Data from: A new species and introgression in eastern Asian hemlocks (Pinaceae: Tsuga)

Species delimitation in Pinaceae is often challenged by limited morphological differentiation and introgression. In Tsuga (hemlocks), species delimitation has been most challenging among northeastern Asian taxa, where the species are weakly marked morphologically and range in number from three to five in previous studies. Two low-copy nuclear four-coumarase-ligase (4CL) genes andmorphology strongly support a clade of the Japanese endemic T. diversifolia and T. sieboldii from Japan and Ulleung island (Ulleungdo) in Korea. This clade is here referred to as the oceanic hemlocks. 4CL strongly supports a sister-group relationship of the widespread northeastern Asian T. chinensis and eastern North American T. caroliniana. In contrast, chloroplast genomes, which are markedly reduced in Tsuga and relatives, strongly support Japanese T. sieboldii as sister to T. chinensis and moderately support T. caroliniana as sister to a clade of T. diversifolia and hemlocks fromUlleungdo. These divergent topologies suggest chloroplast capture of T. chinensis by Japanese T. sieboldii. Ulleungdo hemlocks are distinct from other northeastern Asian species in leaf and cone morphology and phenologically in common-garden observations. We therefore describe these hemlocks as a new species, T. ulleungensis.松科（Pinaceae）的物种界定常受限于有限的形态分化与基因渐渗（introgression）。在铁杉属（Tsuga）中，东北亚类群（taxa）的物种界定难度最高——该类群物种的形态特征辨识度极低，既往研究中其物种数量被界定为3至5种。两项低拷贝核基因4-香豆酸连接酶（4CL）以及形态学证据均强烈支持一支演化支（clade）：该支包含分布于日本以及韩国郁陵岛（Ulleungdo）的日本特有种日本铁杉（T. diversifolia）和西博尔德铁杉（T. sieboldii），本研究将该演化支命名为海洋铁杉类群（oceanic hemlocks）。4CL基因证据强烈支持广布东北亚的中国铁杉（T. chinensis）与北美东部的卡罗莱纳铁杉（T. caroliniana）构成姐妹群关系（sister-group relationship）。与之形成对比的是，铁杉属及其近缘类群的叶绿体基因组（chloroplast genomes）普遍存在显著缩减，该证据强烈支持日本西博尔德铁杉（T. sieboldii）与中国铁杉（T. chinensis）构成姐妹群，同时中等程度支持卡罗莱纳铁杉（T. caroliniana）为日本铁杉（T. diversifolia）与郁陵岛铁杉类群的姐妹群。这些相互矛盾的拓扑结构（topology）表明，日本西博尔德铁杉曾发生过对中国铁杉叶绿体的捕获事件（chloroplast capture）。通过叶部与球果的形态特征，以及同质园观测（common-garden observations）的物候结果，郁陵岛铁杉类群与其他东北亚铁杉物种存在显著差异。基于上述证据，本研究将该类群描述为一新种：郁陵铁杉（T. ulleungensis）。