Abstract
Tryblionella (Bacillariaceae) was described by Smith in 1853, and for many years, the diagnosis of this genus was questionable. Recent molecular analysis based on the rbcL gene marker suggests that Tryblionella is a polyphyletic genus with T. apiculata, T. hungarica, and T. gaoana forming a distinct group from other Tryblionella representatives. Therefore, this study aimed to clarify the diagnosis of Tryblionella as a genus. The focus of this study was a selected group of species previously categorized within Grunow's section Apiculatae, which includes the type species T. acuminata. This classification serves as a foundation for conducting morphological and molecular comparisons with taxa of Tryblionella sensu lato, which are likely to represent distinct and highly diverse genera. Our review includes a detailed examination of frustule ultrastructure and ontogeny combined with a new molecular phylogenetic analysis derived from a three-gene concatenated dataset. The results of our research indicated that among Tryblionella, several monophyletic groups of taxa can be distinguished, including Tryblionella sensu stricto (s.s.), by three key characters: a porose valve cross-section; longitudinal valve undulation, where the peak of the undulation is located on the proximal side of the valve; and the presence of an axial sternum with thickened and relief virgae. Tryblionella s.s. taxa share a similar girdle structure: The girdle is graded, except that the first band bears a single row of poroids in the pars exterior and a crenulate margin on the side corresponding to the distal valve mantle.