海链藻属硅藻的形态分类和分子系统研究

Thalassiosira Cleve is a very species-rich diatom genus with nearly 400 described taxa, mainly based on morphological features. Thalassiosira species are globally distributed and are ecologically important as they often make up a major part of the phytoplankton community and form blooms. They are therefore an excellent choice as model species of the diatoms (Bacillariophyta). Routine examination of Thalassiosira species relies presently mainly on observations of the valve structures using scanning and transmission electron microscopy. Morphological taxonomy of Thalassiosira has thus become more difficult and challenging. However, in Chinese coastal waters, most studies have been based on light microscopy, and the identification of most Thalassiosira species has stopped at the genus level. Therefore, the species diversity has not been accessed accurately. Comprehensive taxonomic studies using EM are rare and scattered. Moreover, recent phylogenetic analyses inferred from 18S rDNA sequences of the Thalassiosirales have indicated the existence of several well-supported clades in this group, but proved less successful in resolving the within-clade relationships. So far, the paraphyly of Thalassiosira is strongly supported, and better resolution of the within-group relationships may be possible with the use of less conserved molecular markers. In present proposal, the taxonomy and systematic studies of Thalassiosira species are recommended to be conducted based on the combination of morphological methods and molecular analyses synchronously. On one hand, following contents and aims will be achieved using morphological methods: (1) Establish a detailed morphological database of Thalassiosira taxa; (2) Clarify the variations of some key morphological characters and analyse their stability within taxa; (3) Reestimate the taxonomic values of key morphological characters for delineation of taxa; (4) Establish/verify the morphological taxonomical criteria among Thalassiosira species and the systematic relationships within Thalassiosira based on morphological data. Simultaneously, molecular phylogenetic analyses will be carried out based on different target genes from the chloroplast and nuclear genomes, such as SSU, LSU, ITS, rbcl, etc. Finally, the following key taxonomic and systematic questions will be answered through the combination and comparison between morphological data and molecular results: (1) How many different paraphyletic clades exist in Thalassiosira and Thalassiosirales; (2) Which morphological characters reveal shared derived traits in the morphological and molecular trees and which ones show mainly homoplasy; (3) Does morphologically perceived species constitute clades and (4) Do these clades contain genetically distinct groups of specimens. Above studies will provide progress and important new knowledge in the taxonomy and systematics of Thalassiosira, and may hence serve as a baseline study for future studies.

海链藻是全球沿岸水域常见的优势浮游植物和赤潮生物，也是硅藻门的模式物种。该属种类繁多，已报道近400个分类单位，是藻类分类学和系统学领域的研究热点和难点。本项目拟以海链藻及其系统学邻近属种为研究对象，采用形态学和分子生物学技术相结合的手段开展分类学和系统学的专题研究。一方面，利用形态学方法，建立完善的形态学数据库和图像资料库；充分研究关键特征的变化范围并统计分析，就其分类学价值进行二次研究和评估；筛选有效而稳定的特征，建立基于形态学的分类标准和系统关系。另一方面，利用基因序列分析等分子生物学技术开展分子分类学和分子系统学研究，构建系统进化发育树。最后，综合考虑形态学和分子生物学的结论，澄清海链藻属内现存的多个系统学类群，提炼各个类群的共享特征，明确模式物种，构建分类依据和分类标准，对现有分类学和系统学体系进行修订，并对（拟）隐形种进行二次研究和修订。

海链藻是全球沿岸水域常见的优势浮游植物和赤潮生物，也是硅藻门的模式物种。该属种类繁多，已报道近400个分类单位。我国关于海链藻的研究相对较少，物种多样性仍不明确。此外，近年分子系统学研究已经证实，目前建立的海链藻属并非单系起源，但是该如何进行系统学修订，仍是难题，未有进展。本项目基于建立的单克隆培养株系，结合形态学和分子生物学数据，开展了海链藻属的分类学和系统学研究。项目执行的四年期间，在中国沿海进行了广泛的活体样品采集。利用毛细管复洗技术，挑取海链藻单细胞或单条链状群体，成功建立422个培养株系。它们隶属于37个分类单位，报道了海链藻属的5个新分类单位：扇形海链藻、隐秘海链藻、中华海链藻、艾伦海链藻肋纹变种、小字海链藻双卫变种。聚焦内壳面（半）连续筛板膜结构，建立了具有该特征的海链藻属物种，它们在基于核糖体编码基因的系统树上呈现单系起源，可依此将该类群独立成属。此外，聚焦双唇形突结构，建立了若干海链藻属和漂流藻属的株系，开展了物种多样性和系统学研究。报道了三个新物种：疑难漂流藻、相似漂流藻和广州海链藻。还发现在基于核糖体编码基因的系统树上，具有双唇形突结构的海链藻科物种呈现单系起源，或可由此进行系统学调整，但仍需更多双唇形突物种的分子生物学信息。本项目报道了海链藻属更为丰富的物种多样性，并尝试基于一些稀有特征，对现存海链藻属进行系统学修订。相关研究丰富了海链藻属的物种多样性，部分澄清了海链藻属，甚至海链藻科的物种认识，为海链藻属的系统学修订提供了有价值线索。