Department of Forest Science, School of Agriculture, Utsunomiya University, 350 Mine-machi, Utsunomiya, Tochigi, 321-8505, Japan.
Department of Forest Molecular Genetics and Biotechnology, Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan.
J Plant Res. 2021 Sep;134(5):907-919. doi: 10.1007/s10265-021-01301-8. Epub 2021 Apr 18.
The Japanese archipelago exhibits a notable difference in snow depth in winter, deep snow on the Sea of Japan side and low snow cover on the Pacific Ocean side. This contrasting pattern has shaped the distribution of infraspecific taxon pairs in a range of woody plants, with taxa found on the Sea of Japan side typically exhibiting a stunted shrub form with multiple decumbent stems. The phylogenetic origin of these taxon pairs is unknown, i.e., whether the two taxa diverged from the same species or if they have different origins. This study aimed to reveal the phylogenetic origin of two varieties of Torreya nucifera (Taxaceae); var. nucifera is a tree found on the Pacific Ocean side, whereas var. radicans is a shrub found on the Sea of Japan side. We examined the phylogenetic relationships of the two varieties and worldwide Torreya taxa using whole chloroplast genomes, chloroplast DNA fragments, and the nuclear ribosomal internal transcribed spacer (ITS). The whole chloroplast genome phylogeny indicated that T. nucifera var. radicans was a sister taxon to Chinese T. grandis, rather than to var. nucifera. In contrast, the nuclear ITS phylogeny indicated that while several haplotypes of T. nucifera var. radicans were closely related to T. grandis, most haplotypes of T. nucifera var. radicans formed a single clade with those of var. nucifera. This implies that the homogenization of the ITS has occurred between the two taxa, while taxon-specific chloroplast DNA haplotypes were retained. These discordant phylogenies suggested that the two taxa have different phylogenetic origins, but have an intricate evolutionary history, involving inter-taxa hybridization and gene flow, possibly when their distributions were confined to sympatric refugia. Given the genetic evidence and distinct difference in growth form, we propose that T. nucifera var. radicans should be taxonomically treated as a distinct species, T. fruticosa.
日本群岛在冬季的积雪深度上表现出显著差异，日本海一侧积雪深厚，而太平洋一侧积雪较少。这种对比模式塑造了许多木本植物的种下分类单元的分布格局，在日本海一侧发现的分类单元通常表现为多枝平卧的矮灌木形态。这些分类单元对的系统发育起源尚不清楚，即它们是来自同一个物种的两个分支，还是具有不同的起源。本研究旨在揭示 Torreya nucifera（红豆杉科）两个变种的系统发育起源；var. nucifera 是一种生长在太平洋一侧的乔木，而 var. radicans 是一种生长在日本海一侧的灌木。我们利用整个叶绿体基因组、叶绿体 DNA 片段和核核糖体内部转录间隔区（ITS），研究了这两个变种和全球 Torreya 分类单元的系统发育关系。整个叶绿体基因组系统发育分析表明，T. nucifera var. radicans 是中国红豆杉 T. grandis 的姊妹分类单元，而不是 var. nucifera。相比之下，核 ITS 系统发育分析表明，尽管 T. nucifera var. radicans 的几个 ITS 单倍型与 T. grandis 密切相关，但大多数 T. nucifera var. radicans 的 ITS 单倍型与 var. nucifera 形成了一个单独的分支。这意味着这两个分类单元的 ITS 已经发生了同质化，而分类单元特异性的叶绿体 DNA 单倍型则被保留了下来。这些不一致的系统发育关系表明，这两个分类单元具有不同的系统发育起源，但它们有着复杂的进化历史，涉及到种间杂交和基因流，可能发生在它们的分布局限于同域避难所的时候。基于遗传证据和明显不同的生长形态，我们建议将 T. nucifera var. radicans 作为一个独特的物种，即 T. fruticosa，进行分类处理。
