The antioxidant role of carotenoids in the living organism was proposed as a possible basis for the honesty of carotenoid‐based signals. However, recent studies have questioned the relevance of carotenoids as powerful antioxidants in vivo. Current evidence does not seem to support the “antioxidant role” hypothesis, but it does not allow us to reject it either. This paper proposes some steps to solve this controversy, such as taking a dynamic approach to antioxidant responses, designing protocols that expose individuals (...) to oxidative challenges, analyzing tissues other than blood, and obtaining measures of antioxidant capacity and oxidative damage simultaneously. However, it should be considered that, irrespective of their antioxidant potential, carotenoids might still give information on oxidative stress levels if they are particularly sensitive to free radicals. Finally, lumping together the immunostimulatory and antioxidant roles of carotenoids should be avoided as these functions are not necessarily associated. (shrink)

Why do some invertebrates store so much carotenoids in their tissues? Storage of carotenoids may not simply be passive and dependent on their environmental availability, as storage variation exists at various taxonomic scales, including among individuals within species. While the strong antioxidant and sometimes immune-stimulating properties of carotenoids may be beneficial enough to cause the evolution of features improving their assimilation and storage, they may also have fitness downsides explaining why massive carotenoid storage is not universal. Here, (...) the functional and ecological implications of carotenoid storage for the evolution of invertebrate innate immune defenses are examined, especially in crustaceans, which massively store carotenoids for unclear reasons. Three testable hypotheses about the role of carotenoid storage in immunological (resistance and tolerance) and life-history strategies (with a focus on aging) are proposed, which may ultimately explain the storage of large amounts of these pigments in a context of host–pathogen interactions. (shrink)

Allocation tradeoffs with the immune system can enforce honesty on sexual signals that act as indicators of individual quality. Such tradeoffs can be brought about by (1) the dual action of testosterone, which stimulates sexual signals but also suppresses immune functions, and/or (2) competition for carotenoids, which can be deposited as ornamental pigments or used as antioxidants in support of immune functions. Recent studies1-3 integrate these two mechanisms by showing that testosterone treatment in male birds upregulates circulating lipoproteins, plasma (...) carriers for carotenoids, and increases bio-availability of carotenoids, which enhances carotenoid-based sexual coloration and immune responsiveness. BioEssays 29:427–430, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

Carotenoids play pivotal roles in vision as light filters and precursor of chromophore. Many vertebrates also display the colorful pigments as ornaments in bare skin parts and feathers. Proteins involved in the transport and metabolism of these lipids have been identified including class B scavenger receptors and carotenoid cleavage dioxygenases. Recent research implicates members of the Aster protein family, also known as GRAM domain‐containing (GRAMD), in carotenoid metabolism. These multi‐domain proteins facilitate the intracellular movement of carotenoids from their (...) site of cellular uptake by scavenger receptors to the site of their metabolic processing by carotenoid cleavage dioxygenases. We provide a model how the coordinated interplay of these proteins and their differential expression establishes carotenoid distribution patterns and function in tissues, with particular emphasis on the human retina. (shrink)

In many vertebrates, the enzymatic oxidation of dietary yellow carotenoids generates red keto‐carotenoids giving color to ornaments. The oxidase CYP2J19 is here a key effector. Its purported intracellular location suggests a shared biochemical pathway between trait expression and cell functioning. This might guarantee the reliability of red colorations as individual quality signals independent of production costs. We hypothesize that the ornament type (feathers vs. bare parts) and production costs (probably CYP2J19 activity compromising vital functions) could have promoted tissue‐specific (...) gene relocation. We review current avian tissue‐specific CYP2J19 expression data. Among the ten red‐billed species showing CYP2J19 bill expression, only one showed strong hepatic expression. Moreover, a phylogenetically‐controlled analysis of 25 red‐colored species shows that those producing red bare parts are less likely to have strong hepatic CYP2J19 expression than species with only red plumages. Thus, both production costs and shared pathways might have contributed to the evolution of red signals. (shrink)

In contrast to vertical gene transfer from parent to offspring, horizontal (or lateral) gene transfer moves genetic information between different species. Bacteria and archaea often adapt through horizontal gene transfer. Recent analyses indicate that eukaryotic genomes, too, have acquired numerous genes via horizontal transfer from prokaryotes and other lineages. Based on this we raise the hypothesis that horizontally acquired genes may have contributed more to adaptive evolution of eukaryotes than previously assumed. Current candidate sets of horizontally acquired eukaryotic genes may (...) just be the tip of an iceberg. We have recently shown that adaptation of the thermoacidophilic red algaGaldieria sulphurariato its hot, acid, toxic‐metal laden, volcanic environment was facilitated by the acquisition of numerous genes from extremophile bacteria and archaea. Other recently published examples of horizontal acquisitions involved in adaptation include ice‐binding proteins in marine algae, enzymes for carotenoid biosynthesis in aphids, and genes involved in fungal metabolism.Editor's suggested further reading in BioEssaysJumping the fine LINE between species: Horizontal transfer of transposable elements in animals catalyses genome evolution Abstract. (shrink)

Two recent hypotheses have proposed that non‐green plant colouration evolved as a defence against herbivores, either as protective colouration promoting handicap signals indicating plant fitness or by undermining their crypsis. The handicap hypothesis posits a co‐evolutionary process between plants and herbivores, whereas the anti‐crypsis hypothesis suggests that an arms race between insects and plants is the evolutionary mechanism. Both explanations assume that insects are the evolutionary origin causing plants' colouration. Here, we propose a different hypothesis, termed the “Defence Indication hypothesis”. (...) This idea focuses on the multiple protective functions of anthocyanins and carotenoids as pigments, and suggests that plant colouration evolved primarily in response to various stressors. Because pigments and defensive compounds share a common biosynthesis, the production of pigments also provides elevated defensive strengths against herbivores, a process termed priming. In effect, the Defence Indication hypothesis predicts that pleiotropic effects of the pigments and, more generally, plants' shared defence responses, explain why insects might react to plant colouration. BioEssays 28:65–71, 2006. © 2005 Wiley Periodicals, Inc. (shrink)

The eyespot organelle of the green alga Chlamydomonas allows the cell to phototax toward (or away) from light to maximize the light intensity for photosynthesis and minimize photo‐damage. At cytokinesis, the eyespot is resorbed at the cleavage furrow and two new eyespots form in the daughter cells 180° from each other. The eyespots are positioned asymmetrically with respect to the microtubule cytoskeleton. Eyespots are assembled from all three chloroplast membranes and carotenoid‐filled granules, which form a sandwich structure overlaid by the (...) tightly apposed plasma membrane. This review describes (1) my interest in cellular asymmetry and organelle biology, (2) isolation of mutations that describe four genes governing eyespot placement and assembly, (3) the characterization of the EYE2 gene, which encodes a thioredoxin superfamily member, and (4) the characterization of the MIN1 gene, which is required for the layered organization of granules and membranes in the eyespot. BioEssays 25:410–416, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Darwin1 introduced the idea that ornamental secondary sexual traits have evolved in response to female preferences for showy males. Among such traits, yellow and red carotenoid‐based ornaments have been considered as particularly good candidates for explaining why and how females would benefit from mating with showy partners. Because carotenoids can be used for promotion of both health and appearance, colourful male ornaments should honestly reveal the vigour of the bearers. Two recent experiments with birds2,3 now show how allocation of (...) bodily carotenoids to sexual signalling is traded off against the use of carotenoids for boosting immune function. BioEssays 25:746–747, 2003. © 2003 Wiley Periodicals, Inc. (shrink)