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Hollow melanosome morphology in iridescent feathers. Images show representative individuals and corresponding TEM images of feather barbule cross sections in violet-backed starlings (a,b) and wild turkeys (c,d ). Insets in (b) and (d) are fast-Fourier transforms (FFTs) of regions of interest (yellow boxes) and upper barbule surface ( portion including top stack of melanosome layers). Scale bars, 500 nm. Photo credits: (a) Ken Clifton, (c) anonymous.
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Developmental constraints and trade-offs can limit diversity, but organisms have repeatedly evolved morphological innovations that overcome these limits by expanding the range and functionality of traits. Iridescent colours in birds are commonly produced by melanin-containing organelles (melanosomes) organized into nanostructured arrays within feat...
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... further confirm nanostructural details, we examined longitudinal sections of unprepared barbules with scanning electron microscopy (SEM). From the TEM images, we measured the following parameters known to be sig- nificant to colour production on two to four barbule regions per individual per species: air space radius in the interior of melano- somes (r air ), melanosome radius (r mel ), thickness of the keratin cortex taken at 10 locations along the barbule edge and number of melanosome layers perpendicular to the feather surface (see electronic supplementary material, figure S1 for sche- matic). The wavelength of peak reflectance (hue) in PCs is largely determined by the spacing between particle centres (lattice con- stant a), whereas brightness is a function of the refractive index contrast (n high /n low ) and relative proportion of low-index material (openness) [15]. ...
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... size esti- mates based on angle-resolved measurements matched the TEM values fairly well (see electronic supplementary material, appendix S5 and table S2). Fast-Fourier transforms (FFTs) of iso- lated regions of melanosomes indicated hexagonal periodicity rspb.royalsocietypublishing.org Proc R Soc B 280 : 20131505 for both species ( figure 1b,d, left insets). However, FFTs of the whole-barbule surface (the complete nanostructure) differed. ...
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... the openness of close-packed hollow melanosomes is determined not only by the spacing between but also by the amount of air within melanosomes, these structures can pro- duce bright colours even when close-packed (figures 3 and 4b). In turn, this configuration produces strong nanoscale order- ing ( figure 1b,d) and, in some cases, remarkable colour changes with angle ( figure 2d). Thus, because close-packed nanostruc- tures are more thermodynamically stable and therefore more likely to form by self-assembly [24], bright nanostructures may evolve more frequently in lineages with hollow melanosomes. ...
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... morphological and spectral results confirmed that hexagonal arrangements of hollow melanosomes in feather bar- bules act as two-dimensional PCs. First, TEM and SEM showed that melanosomes are air-filled and cylindrical (see figure 1 and electronic supplementary material, figure S2). Second, spectral results revealed that primary and secondary reflectance peaks for TE-polarized light were much broader and taller in both species than for TM-polarized light ( figure 2a,b), matching the photonic band structure prediction (see electronic supplemen- tary material, figure S5) and agreeing with theoretical results for similar photonic structures containing air [15]. ...
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... did not observe an abrupt shift in colour for starlings as we observed in the turkey; rather, the primary peak remained visible over a wide range of observation angles ( figure 2c). This may be because starling barbules are strongly curved, whereas turkey barbules are almost flat ( figure 1b,d, right insets). Barbule curvature influences the orientation of mel- anosome layers with respect to the observer and varies along the barbule surface. ...
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Citations
... Additionally, these studies explore shell thicknesses less than 20−25 nm, although thicker shells (33−93 nm) are prevalent in hollow melanosomes found in bird feathers. 16 Previously, Eliason et al. investigated how hollow melanosomes affect structural color production in birds, but the study was limited to hexagonal close-packed assemblies and lacked an experimental component. 16 The present study uses both experiments and simulations to examine the significance of the hollow spherical structure and thickness of the melanin shell to create structural colors relative to core−shell and solid nanoparticles in random closepacked assemblies. ...
... 16 Previously, Eliason et al. investigated how hollow melanosomes affect structural color production in birds, but the study was limited to hexagonal close-packed assemblies and lacked an experimental component. 16 The present study uses both experiments and simulations to examine the significance of the hollow spherical structure and thickness of the melanin shell to create structural colors relative to core−shell and solid nanoparticles in random closepacked assemblies. First, hollow nanoparticles with varying thicknesses of the PDA shell (38−96 nm) were prepared by single-step polymerization of dopamine onto PS templates. ...
... 20 The PDA shell thicknesses examined in the present study (30−100 nm) fall within the shell thickness range observed in natural melanosomes. 16 The thick PDA shells also impart stability to hollow PDA nanoparticles, making them suitable for a wide range of applications, including structural colors, drug delivery, and catalysis. ...
Hollow melanosomes found in iridescent bird feathers, including violet-backed starlings and wild turkeys, enable the generation of diverse structural colors. It has been postulated that the high refractive index (RI) contrast between melanin (1.74) and air (1.0) results in brighter and more saturated colors. This has led to several studies that have synthesized hollow synthetic melanin nanoparticles and fabricated colloidal nanostructures to produce synthetic structural colors. However, these studies use hollow nanoparticles with thin shells (<20 nm), even though shell thicknesses as high as 100 nm have been observed in natural melanosomes. Here, we combine experimental and computational approaches to examine the influence of the varying polydopamine (PDA, synthetic melanin) shell thickness (0–100 nm) and core material on structural colors. Experimentally, a concomitant change in overall particle size and RI contrast makes it difficult to interpret the effect of a hollow or solid core on color. Thus, we utilize finite-difference time-domain (FDTD) simulations to uncover the effect of shell thickness and core on structural colors. Our FDTD results highlight that hollow particles with thin shells have substantially higher saturation than same-sized solid and core–shell particles. These results would benefit a wide range of applications including paints, coatings, and cosmetics.
... Even though melanin is a dark pigment and it is responsible for colors ranging from black to greys and browns, some of the brightest colors in nature are produced by nanoscale arrangements of melanosomes (Maia et al. 2013;Eliason et al. 2013). Further, melanosomes vary greatly in shape and size, and this diversity is particularly large in birds (see above; Fig. 3) where we also see some of the best examples of brilliant iridescence. ...
... Hollow melanosomes introduce an additional low refractive index material (air) into organized nanostructures and thereby produce even brighter colors (Eliason et al. 2013). These types of melanosomes have independently evolved numerous times in some lineages with bright colors (e.g. ...
Many fungi produce melanins, which can play critical roles in virulence and pathogenicity. In fungal pathogenic species, melanin confers protection against host immune defense mechanisms that involve both chemical and physical antimicrobial defense strategies. In this piece, we review the different types of melanin produced by fungi, their structure and localization in the fungal cell, and how melanin interacts with the host to aid the fungus during infection and persistence.
... Even though melanin is a dark pigment and it is responsible for colors ranging from black to greys and browns, some of the brightest colors in nature are produced by nanoscale arrangements of melanosomes (Maia et al. 2013;Eliason et al. 2013). Further, melanosomes vary greatly in shape and size, and this diversity is particularly large in birds (see above; Fig. 3) where we also see some of the best examples of brilliant iridescence. ...
... Hollow melanosomes introduce an additional low refractive index material (air) into organized nanostructures and thereby produce even brighter colors (Eliason et al. 2013). These types of melanosomes have independently evolved numerous times in some lineages with bright colors (e.g. ...
Most organisms can synthesize a variety of natural polymers called melanins. These substances serve protective roles against physical and chemical stressors. These products result from the enzyme-catalyzed oxidation of phenolic and indolic substrates which polymerize to produce melanins such as eumelanin, pheomelanin, pyomelanin, and allomelanins. Tyrosinase and laccase protein families are primarily involved in the production of melanin. The pharmaceutical, cosmetic, optical, and electrical industries all use melanins as functional polymeric materials. The development of biotechnological processes to produce melanins is becoming an attractive alternative compared to their extraction from plant or animal matter, in which they are only present at low concentrations. Numerous types of bacteria are naturally capable of producing melanin. Using genetic engineering techniques, it is currently possible to overexpress in microorganisms the genes for melanin-producing enzymes. These advancements have allowed increasing the productivity of melanogenic organisms and have enabled the creation of novel recombinant microbial strains that can synthesize a variety of melanins. Furthermore, strains capable of completely synthesizing melanins from basic carbon sources on a gram-scale basis have been developed by metabolic engineering of microbial hosts through altering pathways relevant to the availability of melanogenic precursors. The most recent discoveries in the development of recombinant melanin-producing strains and manufacturing methods are compiled and reviewed in this chapter.
... If the shape and/or arrangement of internal nanostructure is anisotropic, the periodicity that the traveling light experiences through the internal structure varies depending on the incident angle. Thus, the pattern of light scattering and the outcome of interference change with the incident angle and iridescence, the change of light properties depending on the viewing angle, is observed [9][10][11][12]. ...
The tail feathers of magpies are iridescent, with hues ranging from navy to violet and green. It has been previously shown that the hexagonal arrangement of melanosomes in the distal barbules is responsible for these colors, but previous simulation models have relied on average values for the parameters associated with this arrangement (e.g., periodicity), and it remains to be studied whether the actual (rather than averaged) structural arrangement and its inherent irregularities reliably predict structural color. Previous studies using unmodified images for the analysis have not focused on the effect of such irregularities on the color production. In this study, we conducted finite-difference time-domain (FDTD) simulations using actual transmission electron microscopy (TEM) images obtained from the distal barbules of a magpie tail feather, compared the reflectance spectra predicted using the FDTD simulation with those measured with a spectrometer, and found a substantial discrepancy between the two. Fourier analysis suggests that the non-uniform arrangement of the melanosomes within the barbule is responsible for this discrepancy by creating variation in the periodicity. Our results suggest that a simple model in which the parameters for internal structures are averaged cannot fully explain the variation in the structural colors observed in biological samples such as the feathers of birds.
... More saturated colours can be produced by increasing the difference between the refractive indices, the degree of order or the thickness of optical nanostructures [29][30][31][32]. Moreover, the varying shapes of the melanosomes (solid rods, hollow rods, solid platelets or hollow platelets) also play a significant role in producing structural colours, as they can vary refractive index profiles [32][33][34][35]. ...
... 6 nm) of blue and green barbules. The radius of a melanosome and the interspatial distance affect the structural colour because they change the refractive index profile of the crystalline melanosome layers [34]. By contrast, the thickness of the keratin cortex affects the structural colour because it provides a refractive index gradient in combination with the melanosomes and causes thin film interference [15,16,36,39,44]. ...
The bright, saturated iridescent colours of feathers are commonly produced by single and multi-layers of nanostructured melanin granules (melanosomes), air and keratin matrices, surrounded by an outer keratin cortex of varying thicknesses. The role of the keratin cortex in colour production remains unclear, despite its potential to act as a thin film or absorbing layer. We use electron microscopy, optical simulations and oxygen plasma-mediated experimental cortex removal to show that differences in keratin cortex thickness play a significant role in producing colours. The results indicate that keratin cortex thickness determines the position of the major reflectance peak (hue) from nanostructured melanosomes of common pheasant (Phasianus colchicus) feathers. Specifically, the common pheasant has appropriate keratin cortex thickness to produce blue and green structural colours. This finding identifies a general principle of structural colour production and sheds light on the processes that shaped the evolution of brilliant iridescent colours in the common pheasant.
... However, the morphology of melanosomes varies between iridescent plums of different species [14]. The iridescent color comes from the melanin-containing melanosome, which is organized in an array form [15]. Melanin color is the standard color in the animal world and is significant at the pigmentary stage. The color concentration is illustrated as a substantial deviation in the brightness of the breast plumage [16]. ...
This manuscript explores the topological and optical properties of a Passeriformes bird feather. Inside the feather, the layers of keratin and melanin are responsible for light reflection,
transmission, and absorption; notably, the miniature composition of melanosome barbules plays a crucial role in its reflective properties. We adopted a multilayer interference model to investigate light propagation throughout the Passeriformes plume. As a result, we obtained all necessary simulated
results, such as resonance band, efficiency, and electromagnetic radiation patterns of the Passeriformes plume, and they were verified with the experimental results reported in the literature study regarding light reflectivity through its internal geometry. Interestingly, we discovered that the interior structure of the Passeriformes plume functions similarly to a UV reflector antenna.
... By contrast, when arranged at a nanometre scale, they can produce bright iridescent colours across the visible spectrum by coherent light scattering (Prum 2006). More importantly, both visible and NIR reflectance of feathers can be affected not only by melanin chemistry in feathers, but also by its morphology and spatial arrangement (Eliason et al. 2013), while melanin content can determine radiative heat gain (Rogalla et al. 2021a). Although in some species melanin pigmentation can be associated with aspects of individual quality (e.g. ...
Plumage coloration can have substantial effects on a bird's energy budget. This is because different colours reflect and absorb light differently, affecting the heat loads acquired from solar radiation. We examine the thermal effects of feather coloration on solar heat gain and flight performance and discuss the potential role of plumage colour on a bird's energy budget. Early investigations of the effects of plumage colour on thermoregulation revealed complex interactions between environmental conditions and physical properties of the plumage that may have led to diverse behavioural and physiological adaptations of birds to their thermal environment. While darker feather surfaces absorb more light, and heat more, than light‐coloured surfaces under exposure to the sun, this relationship is not always straightforward when considering heat transfer to the skin. Heat transfer through plumage varies depending on multiple factors, such as feather density and transmission of light. For instance, higher transmissivity of light‐coloured plumage can increase heat loads reaching skin level, while conduction and convection transfer heat from the surface of dark feathers to the skin. Solar heating can affect the metabolic costs of maintaining a constant body temperature, and depending on environmental conditions, colours can have either a positive or negative effect on a bird's energy budget. More specifically, solar heating can be advantageous in the cold but may increase the energetic costs associated with thermoregulation when ambient temperature is high. More recent studies have further suggested that the thermal properties of feather coloration might reduce the energetic costs of flight. This is because surface heating can affect the ratio between lift and drag on a wing. As concluding remarks, we provide future directions for new lines of research that will aid in improving our understanding of the thermal effects of feather coloration on a bird's energy budget, which can potentially explain factors driving colour evolution and distribution patterns in birds.
... Barbule structural color permits the production of any peak-reflected wavelength by varying the thickness of melanosome arrays, which can produce a diversity of single-peak spectra-hues, such as the unusual diversity of greens, blues, and blue + greens seen in hummingbirds (Fig. 2b). Hummingbird melanosomes are among the most unusual in birds in being both disc-shaped and air-filled [9][10][11][12][13]23 . The air in the center of hummingbird melanosomes approaches the maximum possible biological difference in refractive index (air = 1.0, melanin =~1.7), which results in the efficient production of brilliant colors with the fewest layers of melanosomes, such that resulting spectra are narrow and near saturation 13,24 . ...
A color gamut quantitatively describes the diversity of a taxon’s integumentary coloration as seen by a specific organismal visual system. We estimated the plumage color gamut of hummingbirds (Trochilidae), a family known for its diverse barbule structural coloration, using a tetrahedral avian color stimulus space and spectra from a taxonomically diverse sample of 114 species. The spectra sampled occupied 34.2% of the total diversity of colors perceivable by hummingbirds, which suggests constraints on their plumage color production. However, the size of the hummingbird color gamut is equivalent to, or greater than, the previous estimate of the gamut for all birds. Using the violet cone type visual system, our new data for hummingbirds increases the avian color gamut by 56%. Our results demonstrate that barbule structural color is the most evolvable plumage coloration mechanism, achieving unique, highly saturated colors with multi-reflectance peaks. An analysis of the plumage colors in 114 hummingbird species finds that the breadth of the hummingbird color gamut exceeds or is equivalent to that of the previous estimate of all living birds. These data for hummingbirds increases the known avian color gamut by 56%.
... Micrographs (B) and (D) adapted with permission from[259,260]; © Elsevier, 2009. Image A and micrograph C adapted with permission from[261]; © PLoS, 2011. ...
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... Iridescent plumage color is produced by coherent light scattering from periodic ordered stacks of melanosomes within keratin. Based on extensive studies of the melanosomes in iridescent feathers in extant birds, melanosomes that create iridescent plumage colors are broadly classified into four types: solid cylindrical, solid flattened, hollow cylindrical and hollow flattened [2][3][4]. The flat and hollow forms are found exclusively in iridescent feathers [4]. ...
... hollow melanosomes introduce air with extremely low RI = ∼1.0, and therefore produce brighter colors owing to a higher RI contrast than the solid melanosomes [3]. It has been shown that birds with more complex melanosomes and variable melanosome types forming the photonic crystals could increase the range of color variability [2,4], e.g. the colorful hummingbirds that contain the most complicated hollow and flattened melanosomes [2]. ...
A unique form of melanosomes contributing to brilliant iridescent colors in modern bird feathers, previously unknown in fossil birds, is identified in the Early Cretaceous bird Eoconfuciusornis. The discovery highlights the complexity of plumage color nanostructures utilized early in bird evolution as far back as 130 million years ago.
