Phylogeny, biogeography and taxonomic re-assessment of Multifurca (Russulaceae, Russulales) using three-locus data

Multifurca is a small genus newly established to accommodate lactarioid and russuloid species with some characters reminiscent of corticoid members of Russulaceae. It shows an amphi-pacific distribution with strong preference for the tropical zone of the Northern Hemisphere and thus has particular significance for biogeographical study. Using worldwide samples and three loci (ITS, 28S rDNA and rpb2), we demonstrated that Multifurca is split into two highly supported major clades that are here recognized at the subgeneric level: subg. Furcata subg. nov. exclusively includes lactarioid species, while subg. Multifurca includes species with a russuloid habit. Using phylogenetic species recognition and comparison of genetic distances we recognize five new and six previously described species, almost double the known number of species before this study. Molecular dating using a Bayesian method suggested that Multifurca originated in early Paleocene and diversified in the Eocene. The most recent interspecific divergences occurred both in Asia and America, roughly at the same time around the Pliocene. Ancestral area reconstruction and comparisons of genetic distances and morphology suggested an early divergence within Australasia or tropical Asia. From the early Miocene to Pliocene, multiple dispersals/migrations to Australasia and North America by island hopping or land bridge likely happened. Vicariance at the late Tertiary might be the most likely mechanism accounting for the eastern Asia—southeastern North America and Australasia—tropical Asia disjunct distributions. The shared polymorphisms in the ITS alignment, numerous degenerated base pairs in the rpb2 sequences and weak conflict between the ITS and LSU genealogies of M. subg. Furcata suggest recent speciation. Host specificity of Multifurca species or species pairs is relatively low. Host shifts are believed to have aided establishment in new territories during the dispersals and migrations.多叉菌属（Multifurca）是一个新近建立的小型类群，用以容纳兼具乳菇类（lactarioid）与红菇类（russuloid）特征，且部分性状类似红菇科（Russulaceae）革菌状（corticoid）成员的物种。该类群呈环太平洋分布，且对北半球热带区域具有极强的偏好性，因此在生物地理学研究中具有特殊意义。本研究依托全球采样样本与三个基因位点——内转录间隔区（ITS）、28S核糖体DNA（28S rDNA）以及RNA聚合酶Ⅱ第二亚基（rpb2）——的序列数据，证实多叉菌属可分为两个支持度极高的主要演化支，本研究将其界定为亚属级分类单元：叉状新亚属（subg. Furcata subg. nov.）仅包含乳菇类物种，而多叉菌亚属（subg. Multifurca）则涵盖具有红菇类习性的物种。通过系统发育物种识别与遗传距离比较分析，本研究共界定5个新种与6个已发表物种，使该属已知物种数量较本研究开展前近乎翻倍。采用贝叶斯方法进行分子钟定年的结果显示，多叉菌属起源于古新世早期，并在始新世发生分化。最近的种间分化事件同时发生于亚洲与美洲，大致集中在上新世前后的相近时段。祖先区域重建、遗传距离与形态学比较分析表明，该属的早期分化发生于澳大拉西亚或热带亚洲区域。从中新世早期至上新世，类群可能通过岛屿跳跃或陆桥进行多次扩散/迁移，进而抵达澳大拉西亚与北美洲。第三纪晚期的地理分异事件，或为解释东亚-北美东南部、澳大拉西亚-热带亚洲间断分布格局的最合理机制。叉状新亚属（subg. Furcata）的ITS序列比对中存在共享多态性，rpb2序列中存在大量简并碱基对，且ITS与大亚基核糖体RNA（LSU）的谱系间存在微弱冲突，这些证据均暗示该类群经历了近期的物种形成过程。多叉菌属物种或物种类群的宿主特异性相对较低，学界认为宿主转换事件在其扩散迁移过程中助力了新区域的定殖。