With the increasing application of molecular techniques for diatom species discovery and identification, it is important both from a taxonomic as well as an ecological and applied perspective, to understand in which groups morphological species delimitation is congruent with molecular approaches, or needs reconsideration. Moreover, such studies can improve our understanding of morphological trait evolution in this important group of microalgae. In this study, we used morphometric analysis on light microscopy (LM) micrographs in SHERPA, detailed scanning electron microscopy (SEM), and cytological observations in LM to examine 70 clones belonging to eight distinct molecular lineages of the cosmopolitan terrestrial diatom Pinnularia borealis. Due to high within-lineage variation, no conclusive morphological separation in LM nor SEM could be detected. Morphological stasis due to the “low-morphology” problem or stabilizing selection, as well as parallel/convergent evolution, phenotypic plasticity and structural inheritance are discussed as potential drivers for the observations. Altogether, P. borealis is truly cryptic, in contrast to the majority of other diatom species complexes which turned out to be pseudo-cryptic following detailed morphological analysis.

随着分子技术在硅藻物种发现和鉴定中的应用日益广泛，从分类学以及生态学和应用的角度来看，重要的是要了解哪些物种的形态学界定与分子方法是一致的，还是需要重新考虑。此外，此类研究可以增进我们对这一重要微藻类群的形态特征进化的理解。在这项研究中，我们对SHERPA中的光学显微镜（LM）显微照片，详细的扫描电子显微镜（SEM）和LM中的细胞学观察进行了形态计量学分析，以检查属于大都会陆地硅藻Pinnulariaborealis八个不同分子谱系的70个克隆。由于内部谱系差异很大，因此无法在LM和SEM中检测到最终的形态学分离。讨论了由于“低形态”问题或稳定选择而引起的形态停滞，以及平行/收敛演化，表型可塑性和结构遗传，这是观察的潜在驱动力。总的来说，北极假单胞菌是真正的隐性，与大多数其他硅藻物种复合物形成对比，而这些硅藻物种复合物在经过详细的形态学分析后变成伪隐性。