Abstract
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.
Highlights
Exaggerated 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
Whereas most of the few birds examined to date seem to have a single CYP2J19 gene, two copies have been reported in the zebra finch, one specialised for colour vision (CYP2J19A) and the other for red coloration (CYP2J19B) [8]
We investigated the evolution of CYP2J19 in birds, by conducting a broad analysis of avian genomes for presence, copy number and selection of CYP2J19 with comparison to other Gene encoding a cytochrome P450 enzyme (CYP) loci, and a focussed analysis of selection on CYP2J19 in the weaverbirds (Ploceidae), which vary in yellow and red carotenoid coloration
Summary
Exaggerated 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. 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.). Red C4-ketocarotenoid pigmentation, the main mechanism for redness in birds, In addition to its function in coloration in some lineages, CYP2J19 appears to be widely expressed in the avian retina [9, 10], where it is involved in colour vision by generating the C4-ketocarotenoid astaxanthin in the red oil droplets of longwave-sensitive cones [10, 11]. The ancestral function of the gene was likely for retinal oil droplet pigmentation, with its function in red coloration being subsequently co-opted in certain avian and turtle lineages [13]. Given the importance of duplication events for functional divergence, it is an interesting question if and where CYP1J19 duplication has occurred in other bird lineages
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