Mergansers are riverine and coastal piscivorous ducks that are widespread throughout North America and Eurasia but uncommon in the Southern Hemisphere. One species occurs in South America and at least two extinct species are known from New Zealand. It has been proposed that these Southern Hemisphere merganser lineages were founded by at least two independent dispersal events from the Northern Hemisphere. However, some morphological and behavioural evidence suggests that Southern Hemisphere mergansers may form a monophyletic clade that descended from only a single dispersal event from the Northern Hemisphere. For example, Southern Hemisphere mergansers share several characteristics that differ from Northern Hemisphere mergansers (e.g. non-migratory vs. migratory, sexual monochromatism vs. sexual dichromatism, long vs. short pair bonds). We sequenced complete mitogenomes from the Brazilian merganser and an extinct merganser from New Zealand—the Auckland Island merganser. Our results show that the Brazilian and Auckland Island mergansers are not sister-taxa, and probably descend from two separate colonization events from the Northern Hemisphere at least 7 Mya. Nuclear (palaeo)genomic data may help to further resolve the relationship between living and extinct mergansers, including merganser fossils from New Zealand that have not been subjected to palaeogenetic analysis.

中文翻译：

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古代线粒体基因组揭示了晚中新世秋沙鸭向南半球扩散的证据

秋沙鸭是河流和沿海的食鱼鸭，广泛分布于北美和欧亚大陆，但在南半球并不常见。其中一种出现在南美洲，新西兰至少有两种已灭绝的物种。有人提出，这些南半球秋沙鸭谱系是由至少两次来自北半球的独立扩散事件建立的。然而，一些形态和行为证据表明，南半球秋沙鸭可能形成一个单系进化枝，仅源自北半球的一次扩散事件。例如，南半球秋沙鸭有几个与北半球秋沙鸭不同的特征（例如非迁徙性与迁徙性、性单色性与性二色性、长配对键与短配对键）。我们对巴西秋沙鸭和新西兰已灭绝的秋沙鸭——奥克兰岛秋沙鸭的完整线粒体基因组进行了测序。我们的研究结果表明，巴西秋沙鸭和奥克兰岛秋沙沙鸭不是姊妹类群，可能是来自北半球至少 7 Mya 的两次独立殖民事件的后代。核（古）基因组数据可能有助于进一步解决现存秋沙鸭与已灭绝秋沙鸭之间的关系，包括来自新西兰尚未经过古遗传学分析的秋沙鸭化石。