Pointer Marie A, Mundy Nicholas I
Department of Zoology, University of Cambridge, Cambridge, UK.
BMC Evol Biol. 2008 Sep 12;8:249. doi: 10.1186/1471-2148-8-249.
The MC1R (melanocortin-1 receptor) locus underlies intraspecific variation in melanin-based dark plumage coloration in several unrelated birds with plumage polymorphisms. There is far less evidence for functional variants of MC1R being involved in interspecific variation, in which spurious genotype-phenotype associations arising through population history are a far greater problem than in intraspecific studies. We investigated the relationship between MC1R variation and plumage coloration in swans (Cygnus), which show extreme variation in melanic plumage phenotypes among species (white to black).
The two species with melanic plumage, C. atratus and C. melanocoryphus (black and black-necked swans respectively), both have amino acid changes at important functional sites in MC1R that are consistent with increased MC1R activity and melanism. Reconstruction of MC1R evolution over a newly generated independent molecular phylogeny of Cygnus and related genera shows that these putative melanizing mutations were independently derived in the two melanic lineages. However, interpretation is complicated by the fact that one of the outgroup genera, Coscoroba, also has a putative melanizing mutation at MC1R that has arisen independently but has nearly pure white plumage. Epistasis at other loci seems the most likely explanation for this discrepancy. Unexpectedly, the phylogeny shows that the genus Cygnus may not be monophyletic, with C. melanocoryphus placed as a sister group to true geese (Anser), but further data will be needed to confirm this.
Our study highlights the difficulty of extrapolating from intraspecific studies to understand the genetic basis of interspecific adaptive phenotypic evolution, even with a gene whose structure-function relationships are as well understood as MC1R as confounding variation make clear genotype/phenotype associations difficult at the macroevolutionary scale. However, the identification of substitutions in the black and black-necked swan that are known to be associated with melanic phenotypes, suggests Cygnus may be another example where there appears to be convergent evolution at MC1R. This study therefore provides a novel example where previously described intraspecific genotype/phenotype associations occur at the macroevolutionary level.
MC1R（黑皮质素-1受体）基因座是几种具有羽毛多态性的不相关鸟类中基于黑色素的深色羽毛颜色种内变异的基础。关于MC1R功能变体参与种间变异的证据要少得多，在种间变异中，由种群历史产生的虚假基因型-表型关联比种内研究中的问题要大得多。我们研究了天鹅（天鹅属）中MC1R变异与羽毛颜色之间的关系，天鹅在物种间的黑色素羽毛表型存在极端变异（从白色到黑色）。
两种具有黑色素羽毛的物种，黑天鹅和黑颈天鹅（分别为黑色和黑颈），在MC1R的重要功能位点都有氨基酸变化，这与MC1R活性增加和黑化现象一致。在新生成的天鹅属及相关属的独立分子系统发育树上对MC1R进化的重建表明，这些假定的黑化突变在两个黑色素谱系中是独立产生的。然而，一个外类群属科斯可罗巴也在MC1R处有一个独立产生的假定黑化突变，但其羽毛几乎全白，这一事实使解释变得复杂。其他位点的上位效应似乎是这种差异最可能的解释。出乎意料的是，系统发育树显示天鹅属可能不是单系的，黑颈天鹅被置于真鹅（雁属）的姐妹群位置，但还需要更多数据来证实这一点。
我们的研究强调了从种内研究推断以理解种间适应性表型进化的遗传基础的困难，即使对于一个结构-功能关系像MC1R一样被充分理解的基因，由于混杂变异使得在宏观进化尺度上难以明确基因型/表型关联。然而，在黑天鹅和黑颈天鹅中鉴定出已知与黑色素表型相关的替代突变，表明天鹅属可能是另一个在MC1R处似乎存在趋同进化的例子。因此，本研究提供了一个新的例子，即先前描述的种内基因型/表型关联在宏观进化水平上出现。
