Algae World: Sellaphora

Overview

Individual species of Sellaphora have been known for up to 170 years, but for most of this period they were classified in the genus Navicula, which was then a 'catch-all' grouping of simply-structured, bilaterally symmetrical, raphid diatoms. The species destined to become the type of Sellaphora, Navicula pupula, was described by F.T. Kützing in 1844. At around the same time, two other commonly-recorded species, Sellaphora bacillum and S. laevissima, were also described, again as Navicula species.

During most of the 19th and 20th centuries, diatom taxonomy was based almost entirely on the structure of the frustule (the silicified cell wall of diatoms); other features, such as chloroplast morphology, reproductive characteristics or nuclear behaviour, were not considered. As we explain elsewhere on this site, there is nothing particularly unusual about the structure of the frustule in Sellaphora - there are no unique features that set Sellaphora apart from all other 'naviculoid' diatoms. Hence the classification of Sellaphora species in Navicula is wholly understandable, if frustule characters are used as the sole basis for diatom taxonomy. However, if chloroplast morphology is taken into account, differences between Sellaphora and Navicula species are immediately apparent.

Among 19th and early 20th century diatomists, almost the only one to take chloroplast morphology seriously as a source of taxonomic information in diatoms was the eccentric Russian biologist Konstantin Sergeevich Merezhkovskij (Mereschkowsky), better known today for his prescient ideas on the endosymbiotic origin of chloroplasts (e.g.  Mereschkowsky 1905). In a few years (principally 1900-1906), Mereschkowsky published a series of ground-breaking papers on chloroplast morphology, chloroplast division and sexual reproduction in diatoms, in which he developed new ideas about systematic relationships and diatom evolution. He also described several new genera, including Sellaphora (Mereschkowsky 1902).

Mereschkowsky's papers were ignored or rejected. Otto Müller seems to have been almost the only one to take Mereschkowsky's work seriously, and published a few new species of Sellaphora in his studies of E African diatoms from Lake Malawi (the Nyassa-See) and the surrounding region (Müller 1910).

In the 1970 and 1980s, new information from scanning electron microscopy (SEM) suggested that Navicula was not only large and heterogeneous, but probably also polyphyletic. However, although SEM provided many new insights and suggested how some existing genera might be remodelled (e.g. Medlin & Round 1986), in many cases evolutionary relationships were unclear because of character conflicts. Partly because of this, and partly because the prevailing taxonomic philosophy was then 'phenetics', with its requirement that taxonomic judgments should be based on 'overall similarity', based on all available evidence, several diatomists started to look for alternative sources of information, including cytological and reproductive characteristics. During this phase, Sellaphora was reinstated as a separate genus (Mann 1989). Since then, the phenetic approach has been discredited for phylogenetic analysis and powerful molecular genetic tools have become available for analysing evolutionary relationships, but the concept of Sellaphora developed by Mann (1989) has withstood all tests so far.