A mitochondria-targeted antioxidant and a thyroid hormone affect carotenoid ketolase gene expression and bill redness in zebra finches
Conspicuous ornaments in animals can evolve to reveal individual quality when their production/maintenance costs make them reliable as signals or if their expression level is intrinsically linked to quality by some unfalsifiable mechanism (quality indices). The latter has been mostly associated with traits constrained by body size. However, red ketocarotenoid-based coloured ornaments may also have evolved as quality indices because their production could be closely linked to individual metabolism and, particularly, to the cell respiration at the inner mitochondrial membrane (IMM). This mechanism would supposedly not depend on resource (yellow carotenoids) availability, thus discarding allocation trade-offs. A gene coding for a ketolase enzyme (CYP2J19) responsible for converting dietary yellow carotenoids to red ketocarotenoids has recently been described in birds. It is not known, however, if this ketolase is involved in mitochondrial metabolism and if its expression level and activity is resource independent. Here, we manipulated the metabolism of captive male zebra finches by an antioxidant designed to penetrate the IMM (mitoTEMPO) and a thyroid hormone (triiodothyronine; T3) with known hypermetabolic effects. The expression levels of a ketocarotenoid-based ornament (bill redness) and CYP2J19 were measured. MitoTEMPO downregulated CYP2J19 expression, supporting the mitochondrial involvement in ketolase function. T3 also reduced CYP2J19 expression, but at an intermediate dosage, this effect being buffered by mitoTEMPO. Bill redness seemed to show a similar interacting effect. Nevertheless, this faded when CYP2J19 expression level was controlled for as a covariate. We argue that the well-known mitoTEMPO effect in reducing mitochondrial reactive oxygen species (ROS) production (particularly superoxide) could have interfered on redox signalling mechanisms controlling ketolase transcription. High T3 levels, contrarily, can lead to high ROS production but also trigger compensatory mechanisms, which may explain the U-shaped effect with dosage on CYP2J19 expression levels. Bill CYP2J19 expression values were also positively correlated to redness and circulating substrate carotenoid levels. Nonetheless, treatment effects did not change when controlling for blood carotenoid concentration, suggesting that resource-availability dependence was irrelevant. Finally, our findings reveal a role for thyroid hormones in the expression of carotenoid-based ornaments that has virtually been ignored until now.