Bright-red colors in vertebrates are commonly involved in sexual, social, and interspecific signaling [1-8] and are largely produced by ketocarotenoid pigments. In land birds, ketocarotenoids such as astaxanthin are usually metabolically derived via ketolation of dietary yellow carotenoids [9, 10]. However, the molecular basis of this gene-environment mechanism has remained obscure. Here we use the yellowbeak mutation in the zebra finch (Taeniopygia guttata) to investigate the genetic basis of red coloration. Wild-type ketocarotenoids were absent in the beak and tarsus of yellowbeak birds. The yellowbeak mutation mapped to chromosome 8, close to a cluster of cytochrome P450 loci (CYP2J2-like) that are candidates for carotenoid ketolases. The wild-type zebra finch genome was found to have three intact genes in this cluster: CYP2J19A, CYP2J19B, and CYP2J40. In yellowbeak, there are multiple mutations: loss of a complete CYP2J19 gene, a modified remaining CYP2J19 gene (CYP2J19(yb)), and a non-synonymous SNP in CYP2J40. In wild-type birds, CYP2J19 loci are expressed in ketocarotenoid-containing tissues: CYP2J19A only in the retina and CYP2J19B in the beak and tarsus and to a variable extent in the retina. In contrast, expression of CYP2J19(yb) is barely detectable in the beak of yellowbeak birds. CYP2J40 has broad tissue expression and shows no differences between wild-type and yellowbeak. Our results indicate that CYP2J19 genes are strong candidates for the carotenoid ketolase and imply that ketolation occurs in the integument in zebra finches. Since cytochrome P450 enzymes include key detoxification enzymes, our results raise the intriguing possibility that red coloration may be an honest signal of detoxification ability.

Avian ketocarotenoid pigments occur in both the red retinal oil droplets that contribute to colour vision and bright red coloration used in signalling. Turtles are the only other tetrapods with red retinal oil droplets, and some also display red carotenoid-based coloration. Recently, the CYP2J19 gene was strongly implicated in ketocarotenoid synthesis in birds. Here, we investigate CYP2J19 evolution in relation to colour vision and red coloration in reptiles using genomic and expression data. We show that turtles, but not crocodiles or lepidosaurs, possess a CYP2J19 orthologue, which arose via gene duplication before turtles and archosaurs split, and which is strongly and specifically expressed in the ketocarotenoid-containing retina and red integument. We infer that CYP2J19 initially functioned in colour vision in archelosaurs and conclude that red ketocarotenoid-based coloration evolved independently in birds and turtles via gene regulatory changes of CYP2J19. Our results suggest that red oil droplets contributed to colour vision in dinosaurs and pterosaurs.

Spc72 differential recruitment imparting asymmetric aMT organization represents the most upstream determinant linking SPB historical identity and fate.

Red carotenoid colours in birds are widely assumed to be sexually selected quality indicators, but this rests on a very incomplete understanding of genetic mechanisms and honesty-mediating costs. Recent progress was made by the implication of the gene CYP2J19 as an avian carotenoid ketolase, catalysing the synthesis of red C4-ketocarotenoids from yellow dietary precursors, and potentially a major mechanism behind red coloration in birds. Here, we investigate the role of CYP2J19 in the spectacular colour diversification of African weaverbirds (Ploceidae), represented by five genera and 16 species: eight red, seven yellow and one without carotenoid coloration. All species had a single copy of CYP2J19, unlike the duplication found in the zebra finch, with high expression in the retina, confirming its function in colouring red oil droplets. Expression was weak or undetected in skin and follicles of pigment-depositing feather buds, as well as in beaks and tarsi, including those of the red-billed quelea. In contrast, the hepatic (liver) expression of CYP2J19 was consistently higher (>14-fold) in seven species with C4-ketocarotenoid coloration than in species without (including one red species), an association strongly supported by a phylogenetic comparative analysis. The results suggest a critical role of the candidate ketolase, CYP2J19, in the evolution of red C4-ketocarotenoid colour variation in ploceids. As ancestral state reconstruction suggests that ketocarotenoid coloration has evolved twice in this group (once in Euplectes and once in the Quelea/Foudia clade), we argue that while CYP2J19 has retained its ancestral role in the retina, it has likely been co-opted for red coloration independently in the two lineages, via increased hepatic expression.

BackgroundExaggerated signals, such as brilliant colours, are usually assumed to evolve through antagonistic coevolution between senders and receivers, but the underlying genetic mechanisms are rarely known. Here we explore a recently identified “redness gene”, CYP2J19, that is highly interesting in this context since it encodes a carotenoid-modifying enzyme (a C4 ketolase involved in both colour signalling and colour discrimination in the red (long wavelength) spectral region.)ResultsA single full-length CYP2J19 was retrieved from 43 species out of 70 avian genomes examined, representing all major avian clades. In addition, CYP2J19 sequences from 13 species of weaverbirds (Ploceidae), seven of which have red C4-ketocarotenoid coloration were analysed. Despite the conserved retinal function and pleiotropy of CYP2J19, analyses indicate that the gene has been positively selected throughout the radiation of birds, including sites within functional domains described in related CYP (cytochrome P450) loci. Analyses of eight further CYP loci across 25 species show that positive selection is common in this gene family in birds. There was no evidence for a change in selection pressure on CYP2J19 following co-option for red coloration in the weaverbirds.ConclusionsThe results presented here are consistent with an ancestral conserved function of CYP2J19 in the pigmentation of red retinal oil droplets used for colour vision, and its subsequent co-option for red integumentary coloration. The cause of positive selection on CYP2J19 is unclear, but may be partly related to compensatory mutations related to selection at the adjacent gene CYP2J40.Electronic supplementary materialThe online version of this article (10.1186/s12862-018-1136-y) contains supplementary material, which is available to authorized users.

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