黑色素羽毛装饰反映了鸟类的抗氧化和免疫能力吗?

摘要

鸟类信号系统研究认为,不同于类胡萝卜素,基于黑色素的羽色装饰不需要昂贵的生理代价.然而,羽毛色素细胞的黑色素沉积与抗氧化能力和免疫系统有很强的联系,而抗氧化能力和免疫系统在提高有机体适合度方面具有重要的功能.黑色素细胞的生长对氧化环境压力十分敏感;并且,黑色素本身似乎就具有抗氧化剂的功能.相应地,把抗氧化剂用于以黑色素为基础羽色发育,还是用于其它方面例如免疫调节和免疫刺激等,个体也许必须对此做出权衡.组织中的抗氧化功能大多与代谢活动有关,也就是和自由基的最高水平有关.此外,人们发现,在哺乳动物中调节黑色素沉积的激素,即α-黑素细胞刺激素,在鸟类上皮组织的色素沉积中也具有同样的功能.这种进化保守的激素是免疫和炎症反应的一个重要介体.它下调前炎性细胞因子、免疫介导细胞因子和协同刺激分子,以及主要组织相容性复合体Ⅰ类分子在单核细胞的表达以及抗体的产生,而上调抑制因子白介素.黑色素在羽毛上的大量沉积,也可能反映出免疫系统在局部免疫应答的负调控能力.这些将黑色素沉积和抗氧化剂以及免疫功能联系起来的机制,表明基于黑色素的羽色信号具有一定的生理代价,这些对以往关于黑色素的优势作用的假说提出质疑[动物学报52(1):202-208,2006].%The assumption that melanin-based ornaments, in contrast to those that are carotenoid-based, are not physiologically costly has been pervasive in studies of avian signalling. However, melanin deposition in feather melanocytes has strong links with the capacity of the antioxidant and immune systems, two crucial fitness-enhancing organismic functions.Thus, melanocyte development is very sensitive to oxidative stress and melanin itself seems to function as an antioxidant.Accordingly, individuals may have to trade the use of antioxidants in development of melanin-based coloration against other uses such as immuno-modulation and immuno-stimulation and anti-oxidant function in tissues most involved in metabolic activity and hence with the highest levels of free radicals. Also, the hormone regulating melanin deposition in mammals,α-melanocyte-stimulating hormone, has been found to work in birds as well and with the same function with respect to pigmentation of tegumentary structures. This evolutionarily conserved hormone is produced locally and functions as an important mediator of immunity and inflammation. Its action down-regulates the production of proinflammatory and immuno-modulating cytokines and co-stimulatory molecules, the expression of major histocompatibility complex class Ⅰmolecules on monocytes and antibody production, and up-regulates the suppressor factor interleukin 10. The deposition of melanin in large tracts of plumage may signal the capacity of the immune system to function properly in a situation of down-regulation of the local immune response. These different mechanisms linking melanin deposition to antioxidant and immune function, suggest a physiological cost involved in melanin-based signals and weakens the assumption of their arbitrariness [Acta Zoologica Sinica 52 (1): 202-208, 2006].