Nikula Raisa, Strelkov Petr, Väinölä Risto
Department of Biological and Environmental Sciences, P.O. Box 65, FI-00014, University of Helsinki, Finland.
Evolution. 2007 Apr;61(4):928-41. doi: 10.1111/j.1558-5646.2007.00066.x.
The history of repeated inter- or transoceanic invasions in bivalve mollusks of the circumpolar Macoma balthica complex was assessed from mtDNA COIII sequences. The data suggest that four independent trans-Arctic invasions, from the Pacific, gave rise to the current lineage diversity in the North Atlantic. Unlike in many other prominent North Atlantic littoral taxa, no evidence for (postinvasion) trans-Atlantic connections was found in the M. balthica complex. The earliest branch of the mtDNA tree is represented by the temperate-boreal North American populations (=Macoma petalum), separated from the M. balthica complex proper in the Early Pliocene at latest. The ensuing trans-Arctic invasions established the North European M. b. rubra, which now prevails on the North Sea and northeast Atlantic coasts, about two million years ago, and the currently northwest Atlantic M. balthica lineage in the Canadian Maritimes, in the Middle Pleistocene. The final reinvasion(s) introduced a lineage that now prevails in a number of North European marginal seas and is still hardly distinguishable from North Pacific mtDNA (M. b. balthica). We used coalescence simulation analyses to assess the age of the latest invasion from the Pacific to the northeast Atlantic. The results refute the hypothesis of recent, human-mediated reintroductions between northeast Pacific and the North European marginal seas in historical times. Yet they also poorly fit the alternative hypotheses of an early postglacial trans-Arctic invasion (< 11 thousand years ago), or an invasion during the previous Eemian interglacial (120 thousand years ago). Divergence time estimates rather fall in the Middle Weichselian before the Last Glacial Maximum, in conflict with the conventional thinking of trans-Arctic biogeographical connections; an early Holocene reinvasion may still be regarded as the most plausible scenario. Today, the most recently invaded Pacific mtDNA lineage is found admixed with the earlier established European Atlantic "rubra" lineage in the Baltic Sea and in Barents Sea populations east of the Varanger peninsula, and it is practically exclusive in the White and Pechora seas. Yet mtDNA does not always constitute an unequivocal taxonomic marker at individual level; the marginal populations represent hybrid swarms of the Atlantic and Pacific lineages in their nuclear genes.
基于线粒体DNA细胞色素氧化酶亚基III（mtDNA COIII）序列，评估了环北极区波罗的海蛤仔复合体双壳贝类反复进行的跨洋或跨北冰洋入侵历史。数据表明，来自太平洋的四次独立跨北极入侵产生了北大西洋目前的谱系多样性。与许多其他著名的北大西洋沿岸类群不同，在波罗的海蛤仔复合体中未发现（入侵后）跨大西洋联系的证据。线粒体DNA树的最早分支由温带 - 寒带北美种群（=花瓣蛤仔）代表，最晚在早上新世与波罗的海蛤仔复合体主体分离。随后的跨北极入侵在约200万年前形成了北欧的红波罗的海蛤仔，它现在在北海和东北大西洋海岸占主导地位，并在中更新世形成了加拿大海洋省份目前的西北大西洋波罗的海蛤仔谱系。最后的再次入侵引入了一个谱系，该谱系现在在一些北欧边缘海域占主导地位，并且仍然与北太平洋线粒体DNA（波罗的海蛤仔指名亚种）难以区分。我们使用溯祖模拟分析来评估最近一次从太平洋到东北大西洋入侵的时间。结果反驳了历史时期东北太平洋与北欧边缘海域之间近期由人类介导的重新引入的假设。然而，它们也不太符合早期冰后期跨北极入侵（<11000年前）或上一个埃姆间冰期（120000年前）入侵的替代假设。分歧时间估计相当落在末次盛冰期之前的魏克塞尔间冰期，这与跨北极生物地理联系的传统观念相冲突；全新世早期的再次入侵仍可能被视为最合理的情况。如今，最近入侵的太平洋线粒体DNA谱系在波罗的海以及瓦朗厄尔半岛以东的巴伦支海种群中与较早建立的欧洲大西洋“红”谱系混合存在，并且实际上在白海和伯朝拉海是唯一的。然而，线粒体DNA在个体水平上并不总是构成明确的分类标记；边缘种群在其核基因中代表了大西洋和太平洋谱系的杂交群。
