The diatom genus Orthoseira Thwaites (Bacillariophyta) is a ubiquitous taxon in aerial diatom assemblages, with species found globally. Cylindrical cell shape and radial symmetry of this genus has led to its historical placement in the Coscinodiscophyceae (‘radial centric’ diatoms), but its systematic relationships have remained uncertain. We present a five-gene phylogeny, based on nuclear (nSSU rDNA) and chloroplast (rbcL, psbC, psbA, and psaB) genes to determine the phylogenetic placement of Orthoseira among the diatoms. The concatenated multi-gene phylogenies and nSSU-only gene tree demonstrate that Orthoseira is deeply embedded within a clade of the Mediophyceae (‘multipolar centric’ diatoms). Throughout all phylogenetic analyses, Orthoseira was shown to be sister to the genera Terpsinoë and Hydrosera. Through comparison of topologies reflecting competing hypotheses about the placement of Orthoseira, it was determined that the hypothesis that Orthoseira, represented here by O. dendroteres and O. roeseana, is a member of the Melosirales should be rejected. Therefore, lack of morphological similarity between Hydrosera, Orthoseira, and Terpsinoë is hypothesized to be the result of changes in habitat preferences that lead to an ancient divergence event between the Orthoseirales and the Hydrosera, Terpsinoë clade.

硅藻属Orthoseira Thwaites（Bacillariophyta）是空中硅藻群落中普遍存在的分类群，其物种遍布全球。该属的圆柱状细胞形状和径向对称性导致其在Coscinodiscophyceae（“径向中心”硅藻）中的历史定位，但其系统关系仍不确定。我们提出了一个基于核（nSSU rDNA）和叶绿体（rbcL，psbC，psbA和psaB）基因的五基因系统发育，以确定Orthoseira在硅藻中的系统发生位置。级联的多基因系统发育和仅nSSU的基因树表明Orthoseira被深深地嵌入到地中海植物科（“多极中心”硅藻）内。在所有的系统发育分析中，Orthoseira被证明是Terpsinoë和Hydrosera属的姐妹。通过拓扑反射大约的放置竞争假设的比较Orthoseira，它被确定的是，假设Orthoseira，表示这里由O. dendroteres和O. roeseana，是Melosirales的构件应该被拒绝。因此，Hydrosera，Orthoseira和Terpsinoë之间缺乏形态相似性假设是在栖息地的偏好导致Orthoseirales和之间的分歧古代事件变化的结果Hydrosera，Terpsinoë分支。