Department of Biosciences, University of Milan, Milan, Italy.
PLoS One. 2013;8(4):e60426. doi: 10.1371/journal.pone.0060426. Epub 2013 Apr 3.
Pigmentation of body surface in animals can have multiple determinants and accomplish diverse functions. Eumelanin and pheomelanin are the main animal pigments, being responsible of yellow, brownish-red and black hues, and have partly common biosynthetic pathways. Many populations of vertebrates show individual variation in melanism, putatively with large heritable component. Genes responsible for eu- or pheomelanogenesis have pleiotropic but contrasting effects on life-history traits, explaining the patterns of covariation observed between melanization and physiology (e.g. immunity and stress response), sexual behavior and other characters in diverse taxa. Yet, very few studies in the wild have investigated if eu- and pheomelanization predict major fitness traits like viability or fecundity. In this correlative study, by contrasting adult barn swallows (Hirundo rustica) matched for age, sex, breeding site, and year and date of sampling, we show that males but not females that survived until the next year had paler, relatively more eu- than pheomelanic pigmentation of ventral body feathers. Better performance of individuals that allocate relatively more to eumelanogenesis was expected based on previous evidence on covariation between eumelanic pigmentation and specific traits related to immunity and susceptibility to stress. However, together with the evidence of no covariation between viability and melanization among females, this finding raises the question of the mechanisms that maintain variation in genes for melanogenesis. We discuss the possibility that eu- and pheomelanization are under contrasting viability and sexual selection, as suggested by larger breeding and sperm competition success of darker males from other barn swallow subspecies.
动物体表的色素沉着可能有多种决定因素，并具有多种功能。真黑素和褐黑素是动物的主要色素，负责产生黄色、红棕色和黑色调，并且具有部分共同的生物合成途径。许多脊椎动物群体表现出黑色素沉着的个体变异，可能具有较大的遗传成分。负责真黑素或褐黑素生成的基因对生活史特征具有多效但相反的影响，解释了在不同分类群中观察到的黑色素沉着与生理（如免疫和应激反应）、性行为和其他特征之间的协同变化模式。然而，在野外进行的很少研究调查了真黑素和褐黑素是否预测存活率或繁殖力等主要适应性特征。在这项对比研究中，我们通过对比年龄、性别、繁殖地点和采样年份和日期匹配的成年家燕（Hirundo rustica），表明只有那些能活到下一年的雄性个体，其腹部羽毛的真黑素相对较多，而褐黑素相对较少。根据真黑素沉着与与免疫和应激易感性相关的特定特征之间的协同变化的先前证据，预计个体将更多地分配给真黑素生成的表现更好。然而，与雌性之间的存活力和黑色素沉着没有协同变化的证据一起，这一发现提出了维持黑色素生成基因变异的机制问题。我们讨论了真黑素和褐黑素可能受到不同的生存和性选择的影响的可能性，因为其他家燕亚种的深色雄性在繁殖和精子竞争中具有更大的成功机会。
