Polyphyly and homoplasic structures in rhizosolenioid diatom genera: A review

Abstract

Background and aims – Traditionally, extant rhizosolenioid diatom genera have been placed in a single family, the Rhizosoleniaceae. However, preliminary molecular data suggested that the family might be polyphyletic. Therefore, a literature review of the morphological, ultrastructural and molecular data of the rhizosolenioid genera was undertaken. Methods – In addition to the literature survey, the location of the rimoportula in a number of rhizosolenioid genera was investigated by breaking the valves and observing the fragments in the scanning electron microscope. Key results – The data provides strong support for the previous separation of Proboscia and Rhizosolenia at the family level (Probosciaceae vs. Rhizosoleniaceae), with the rimoportula being located at the tip of the proboscis in Proboscia, or with an internal labia at the base of the hollow tubular rimoportula (= spine or process) in Rhizosolenia and Pseudosolenia. Conclusions – The data suggests that a number of rhizosolenioid genera should be transferred to other families, and that gene sequences of two genera (Dactyliosolen and Neocalyptrella) are needed as their morphological features differ markedly from those of the Rhizosoleniceae s. str. (Rhizosolenia, Guinardia, Pseudosolenia).