Interactions between avian viruses and skin in farm birds

This article reviews the avian viruses that infect the skin of domestic farm birds of primary economic importance: chicken, duck, turkey, and goose. Many avian viruses (e.g., poxviruses, herpesviruses, Influenza viruses, retroviruses) leading to pathologies infect the skin and the appendages of these birds. Some of these viruses (e.g., Marek's disease virus, avian influenza viruses) have had and/or still have a devasting impact on the poultry economy. The skin tropism of these viruses is key to the pathology and virus life cycle, in particular for virus entry, shedding, and/or transmission. In addition, for some emergent arboviruses, such as flaviviruses, the skin is often the entry gate of the virus after mosquito bites, whether or not the host develops symptoms (e.g., West Nile virus). Various avian skin models, from primary cells to three-dimensional models, are currently available to better understand virus-skin interactions (such as replication, pathogenesis, cell response, and co-infection). These models may be key to finding solutions to prevent or halt viral infection in poultry.

Identifying and locating carotenoids in supra-orbital combs of male black grouse (Lyurus tetrix) using Raman and transmission electron microscopy: A histological study using rehydrated tissue samples

Colourful signals have long been implicated as indicators of individual quality in animals. Bare-skin signals are an understudied aspect of avian colouration compared with plumage studies, despite displaying rapid changes in size and colour in response to different environmental or physiological stressors. Even fewer studies have focused on the underlying histology of these structures and the importance this plays in the resulting skin colour. Using the Black Grouse (Lyrurus tetrix), we identified the underlying structure of individual dermal spikes, which make up the red supra-orbital comb (a known integumentary signal of male quality), and highlight visual structural differences between combs of different sizes. In addition, we used Raman spectroscopy to indicate the presence of carotenoids within the tissue, something that had previously only been inferred through characteristic reflectance patterns. An increased understanding of the structural basis of colour of featherless parts of the skin opens up exciting new avenues for interpreting the information content of integumentary signals.

Phylogenetic and ecological reevaluation of the order Onygenales

The order Onygenales is classified in the class Eurotiomycetes of the subphylum Pezizomycotina. Families in this order have classically been isolated from soil and dung, and two lineages contain causative agents of superficial, cutaneous and systemic infections in mammals. The ecology and habitat choices of the species are driven mainly by the keratin and cellulose degradation abilities. The present study aimed to investigate whether the ecological trends of the members of Onygenales can be interpreted in an evolutionary sense, linking phylogenetic parameters with habitat preferences, to achieve polyphasic definitions of the main taxonomic groups. Evolutionary processes were estimated by multiple gene genealogies and divergence time analysis. Previously described families, namely, Arthrodermataceae, Ajellomycetaceae, Ascosphaeraceae, Eremascaceae, Gymnoascaceae, Onygenaceae and Spiromastigoidaceae, were accepted in Onygenales, and two new families, Malbrancheaceae and Neogymnomycetaceae, were introduced. A number of species could not be assigned to any of the defined families. Our study provides a revised overview of the main lines of taxonomy of Onygenales, supported by multilocus analyses of ITS, LSU, TUB, TEF1, TEF3, RPB1, RPB2, and ribosomal protein 60S L10 (L1) (RP60S) sequences, combined with available data on ecology, physiology, morphology, and genomics.

Properties, Genetics and Innate Immune Function of the Cuticle in Egg-Laying Species

Cleidoic eggs possess very efficient and orchestrated systems to protect the embryo from external microbes until hatch. The cuticle is a proteinaceous layer on the shell surface in many bird and some reptile species. An intact cuticle forms a pore plug to occlude respiratory pores and is an effective physical and chemical barrier against microbial penetration. The interior of the egg is assumed to be normally sterile, while the outer eggshell cuticle hosts microbes. The diversity of the eggshell microbiome is derived from both maternal microbiota and those of the nesting environment. The surface characteristics of the egg, outer moisture layer and the presence of antimicrobial molecules composing the cuticle dictate constituents of the microbial communities on the eggshell surface. The avian cuticle affects eggshell wettability, water vapor conductance and regulates ultraviolet reflectance in various ground-nesting species; moreover, its composition, thickness and degree of coverage are dependent on species, hen age, and physiological stressors. Studies in domestic avian species have demonstrated that changes in the cuticle affect the food safety of eggs with respect to the risk of contamination by bacterial pathogens such as Salmonella and Escherichia coli. Moreover, preventing contamination of internal egg components is crucial to optimize hatching success in bird species. In chickens there is moderate heritability (38%) of cuticle deposition with a potential for genetic improvement. However, much less is known about other bird or reptile cuticles. This review synthesizes current knowledge of eggshell cuticle and provides insight into its evolution in the clade reptilia. The origin, composition and regulation of the eggshell microbiome and the potential function of the cuticle as the first barrier of egg defense are discussed in detail. We evaluate how changes in the cuticle affect the food safety of table eggs and vertical transmission of pathogens in the production chain with respect to the risk of contamination. Thus, this review provides insight into the physiological and microbiological characteristics of eggshell cuticle in relation to its protective function (innate immunity) in egg-laying birds and reptiles.

Body coloration and mechanisms of colour production in Archelosauria: the case of deirocheline turtles

Animal body coloration is a complex trait resulting from the interplay of multiple mechanisms. While many studies address the functions of animal coloration, the mechanisms of colour production still remain unknown in most taxa. Here we compare reflectance spectra, cellular, ultra- and nano-structure of colour-producing elements, and pigment types in two freshwater turtles with contrasting courtship behaviour, Trachemys scripta and Pseudemys concinna. The two species differ in the distribution of pigment cell-types and in pigment diversity. We found xanthophores, melanocytes, abundant iridophores and dermal collagen fibres in stripes of both species. The yellow chin and forelimb stripes of both P. concinna and T. scripta contain xanthophores and iridophores, but the post-orbital regions of the two species differ in cell-type distribution. The yellow post-orbital region of P. concinna contains both xanthophores and iridophores, while T. scripta has only xanthophores in the yellow-red postorbital/zygomatic regions. Moreover, in both species, the xanthophores colouring the yellow-red skin contain carotenoids, pterins and riboflavin, but T. scripta has a higher diversity of pigments than P. concinna. Trachemys s. elegans is sexually dichromatic. Differences in the distribution of pigment cell types across body regions in the two species may be related to visual signalling but do not match predictions based on courtship position. Our results demonstrate that archelosaurs share some colour production mechanisms with amphibians and lepidosaurs (i.e. vertical layering/stacking of different pigment cell types and interplay of carotenoids and pterins), but also employ novel mechanisms (i.e. nano-organization of dermal collagen) shared with mammals.

Olfactory Communication via Microbiota: What Is Known in Birds?

Animal bodies harbour a complex and diverse community of microorganisms and accumulating evidence has revealed that microbes can influence the hosts' behaviour, for example by altering body odours. Microbial communities produce odorant molecules as metabolic by-products and thereby modulate the biochemical signalling profiles of their animal hosts. As the diversity and the relative abundance of microbial species are influenced by several factors including host-specific factors, environmental factors and social interactions, there are substantial individual variations in the composition of microbial communities. In turn, the variations in microbial communities would consequently affect social and communicative behaviour by influencing recognition cues of the hosts. Therefore, microbiota studies have a great potential to expand our understanding of recognition of conspecifics, group members and kin. In this review, we aim to summarize existing knowledge of the factors influencing the microbial communities and the effect of microbiota on olfactory cue production and social and communicative behaviour. We concentrate on avian taxa, yet we also include recent research performed on non-avian species when necessary.

Candidate genes for carotenoid coloration in vertebrates and their expression profiles in the carotenoid-containing plumage and bill of a wild bird

Carotenoid-based coloration has attracted much attention in evolutionary biology owing to its role in honest, condition-dependent signalling. Knowledge of the genetic pathways that regulate carotenoid coloration is crucial for an understanding of any trade-offs involved. We identified genes with potential roles in carotenoid coloration in vertebrates via (i) carotenoid uptake (SR-BI, CD36), (ii) binding and deposition (StAR1, MLN64, StAR4, StAR5, APOD, PLIN, GSTA2), and (iii) breakdown (BCO2, BCMO1). We examined the expression of these candidate loci in carotenoid-coloured tissues and several control tissues of the red-billed quelea (Quelea quelea), a species that exhibits a male breeding plumage colour polymorphism and sexually dimorphic variation in bill colour. All of the candidate genes except StAR1 were expressed in both the plumage and bill of queleas, indicating a potential role in carotenoid coloration in the quelea. However, no differences in the relative expression of any of the genes were found among the quelea carotenoid phenotypes, suggesting that other genes control the polymorphic and sexually dimorphic variation in carotenoid coloration observed in this species. Our identification of a number of potential carotenoid genes in different functional categories provides a critical starting point for future work on carotenoid colour regulation in vertebrate taxa.

Water exchange and permeability properties of the skin in three species of amphibious sea snakes (Laticauda spp.)

Evolutionary transitions between different environmental media such as air and water pose special problems with respect to skin permeability because of the dramatic changes in the driving gradients and nature of water exchange processes. Also, during the transitional periods prior to complete adaptation to a new medium, the skin is exposed to two very different sets of environmental conditions. Here, we report new data for transepidermal evaporative water loss (TEWL) and cutaneous resistance to evaporative water loss (Rs) of sea snakes that are transitional in the sense of being amphibious and semi-terrestrial. We investigated three species of sea kraits (Elapidae: Laticaudinae) that are common to Orchid Island (Lanyu),Taiwan. Generally, Rs of all three species is lower than that characteristic of terrestrial/xeric species of snakes measured in other taxa. Within Laticauda, Rs is significantly greater (TEWL lower) in the more terrestrial species and lowest (TEWL highest)in the more aquatic species. Previously reported losses of water from snakes kept in seawater exhibit a reversed trend, with lower rates of loss in the more aquatic species. These data suggest selection for adaptive traits with respect to increasing exposure to the marine environment. Thus, a countergradient of traits is reflected in decreased TEWL in aerial environments and decreased net water efflux in marine environments, acting simultaneously in the three species. The pattern for TEWL correlates with ultrastructural evidence for increased lipogenesis in the stratum corneum of the more terrestrial species. The skin surfaces of all three species are hydrophobic. Species differences in this property possibly explain the pattern for water efflux when these snakes are in seawater, which remains to be investigated.

Cosmetic coloration in birds: occurrence, function, and evolution

Colorful plumages are conspicuous social signals in birds, and the expression of these colors often reflects the quality of their bearers. Since mature feathers are dead structures, plumage color is often considered a static signal that does not change after molt. Feathers, however, can and do deteriorate between molts, and birds need to invest heavily in plumage maintenance. Here we argue that this need for preserving plumage condition and hence signaling content might have given rise to a novel type of sexual signal: cosmetic coloration. Cosmetic coloration occurs when the substances used for plumage maintenance change the color of the feathers, thereby becoming a signal themselves. Our review of cosmetic coloration in birds demonstrates that it is more widespread than currently realized, occurring in at least 13 bird families. Cosmetics have varied origins: they can be produced by the bird itself (uropygial and skin secretions, feather powder) or obtained from the environment (soil, iron oxide). Intraspecific patterns of cosmetic use (sex, age, and seasonal dimorphism) suggest that in many cases it may act as a sexual signal. However, more information is required on function, mechanisms, and costs to understand the evolution of cosmetic coloration and to confirm its signaling role.

Cutaneous water loss and sphingolipids in the stratum corneum of house sparrows, Passer domesticus L., from desert and mesic environments as determined by reversed phase high-performance liquid chromatography coupled with atmospheric pressure photospray ionization mass spectrometry

Because cutaneous water loss (CWL) represents half of total water loss in birds, selection to reduce CWL may be strong in desert birds. We previously found that CWL of house sparrows from a desert population was about 25% lower than that of individuals from a mesic environment. The stratum corneum (SC), the outer layer of the epidermis, serves as the primary barrier to water vapor diffusion through the skin. The avian SC is formed by layers of corneocytes embedded in a lipid matrix consisting of cholesterol, free fatty acids and two classes of sphingolipids, ceramides and cerebrosides. The SC of birds also serves a thermoregulatory function; high rates of CWL keep body temperatures under lethal limits in episodes of heat stress. In this study, we used high-performance liquid chromatography coupled with atmospheric pressure photoionization-mass spectrometry (HPLC/APPI-MS) to identify and quantify over 200 sphingolipids in the SC of house sparrows from desert and mesic populations. Principal components analysis (PCA) led to the hypotheses that sphingolipids in the SC of desert sparrows have longer carbon chains in the fatty acid moiety and are more polar than those found in mesic sparrows. We also tested the association between principal components and CWL in both populations. Our study suggested that a reduction in CWL found in desert sparrows was, in part, the result of modifications in chain length and polarity of the sphingolipids, changes that apparently determine the interactions of the lipid molecules within the SC.

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.

Cutaneous water loss and lipids of the stratum corneum in house sparrowsPasser domesticusfrom arid and mesic environments

Birds that live in hot, dry environments must balance water intake with losses in order to maintain water homeostasis. The outer layer of the integument, called the stratum corneum (SC), consists of corneocytes embedded in a matrix of lipids. The SC serves as a barrier to water vapor diffusion through the skin. We measured cutaneous water loss (CWL) in two populations of house sparrow Passer domesticus L., one living in a desert environment in Saudi Arabia, and another living in a mesic environment in Ohio, USA. We found that CWL rates at 30°C were lower in desert individuals (11.9±2.2 mg H2O cm-2day-1; N=11) than in mesic birds (16.0±2.6 mg H2O cm-2 day-1; N=14). We hypothesized that changes in the lipid composition of the SC could affect CWL. We analyzed four classes of lipids in the SC: ceramides, cerebrosides,cholesterol and free fatty acids, by thin layer chromatography. Compared to mesic sparrows, desert birds had a higher amount of ceramides(49.2±10.3 mg g-1 SC dry mass in Saudi Arabia;38.2±18.0 mg g-1 SC dry mass in Ohio) and cerebrosides(101.2±48.9 mg g-1 SC dry mass in Saudi Arabia;56.5±34.0 mg g-1 SC dry mass in Ohio), and a lower percentage of cholesterol (4.1±3.6% in Saudi Arabia; 5.4±2.5% in Ohio) in their SC.

Although CWL was lower in sparrows from Arabia, and lipid composition of their SC differed, we could not detect differences between rates of water loss through non-living skin attached to glass vials (46.0±15.7 mg H2O cm-2 day-1 for sparrows in Saudi Arabia;45.8±27.2 mg H2O cm-2 day-1 for sparrows in Ohio). These results suggest that biological control mechanisms interact with layers of lipids in the stratum corneum to adjust CWL to the environment.

Lipids of the Stratum Corneum Vary with Cutaneous Water Loss among Larks along a Temperature‐Moisture Gradient

We explored the relationship between lipids of the stratum corneum (SC), the barrier to water-vapor diffusion of the skin, and cutaneous water loss (CWL) of species of free-living larks along a temperature-moisture gradient. Our results showed that free fatty acids, cholesterol, and ceramides were the major constituents of SC in larks from different environments including the Netherlands, a mesic environment; Iran, a semiarid region; and several areas in Saudi Arabia, a hot dry desert. We found that CWL was reduced among larks inhabiting deserts, but our data did not support the hypothesis that birds from desert environments have larger quantities of lipids per unit dry mass of the SC than larks from more mesic environments. Instead, larks in arid environments had a higher proportion of ceramides, especially the more polar fractions 4-6, and a smaller proportion of free fatty acids in their SC, an adjustment that apparently reduced their CWL. Subtle changes in the ratios of lipid classes can apparently alter the movement of water vapor through the skin. We hypothesize that desert birds have higher proportions of ceramides in their SC and lower proportions of free fatty acids because this combination allows the lipid lamellae to exist in a more highly ordered crystalline phase and consequently creates a tighter barrier to water-vapor diffusion.

Phenotypic flexibility in cutaneous water loss and lipids of the stratum corneum

When vertebrates invaded land during the Carboniferous period, they were exposed not only to new ecological opportunities but also to a desiccating environment. To maintain cellular water homeostasis, natural selection modified the integument of pioneering terrestrial animals, enabling them to reduce water loss through the skin. In desert environments, where ambient temperatures (T(a)) can reach 50 degrees C, relative humidities are low and drinking water is scarce, integumentary modifications that reduce cutaneous water loss (CWL) could be fundamental to survival. Previous research has shown that hoopoe larks (Alaemon alaudipes) from the Arabian desert reduced CWL when acclimated to 35 degrees C compared with individuals at 15 degrees C, but skylarks (Alauda arvensis) and woodlarks (Lullula arborea), from The Netherlands, and Dunn's larks (Eremalauda dunni), also from the Arabian desert, did not. Here, we test the idea that hoopoe larks acclimated to 35 degrees C would alter the lipid composition of their stratum corneum (SC), resulting in a decrease in CWL, but that skylarks, woodlarks and Dunn's larks would not. Specifically, we hypothesized that hoopoe larks, acclimated to 35 degrees C, would increase the proportions of polar ceramide content and decrease the proportions of free fatty acids in their SC compared with individuals acclimated to 15 degrees C. Results showed that hoopoe larks at 35 degrees C had lower CWL and higher proportions of total ceramides but lower proportions of free fatty acids and sterols in their SC. We demonstrate that adjustments in ratios of lipid classes in the SC are associated with changes in CWL in hoopoe larks.

Structural colouration of avian skin: convergent evolution of coherently scattering dermal collagen arrays

Structural colours of avian skin have long been hypothesized to be produced by incoherent (Rayleigh/Tyndall) scattering. We investigated the colour, anatomy, nanostructure and biophysics of structurally coloured skin, ramphotheca and podotheca from 31 species of birds from 17 families in 10 orders from across Aves. Integumentary structural colours of birds include ultraviolet, dark blue, light blue, green and yellow hues. The discrete peaks in reflectance spectra do not conform to the inverse fourth power relationship predicted by Rayleigh scattering. The dermis of structurally coloured skin consists of a thick (100-500 micro m) layer of collagen that is usually underlain by a layer of melanin granules. Transmission electron micrographs (TEMs) of this colour-producing dermal collagen layer revealed quasi-ordered arrays of parallel collagen fibres. Two-dimensional (2-D) Fourier analysis of TEMs of the collagen arrays revealed a ring of peak spatial frequencies in the spatial variation in refractive index that are the appropriate size to make the observed ultraviolet-yellow colours by coherent scattering alone. One species, Philepitta castanea (Eurylaimidae), has exceptionally ordered, hexagonal arrays of collagen fibres that produce a hexagonal pattern of spatial frequency peaks in the power spectra. Ultraviolet, blue, green and yellow structural colours of avian skin are produced by coherent scattering (i.e. constructive interference) by arrays of collagen fibres in the dermis. Some yellow and orange skin colours are produced with a combination of structural and pigmentary mechanisms. These combined colours can have reflectance spectra with discrete peaks that are more saturated than hues produced by carotenoid pigments alone. Bluish facial skin from two species of Neotropical antbirds (Thamnophilidae) are nanostructurally too small to produce visible light by coherent scattering, and the colour production mechanism in these species remains unknown. Based on the phylogenetic distribution of structurally coloured skin in Aves, this mechanism of colour production has evolved convergently more than 50 independent times within extant birds.

Postnatal ecdysis establishes the permeability barrier in snake skin: new insights into barrier lipid structures

A competent barrier to transepidermal water loss (TEWL) is essential for terrestrial life. In various vertebrates, epidermal water barriers composed of lipids prevent excessive TEWL, which varies inversely with habitat aridity. Little is known, however, about the mechanisms and regulation of permeability relative to natal transition from the `aqueous' environments of gestation to the `aerial' environments of terrestrial neonates. We investigated newly hatched California king snakes Lampropeltis getula to test the hypothesis that the first ecdysis is important for establishing the barrier to TEWL. We found that skin resistance to TEWL increases twofold following the first postnatal ecdysis, corresponding with a roughly twofold increase in thickness and deposition of lamellar lipids in the mesos layer, the site of the skin permeability barrier in snakes. In addition, novel observations on lipid inclusions within the alpha layer of epidermis suggest that this layer has functional similarities with avian epidermis. It appears that emergence of the integument from embryonic fluids, and its subsequent pan-body replacement following contact with air, are essential for completion of barrier competence in the newborn. These conditions provide a potentially useful model for investigations on the mechanism of barrier formation. We also found that hatchling snakes are transiently endothermic, with skin temperatures elevated by approximately 0.6°C above ambient air temperature during the period of barrier formation. Behaviourally, hatchlings showed a higher tendency to seek humid microenvironments before the first ecdysis than after. The degree of water movement across the integument might explain the switch from reclusive to dispersive behaviours associated with postnatal ecdysis in snakes.