Evolution of long-term coloration trends with biochemically unstable ingredients Item Type Article Authors Higginson, Dawn M.; Belloni, Virginia; Davis, Sarah N.; Morrison, Erin S.; Andrews, John E.; Badyaev, Alexander V. Citation Dawn M. Higginson, Virginia Belloni, Sarah N. Davis, Erin S. Morrison, John E. Andrews, Alexander V. Badyaev Proc. R. Soc. B 2016 283 20160403; DOI: 10.1098/rspb.2016.0403. Published 18 May 2016 DOI 10.1098/rspb.2016.0403 Publisher ROYAL SOC Journal PROCEEDINGS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES Rights © 2016 The Author(s) Published by the Royal Society. All rights reserved. Download date 28/09/2021 13:15:41 Item License http://rightsstatements.org/vocab/InC/1.0/ Version Final accepted manuscript Link to Item http://hdl.handle.net/10150/620830 1 2 Evolution of long-term coloration trends with 3 biochemically unstable ingredients 4 5 6 7 Dawn M. Higginson*, Virginia Belloni¶, Sarah N. Davis, Erin S. Morrison, John E. 8 Andrews, and Alexander V. Badyaev 9 Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, US 10 ¶ Present address: Center for Mind/Brain Sciences, University of Trento, Rovereto, TN, Italy 11 12 13 *Corresponding author: [email protected] 14 15 Running head: Biochemical stability of ornamentation 16 Key words: biochemical stability, carotenoids, evolutionary transitions, integration, molt 17 18 Word count: 6176 19 Figure count: 4 20 21 22 The evolutionarily persistent and widespread use of carotenoid pigments in animal coloration contrasts 23 with their biochemical instability. Consequently, evolution of carotenoid-based displays should include 24 mechanisms to accommodate or limit pigment degradation. In birds, this could involve two strategies: 1) 25 evolution of a molt immediately prior to the mating season, enabling the use of particularly fast-degrading 26 carotenoids, and 2) evolution of the ability to stabilize dietary carotenoids through metabolic modification 27 or association with feather keratins. Here we examine evolutionary lability and transitions between the 28 two strategies across 126 species of birds. We report that species that express mostly unmodified, fast- 29 degrading, carotenoids have pre-breeding molts, and a particularly short time between carotenoid 30 deposition and the subsequent breeding season. Species that expressed mostly slow-degrading carotenoids 31 in their plumage accomplished this through increased metabolic modification of dietary carotenoids, and 32 the selective expression of these slow-degrading compounds. In these species, the timing of molt was not 33 associated with carotenoid composition of plumage displays. Using repeated samples from individuals of 34 one species, we found that metabolic modification of dietary carotenoids significantly slowed their 35 degradation between molt and breeding season. Thus, the most complex and colorful ornamentation is 36 likely the most biochemically stable in birds, and depends less on ecological factors, such as molt timing 37 and migration tendency. We suggest that coevolution of metabolic modification, selective expression, and 38 biochemical stability of plumage carotenoids enables the use of unstable pigments in long-term 39 evolutionary trends in plumage coloration. 40 41 42 43 3 44 1. INTRODUCTION 45 How do the contingent processes of development and function produce long-term evolutionary trends? 46 Darwin considered this question to be central to his theory of evolution because it would provide a crucial 47 insight into the relationship between inheritance and natural selection [1], and between the developmental 48 stability of complex phenotypes and their ability to accommodate and integrate novel inputs [2-4]. There 49 are several insightful conceptual resolutions of this question [5-10], but empirical tests are rare because 50 they require investigation of evolutionary trajectories where an environmentally-contingent trait gets 51 reliably incorporated into the phenotype and stabilized over evolutionary time scales. 52 One of the most striking examples of the recurrent incorporation and accommodation of 53 environmentally-contingent elements is the evolution of diet-dependent coloration in animals. For 54 example in birds, complex and lineage-specific carotenoid-based ornamentation requires carotenoids 55 obtained through the diet that are subsequently incorporated into the organismal phenotype [11]. Unbound 56 carotenoids (and, in particular, dietary carotenoids) degrade rapidly with UV and oxygen exposure [12- 57 14]. Thus, differences among carotenoids in biochemical stability, together with the empirical 58 investigation of mechanisms by which birds stabilize and integrate these compounds into their feathers, 59 can provide important insight into the evolutionary integration of externally obtained elements into 60 phenotypes. 61 Plumage-bound carotenoids are acquired through the diet during the short period of feather 62 growth (molt), and can be either deposited directly or biochemically modified prior to deposition in the 63 feather [18, 19]. Once carotenoids become incorporated into the keratin matrix of a growing feather, no 64 further addition of pigment is possible. Numerous observations of change in feather color between 65 subsequent molts showed that abrasion or other alterations of feather structure (e.g., by microbial activity, 66 physical damage, UV exposure) progressively exposes embedded carotenoids to additional oxidization, 67 and thus produces color change despite the lack of additional pigment input [16, 20-23]. An important 68 discovery is that different carotenoid types within a feather have different potential for UV and oxygen 69 modification, with some carotenoids remaining remarkably stable once deposited in feathers whereas 70 others change readily, considerably modifying the appearance of the plumage [17, 22, 23]. The 71 biochemical stability of carotenoids typically increases with metabolic modification [24, 25], including 72 those commonly accomplished by birds [26]. Thus, metabolic modification of carotenoids prior to 73 deposition, the selective deposition of biochemically stable carotenoids, and the duration of post- 74 deposition exposure prior to the breeding season (i.e., timing of molt) should coevolve. 75 Carotenoids can be a priori classified into degradation propensity groups based on their structural 76 features [Electronic Supplementary Material Appendix 1; 13, 27]. For example, carotenoids with different 77 structures vary in their potential for photo-oxidation, reactivity with free radicals, and in their associated 78 tendency to produce carbonyl compounds and highly reactive epoxides [12, 28-30]. Reduction in the 79 number of conjugated double bonds in a carotenoid produces a shift towards lower wavelength 80 absorbance, and produces corresponding changes in hue and intensity of the associated color [14, 31]. 81 Differences between yellow and red carotenoids in their degradation propensity and photo-bleaching are 82 consistent with the observed relative biochemical instability of yellow feather pigments compared to red 83 and purple pigments [e.g., 15, 32]. 84 To accommodate the biochemical instability of dietary carotenoids birds could employ several 85 strategies. Species could 1) evolve enzymatic pathways to convert these compounds to slower-degrading 86 forms prior to deposition in feathers, 2) evolve compound-specific integration with feather proteins that 87 minimizes carotenoid instability, 3) selectively consume more slow-degrading dietary carotenoids or 88 selectively express slow-degrading carotenoids, or 4) optimize the time between carotenoid acquisition 89 (i.e., molt) and plumage display (i.e., breeding season). These strategies may be sequential evolutionary 90 stages or alternative tactics pursued by different avian lineages. Optimizing the timing of carotenoid 91 acquisition and display might involve the evolution of a pre-breeding molt for species that deposit mostly 92 fast-degrading carotenoids. Alternatively, species might capitalize on the effects of feather abrasion, 5 93 subsequent photo-oxidation and associated color change to arrive at the most advantageous color at the 94 time of mating [16, 21, 33]. Species with only a post-breeding molt might have a greater prevalence of 95 feather protective structures and modifications (e.g., unpigmented feather tips and barbules, denser keratin 96 matrix) that enable pigment preservation and optimal expression during the subsequent mating season. 97 Here we examine coevolution and evolutionary lability of metabolic modification of consumed 98 carotenoids, their expression in the plumage, and timing of molt in 126 bird species. The timing of molt, 99 its duration, and intensity coevolves with migratory tendency in birds [33-36], which imposes an 100 additional constraint on the acquisition, metabolism, and subsequent duration of environmental exposure 101 of the plumage carotenoids [37-40]. In turn, the juxtaposition of molt, breeding season, and migration 102 varies with geographical distribution and local seasonality. We statistically controlled for the effects of 103 migration tendencies and geographical distribution on molt strategies and biochemical stability of 104 plumage carotenoids. We also examine the mechanisms associated with evolutionary transitions from the 105 expression of fast-degrading to slow-degrading carotenoids in plumage and use repeated sampling of 106 individuals of one species to directly examine the