Molecular phylogeny of the marine diatom genus Druehlago (Bacillariophyceae) from Japan

The diatom genus Druehlago was established to differentiate the type species from Craspedostauros and Achnanthes, on the basis of unique heteropolar, cuneate frustules and numerous lenticular plastids. The phylogenetic placement of Druehlago has, until now, relied solely on morphological evidence, suggesting a close relationship with the genus Craspedostauros. In the present study, we discovered a population of Druehlago cf. cuneata along the coast of Japan and established a cultured strain. Molecular phylogenetic analysis based on rbcL, psbC and 18S rRNA genes was conducted to explore the phylogenetic position and classification of Druehlago. Our results revealed that D. cf. cuneata belongs to the Craspedostauros clade, but with weak statistical support, which is, in turn, sister to Achnanthes. Given the lack of support for various nodes within the Craspedostauros clade, constrained trees were constructed based on several hypotheses regarding the phylogenetic position of Druehlago. The hypothesis inferring monophyly of Druehlago and Achnanthes was statistically rejected, which highlights the close relationship between Druehlago and Craspedostauros. However, our topology tests provided no further resolution regarding the phylogenetic position of Druehlago within or outside Craspedostauros. Furthermore, the treatment of the specimens is discussed through a morphological comparison with the type of material.