Symmetry and structure re-visited: is symmetry still over-emphasised as a taxonomic character for naviculoid diatoms?

Background and aims – Over 40 years ago, I explored the use of symmetry and structure as taxonomic criteria for naviculoid diatoms, arguing that structure was the more informative character. This paper examines subsequent progress in the understanding and systematics of naviculoid diatoms with particular reference to the treatment of symmetry as a taxonomic criterion. Methods and key results – The diatom literature since 1979 has been reviewed showing how SEM studies and data on wall morphogenesis have improved our knowledge of diatom wall structure and ontogenetic pathways. Understanding the inherent asymmetry in raphid diatom valves led to the recognition of two types of dorsiventrality within the Cymbellales and underpins the separation of cymbelloid taxa in different families. Hypotheses of character homology in the Naviculaceae have been supported by more recent molecular phylogenetic analyses, also revealing how shape and symmetry can vary within this family. Over-emphasising the significance of a particular character (out of familiarity or a priori preference?) can lead to relevant discriminative characteristics being ignored. The validity of some new genera is questioned and generic descriptions should be revised. Conclusion – Whereas valve shape and symmetry remain useful features when identifying taxa, systematic relationships are revealed through wall structure and should be the basis for diatom systematics. Nevertheless, it is important to discriminate between homologous and homoplasious characters and to apply terminology accurately.