Re-examination and assessment of the morphological traits of the diatom genus Thalassiosira Cleve, a case study of Thalassiosira allenii Takano
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摘要: 海链藻属（Thalassiosira Cleve）是硅藻门中物种多样性最为丰富的大属之一，广泛分布于全球各类水体。目前沿用的海链藻属分类体系是基于形态特征建立的，尤其是电镜下的超微特征具有重要的分类价值。然而，也有研究认为某些形态特征存在不稳定或变化的情况，应对其开展二次研究和重新评估。故此，本文选择全球近岸广布种艾伦海链藻（T.allen Takano）作为典型代表，基于建立的单克隆培养株系，结合传统形态学方法和现代分子生物学技术，对其重要形态特征进行了二次研究。结果显示：（1）艾伦海链藻壳面孔纹的排列方式存在一定的变化，呈切线形或偏心状；壳缘唇形突的位置多有变化，占据一个壳缘支持突的位置、或位于两个壳缘支持突正中间、或靠近其中一个。艾伦海链藻这两个特征并不具有较高的稳定性，其分类学意义或应降低；（2）壳缘不具有肋纹，此特征相对稳定，可作为艾伦海链藻的识别特征之一；（3）艾伦海链藻具有较保守的遗传多样性，尚未发现（拟）隐形种。结合其它海链藻的形态学报道，我们认为孔纹排列方式易受环境的影响，稳定性较差，在分类时应慎重应用；壳缘唇形突的位置在少数海链藻属物种中易发生变化，分类时应视具体物种而针对性应用；支持突等突起特征较为稳定，在海链藻属物种的分类体系中仍可作为重要依据；艾伦海链藻中壳缘肋纹具有较好的稳定性，可作为分类依据，但在其它海链藻属物种中是否也具有很好的分类价值，还需要更多的研究加以验证；分子信息在形态特征上的二次研究和评价中具有重要应用价值，但目前相关分子数据的缺乏和不足限制了对其进一步的研究。本研究对重新评估艾伦海链藻形态特征的分类学价值、完善现存的分类学体系提供了参考。Abstract: Thalassiosira Cleve is one of the largest genera of diatoms, and its taxa are widely distributed in global waters. The present taxonomic and systematic theories of Thalassiosira have been established on the basis of morphology. Most morphological traits provide important taxonomic significance, especially ultra-morphology under electron microscopy. However, variation in some key morphological features has been reported, and the value in taxonomic application has become somewhat suspect. Here, a widely distributed species, T. allen Takano, was selected to re-examine certain traits inferred from morphological and molecular analysis based on established monoclonal strains. Results showed that: (1) The arrangement pattern of the areolae in T. allen varied normally, exhibiting tangential or eccentric array. Different locations of rimoportulae were found, replacing the place of one marginal fultoportula, or at the middle between two, or close to one marginal fultoportula. These two features varied considerably, and thus their taxonomic significance has become somewhat limited. (2) No rib was observed at the valve margin, which could be applied as a typical trait to distinguish T. allen and close taxa. (3) No pseudo-cryptic or cryptic species were found in T. allen due to their conservative genetic divergence. Combined with previous studies, some morphological traits were reassessed. Areolae arrangement varied frequently and was easily affected by environmental factors, and thus was not a good taxonomic trait for Thalassiosira taxa classification. Processes were key features for Thalassiosira taxa, because most traits were steady, especially the amount, location, and density of the fultoportulae. However, the location of the rimoportulae varied in some Thalassiosira species, and should be cautiously applied. Ribs were a relatively stable trait for T. allen and several other Thalassiosira taxa, and could be applied in taxonomic classification. Molecular data can play a key role in the re-examination and re-evaluation of morphological traits, while inadequate information remains a limitation for further study. In summary, this research provides a useful reference for the re-evaluation of the taxonomic significance of key morphological traits and renews the usage of these traits in present taxonomy.
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Keywords:
- Thalassiosira /
- Thalassiosira allenii /
- Morphological traits /
- Re-examination /
- Assessment
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