Maria Prager

Convergent evolution of red carotenoid coloration in widowbirds and bishops (Euplectes spp.).

Avian carotenoid‐based signals are classic examples of sexually selected, condition‐dependent threat displays or mate choice cues. In many species, male dominance or mating success is associated with redder (i.e., longer wavelength) color hues, suggesting that red colors are either more efficient or more reliable signals than yellow colors. Few studies, however, have investigated selection for redness in a macroevolutionary context. Here, we phylogenetically reconstruct the evolution of carotenoid coloration in the African widowbirds and bishops (Euplectes spp.), for which agonistic selection for redder hues, as well as pigmentary mechanisms, is well documented. Using reflectance spectrometry for objective color quantification, and accounting for phylogenetic uncertainty, we find that yellow plumage color is a retained ancestral state in Euplectes, and that red color hues have convergently evolved two or three times. Results are discussed in relation to a known diversity in pigment mechanisms, supporting independent origins of red color, and suggesting that agonistic selection and physiological constraints have interacted to generate color diversity in Euplectes.

Differential ability of carotenoid C4-oxygenation in yellow and red bishop species (Euplectes spp.)

Male breeding plumages of African widowbirds and bishops (Euplectes spp.) show striking variation in carotenoid-based plumage coloration, with saturated yellow or orange-red patches of different size. Yet, from observations and experiments, agonistic signaling appears to have been a generalized sexual selection pressure for redness in the genus. Recent results show that yellow and red widowbird colors derive from distinctly different pigment profiles, and suggest that species vary in their ability to metabolize ingested carotenoids. We used reflectance spectrometry and High Performance Liquid Chromatography (HPLC) to describe the species-specific colors and plumage carotenoids of the congeneric yellow-crowned bishop (E. afer) and southern red bishop (E. orix). Results show that the yellow rump color of E. afer primarily derives from direct-deposited, dietary yellow pigments, i.e. lutein and zeaxanthin. In the red breast of E. orix, these are complemented by smaller amounts of derived red C4-keto-carotenoids: mainly alpha-doradexanthin, but also beta-doradexanthin, canthaxanthin, astaxanthin and adonirubin. We also performed a diet supplementation experiment to investigate the relative importance of nutritional and metabolic constraints in determining the differential occurrence of C4-keto-carotenoids, and thus red plumage color, in the two species. Our results indicate that E. orix, but not E. afer, can manufacture red C4-keto-carotenoids (alpha-doradexanthin and canthaxanthin) from yellow dietary precursors (lutein and beta-carotene).

Phylogeny and evolution of sexually selected tail ornamentation in widowbirds and bishops (Euplectes spp.)

Despite similar ecology, mating systems and female preferences for supernormal tails, the 17 species of African widowbirds and bishops (Euplectes spp.) show astonishing variation in male tail ornamentation. Whereas bishops retain their brown nonbreeding tails in nuptial plumage, widowbirds grow black nuptial tails, varying in length from a few centimetres in E. axillaris to the extreme half metre train of E. progne. Here, we phylogenetically reconstruct the evolution of the discrete trait, nuptial tail and the continuous trait, tail length, using a molecular phylogeny of 33 Euplectes subspecies. Unlike many recent findings of labile evolution of plumage ornaments, our results suggest that the nuptial tail of Euplectes is a derived and phylogenetically conserved ornamental trait that, once gained, shows directional evolution in its expression. Directionality is demonstrated in the trivial sense of a short‐tailed ancestor, and by contingency and randomization tests suggesting that branches with increasing tail length are overrepresented. This supports an early origin and strong retention of directional female mate choice in widowbirds and bishops, as previously indicated by empirical and experimental results, and provides a less labile, yet rapid scenario of sexually selected diversification.

A novel method for screening a vertebrate transcriptome for genes involved in carotenoid binding and metabolism

Carotenoid‐based colour signals are widespread in the animal kingdom and common textbook examples of sexually selected traits. Carotenoid pigments must be obtained through the diet as all animals lack the enzymatic machinery necessary to synthesize them from scratch. Once ingested, carotenoids are metabolized, stored, transported and deposited, and some or all of these processes may be limiting for signal production and thus subjected to social or sexual selection on phenotypic coloration. Very little is known about which genes and physiological pathways are involved in carotenoid pigmentation which is unfortunate, as genetic information would allow us to investigate the biochemical consequences of sexual selection. In this study, we present a transcriptome‐screening technique and apply it to a carotenoid‐signalling bird species, the southern red bishop Euplectes orix, to uncover the gene(s) responsible for the conversion of dietary β‐carotene (orange) to canthaxanthin (bright red). The transcriptome, extracted from the liver of a male entering his breeding moult, is expressed within bacterial cells genetically modified to synthesize beta‐carotene. Effects of expressed E. orix proteins on the structure or amount of β‐carotene are initially detected by eye (based on colour change) and subsequently confirmed by high‐performance liquid chromatography. Here, we demonstrate the validity of the technique and provide a list of candidate genes involved in the carotenoid pigmentation pathway. We believe that this method could be applied to other species and tissues and that this may help researchers uncover the genetic basis of carotenoid coloration in vertebrates.