this species, males within a single population can flexibly produce either red/black nuptial plumage or female-like brown plumage. Combining correlational analyses with a field-based testosterone implant experiment and quantitative polymerase chain reaction, we show that testosterone mediates expression of carotenoid-based plumage in part by regulating expression of CYP2J19, a ketolase gene associated with ketocarotenoid metabolism and pigmentation in birds. This is, to our knowledge, the first time that hormonal regulation of a specific genetic locus has been linked to carotenoid production in a natural context, revealing how endocrine mechanisms produce sexual signals that shape reproductive success.