Plasticity for colour adaptation in vertebrates explained by the evolution of the genes pomc, pmch and pmchl

Effects of Water Temperature on the Body Color and Expression of the Genes Related to Body Color Regulation in the Goldfish

Melanin-concentrating hormone (MCH), melanocyte-stimulating hormone (MSH), and somatolactin (SL) in the hypothalamus-pituitary axis are associated with body color regulation in teleost fish. Although these hormones' production and secretion respond well to light environments, such as background color, little is known about the effects of different water temperatures. We investigated the effects of water temperature, 10°C, 20°C, and 30°C, on body color and the expression of these genes and corresponding receptor genes in goldfish. The body color in white background (WBG) becomes paler at the higher water temperature, although no difference was observed in black background (BBG). Brain mRNA contents of proMCH genes (pmch1 and pmch2) increased at 30°C and 20°C compared to 10°C in WBG, respectively. Apparent effects of background color and temperature on the pituitary mRNA contents of a POMC gene (pomc) were not observed. The pituitary mRNA contents of the SLα gene were almost double those on a WBG at any temperature, while those of the SLβ gene (slb) at 30°C tended to be higher than those at 10°C and 20°C on WBG and BBG. The scale mRNA contents of the MCH receptor gene (mchr2) in WBG were higher than those in BBG at 30°C. The highest scale mRNA contents of MSH receptor (mc1r and mc5r) on BBG were observed at 20°C, while the lowest respective mRNA levels were observed at 30°C on WBG. These results highlight the importance of temperature for the endocrinological regulation of body color, and darker background color may stabilize those endocrine functions.

Requirement of Zebrafish Adcy3a and Adcy5 in Melanosome Dispersion and Melanocyte Stripe Formation

cAMP-PKA signaling plays a pivotal role in melanin synthesis and melanosome transport by responding to the binding of the α-melanocyte-stimulating hormone (α-MSH) to melanocortin-1 receptor (MC1R). Adenylate cyclases (ADCYs) are the enzymes responsible for the synthesis of cAMP from ATP, which comprises nine transmembrane isoforms (ADCYs 1-9) and one soluble adenylate cyclase (ADCY 10) in mammals. However, little is known about which and how ADCY isoforms regulate melanocyte generation, melanin biosynthesis, and melanosome transport in vivo. In this study, we have generated a series of single and double mutants of Adcy isoforms in zebrafish. Among them, adcy3a^-/- and adcy5^-/- double mutants cause defects in melanosome dispersion but do not impair melanoblast differentiation and melanocyte regeneration during the embryonic or larval stages. Activation of PKA, the main effector of cAMP signaling, significantly ameliorates the defects in melanosome dispersion in adcy3a^-/- and adcy5^-/- double mutants. Mechanistically, Adcy3a and Adcy5 regulate melanosome dispersion by activating kinesin-1 while inhibiting cytoplasmic dynein-1. In adult zebrafish, Adcy3a and Adcy5 participate in the regulation of the expression of microphthalmia transcription factor (Mitfa) and melanin synthesis enzymes Tyr, Dct, and Trp1b. The deletion of Adcy3a and Adcy5 inhibits melanin production and reduces pigmented melanocyte numbers, causing a defect in establishing adult melanocyte stripes. Hence, our studies demonstrate that Adcy3a and Adcy5 play essential but redundant functions in mediating α-MSH-MC1R/cAMP-PKA signaling for regulating melanin synthesis and melanosome dispersion.

Skin patterning and internal anatomy in a fossil moonfish from the Eocene Bolca Lagerstätte illuminate the ecology of ancient reef fish communities

Colour patterning in extant animals can be used as a reliable indicator of their biology and, in extant fish, can inform on feeding strategy. Fossil fish with preserved colour patterns may thus illuminate the evolution of fish behaviour and community structure, but are understudied. Here we report preserved melanin-based integumentary colour patterning and internal anatomy of the fossil moonfish Mene rhombea (Menidae) from the Bolca Lagerstätte (Eocene (Ypresian), north-east Italy). The melanosome-based longitudinal stripes of M. rhombea differ from the dorsal rows of black spots in its extant relative M. maculata, suggesting that the ecology of moonfish has changed during the Cenozoic. Extant moonfish are coastal schooling fish that feed on benthic invertebrates, but the longitudinal stripes and stomach contents with fish remains in M. rhombea suggest unstructured open marine ecologies and a piscivorous diet. The localized distribution of extant moonfish species in the Indo-Pacific Ocean may reflect, at least in part, tectonically-driven reorganization of global oceanographic patterns during the Cenozoic. It is likely that shifts in habitat and colour patterning genes promoted colour pattern evolution in the menid lineage.

Type II Opsins in the Eye, the Pineal Complex and the Skin of Xenopus laevis: Using Changes in Skin Pigmentation as a Readout of Visual and Circadian Activity

The eye, the pineal complex and the skin are important photosensitive organs. The African clawed frog, Xenopus laevis, senses light from the environment and adjusts skin color accordingly. For example, light reflected from the surface induces camouflage through background adaptation while light from above produces circadian variation in skin pigmentation. During embryogenesis, background adaptation, and circadian skin variation are segregated responses regulated by the secretion of α-melanocyte-stimulating hormone (α-MSH) and melatonin through the photosensitivity of the eye and pineal complex, respectively. Changes in the color of skin pigmentation have been used as a readout of biochemical and physiological processes since the initial purification of pineal melatonin from pigs, and more recently have been employed to better understand the neuroendocrine circuit that regulates background adaptation. The identification of 37 type II opsin genes in the genome of the allotetraploid X. laevis, combined with analysis of their expression in the eye, pineal complex and skin, is contributing to the elucidation of the role of opsins in the different photosensitive organs, but also brings new questions and challenges. In this review, we analyze new findings regarding the anatomical localization and functions of type II opsins in sensing light. The contribution of X. laevis in revealing the neuroendocrine circuits that regulate background adaptation and circadian light variation through changes in skin pigmentation is discussed. Finally, the presence of opsins in X. laevis skin melanophores is presented and compared with the secretory melanocytes of birds and mammals.

Pharmacological Modulation of Melanocortin 1 Receptor Signaling by Mrap Proteins in Xenopus tropicalis

The melanocortin system consists of five G protein-coupled receptors (MC1R-MC5R), the bidirectional endogenous ligands (MSH and Agouti families), and accessory proteins (MRAP1 and MRAP2). Accumulative studies of vertebrate species find high expression level of melanocortin 1 receptor (MC1R) in the dermal melanocyte and elucidate the essential roles in the skin and fur pigmentation, morphological background adaptation, and stress response. The diploid amphibian Xenopus tropicalis (xt) has been utilized as a fantastic animal model for embryonic development and studies of physiological cryptic colouring and environmental adaptiveness. However, the interaction of xtMc1r signaling with xtMrap proteins has not been assessed yet. In this study, we carried out in silico evolutionary analysis of protein alignment and genetic phylogenetic and genomic synteny of mc1r among various vertebrates. Ubiquitous expression of mrap1 and mrap2 and the co-expression with mc1r transcripts in the skin were clearly observed. Co-immunoprecipitation (ip) and fluorescent complementary approach validated the direct functional interaction of xtMc1r with xtMrap1 or xtMrap2 proteins on the plasma membrane. Pharmacological assay showed the improvement of the constitutive activity and alpha melanocyte-stimulating hormone (α-MSH) stimulated plateau without dramatic alteration of the cell surface translocation of xtMc1r in the presence of xtMrap proteins. Overall, the pharmacological modulation of xtMc1r by dual xtMrap2 proteins elucidated the potential role of this protein complex in the regulation of proper dermal function in amphibian species.

Distinct type II opsins in the eye decode light properties for background adaptation and behavioural background preference

Crypsis increases survival by reducing predator detection. Xenopus laevis tadpoles decode light properties from the substrate to induce two responses: a cryptic coloration response where dorsal skin pigmentation is adjusted to the colour of the substrate (background adaptation) and a behavioural crypsis where organisms move to align with a specific colour surface (background preference). Both processes require organisms to detect reflected light from the substrate. We explored the relationship between background adaptation and preference and the light properties able to trigger both responses. We also analysed which retinal photosensor (type II opsin) is involved. Our results showed that these two processes are segregated mechanistically, as there is no correlation between the preference for a specific background with the level of skin pigmentation, and different dorsal retina-localized type II opsins appear to underlie the two crypsis modes. Indeed, inhibition of melanopsin affects background adaptation but not background preference. Instead, we propose pinopsin is the photosensor involved in background preference. pinopsin mRNA is co-expressed with mRNA for the sws1 cone photopigment in dorsally located photoreceptors. Importantly, the developmental onset of pinopsin expression aligns with the emergence of the preference for a white background, but after the background adaptation phenotype appears. Furthermore, white background preference of tadpoles is associated with increased pinopsin expression, a feature that is lost in premetamorphic froglets along with a preference for a white background. Thus, our data show a mechanistic dissociation between background adaptation and background preference, and we suggest melanopsin and pinopsin, respectively, initiate the two responses.

Fish skin pigmentation in aquaculture: The influence of rearing conditions and its neuroendocrine regulation

Skin pigmentation pattern is a species-specific characteristic that depends on the number and the spatial combination of several types of chromatophores. This feature can change during life, for example in the metamorphosis or reproductive cycle, or as a response to biotic and/or abiotic environmental cues (nutrition, UV incidence, surrounding luminosity, and social interactions). Fish skin pigmentation is one of the most important quality criteria dictating the market value of both aquaculture and ornamental species because it serves as an external signal to infer its welfare and the culture conditions used. For that reason, several studies have been conducted aiming to understand the mechanisms underlying fish pigmentation as well as the influence exerted by rearing conditions. In this context, the present review focuses on the current knowledge on endocrine regulation of fish pigmentation as well as on the aquaculture conditions affecting skin coloration. Available information on Iberoamerican fish species cultured is presented.

Color as an important biological variable in zebrafish models: Implications for translational neurobehavioral research

Color is an important environmental factor that in multiple ways affects human and animal behavior and physiology. Widely used in neuroscience research, various experimental (animal) models may help improve our understanding of how different colors impact brain and behavioral processes. Complementing laboratory rodents, the zebrafish (Danio rerio) is rapidly emerging as an important novel model species to explore complex neurobehavioral processes. The growing utility of zebrafish in biomedicine makes it timely to consider the role of colors in their behavioral and physiological responses. Here, we summarize mounting evidence implicating colors as a critical variable in zebrafish models and neurobehavioral traits, with a particular relevance to CNS disease modeling, genetic and pharmacological modulation, as well as environmental enrichment and animal welfare. We also discuss the growing value of zebrafish models to study color neurobiology and color-related neurobehavioral phenomics, and outline future directions of research in this field.

Melanin-concentrating hormone like and somatolactin. A teleost-specific hypothalamic-hypophyseal axis system linking physiological and morphological pigmentation

Plastic adaptation to match the skin colour to the surrounding is key to survival. Two biological responses in skin colour are associated with background adaptation. A fast "physiological response" that aggregates/disperses the pigment organelles of skin chromatophores, and a slow "morphological response" that alters the type and/or density of pigment cells in the skin. Both responses are linked by unknown mechanisms. In this review, we discuss the role in skin colour regulation of two molecules that form part of a hypothalamic-hypophyseal pathway unique to teleosts, melanin-concentrating hormone "like" (MCHL) (previously known as MCH), and somatolactin. MCHL neurons project to the neurohypophysis and to the pars intermedia pituitary, where they interact with somatolactin-expressing cells. With a white background MCHL is released into the circulation to induce rapid melanosome aggregation and skin lightening. Somatolactin is also a fish-specific peptide whose expression and secretion are altered in organisms adapted chronically to white/black backgrounds, and that regulates morphological pigmentation. We discuss the evidence for a model whereby in teleosts, MCHL and somatolactin provide the previously unknown link between physiological and morphological pigmentation.

Plasticity for colour adaptation in vertebrates explained by the evolution of the genes pomc, pmch and pmchl

Different camouflages work best with some background matching colour. Our understanding of the evolution of skin colour is based mainly on the genetics of pigmentation ("background matching"), with little known about the evolution of the neuroendocrine systems that facilitate "background adaptation" through colour phenotypic plasticity. To address the latter, we studied the evolution in vertebrates of three genes, pomc, pmch and pmchl, that code for α-MSH and two melanin-concentrating hormones (MCH and MCHL). These hormones induce either dispersion/aggregation or the synthesis of pigments. We find that α-MSH is highly conserved during evolution, as is its role in dispersing/synthesizing pigments. Also conserved is the three-exon pmch gene that encodes MCH, which participates in feeding behaviours. In contrast, pmchl (known previously as pmch), is a teleost-specific intron-less gene. Our data indicate that in zebrafish, pmchl-expressing neurons extend axons to the pituitary, supportive of an MCHL hormonal role, whereas zebrafish and Xenopus pmch+ neurons send axons dorsally in the brain. The evolution of these genes and acquisition of hormonal status for MCHL explain different mechanisms used by vertebrates to background-adapt.