Geographic patterns of mtDNA and Z-linked sequence variation in the Common Chiffchaff and the ‘chiffchaff complex’

The Common Chiffchaff Phylloscopus collybita is an abundant, polytypic Palearctic bird. Validity of some of its subspecies is controversial and birds from some parts of the species range remain unclassified taxonomically. The relationships among populations from different geographic areas have not been sufficiently explored with molecular data. In this study we analyzed the relationships among the four species in the ‘chiffchaff complex’ (Common Chiffchaff, Iberian Chiffchaff P. ibericus, Canary Islands Chiffchaff P. canariensis and Mountain Chiffchaff P. sindianus), and the patterns of intraspecific geographic variation in the mtDNA ND2 gene and intron 9 of the Z-linked aconitase gene (ACO1I9) across the Common Chiffchaff range, including a recently discovered population breeding on Mt. Hermon (Anti-Lebanon mountains). Our data supported the monophyly of the chiffchaff complex and its current systematics at the species level. Within the Common Chiffchaff, the Siberian race P. c. tristis was the most differentiated subspecies and may represent a separate or incipient species. Other Common Chiffchaff subspecies also were differentiated in their mtDNA, however, lineages of neighboring subspecies formed wide zones of introgression. The Mt. Hermon population was of mixed genetic origin but contained some birds with novel unique lineage that could not be assigned to known subspecies. All Common Chiffchaff lineages diverged at the end of the Ionian stage of Pleistocene. Lineage sorting of ACO1I9 alleles was not as complete as that of mtDNA. Chiffchaff species were mostly distinct at ACO1I9, except the Common and Canary Islands Chiffchaffs that shared multiple alleles. An AMOVA identified geographic structure in Common Chiffchaff ACO1I9 variation that was broadly consistent with that of mtDNA ND2 gene. The genetic and other data suggest the chiffchaff complex to be a group of evolutionarily young taxa that represent a paradigm of ‘species evolution in action’ from intergrading subspecies through to apparently complete biological speciation.

普通柳莺（Common Chiffchaff, *Phylloscopus collybita*）是一类数量丰富的多型古北界鸟类。其部分亚种的分类有效性尚存争议，分布区内的部分种群尚未得到明确的分类学归类。目前学界尚未利用分子数据充分探究该物种不同地理种群间的亲缘关系。本研究分析了“柳莺复合群”（chiffchaff complex）内的4个物种——普通柳莺、伊比利亚柳莺（*P. ibericus*）、加那利柳莺（*P. canariensis*）与山柳莺（*P. sindianus*）——的亲缘关系，并针对普通柳莺的整个分布范围，探究了其线粒体DNA ND2基因（mtDNA ND2 gene）与Z连锁乌头酸酶基因内含子9（Z-linked aconitase gene intron 9, ACO1I9）的种内地理变异模式，研究样本包含了近期在赫尔蒙山（前黎巴嫩山脉）发现的繁殖种群。研究数据支持柳莺复合群为单系群，且其当前基于物种级别的分类系统合理有效。在普通柳莺内部，西伯利亚亚种*P. c. tristis*的分化程度最高，可能代表一个独立物种或处于物种形成初期的类群。其余普通柳莺亚种的线粒体DNA同样存在分化，但相邻亚种的线粒体谱系形成了广泛的基因渐渗区域。赫尔蒙山种群具有混合的遗传起源，其中部分个体拥有独特的新谱系，无法归入已知的任何亚种。所有普通柳莺谱系均在更新世爱奥尼亚阶末期发生分化。ACO1I9等位基因的谱系分选程度不及线粒体DNA。除普通柳莺与加那利柳莺共享多个等位基因外，柳莺复合群各物种在ACO1I9位点上大多存在明显分化。分子方差分析（Analysis of Molecular Variance, AMOVA）结果显示，普通柳莺ACO1I9变异的地理结构与线粒体DNA ND2基因的地理结构大体一致。综合遗传与其他相关数据，柳莺复合群属于演化时间较短的类群，展现了“正在进行的物种演化”的典型范例：从渐杂交的亚种逐步过渡到近乎完全的生物学物种形成。