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Molecular phylogeny of Schizostauron Grunow reveals another lineage of monoraphid diatoms related to Stauroneidaceae D.G. Mann in Round, Crawford & Mann 1990

The monoraphid diatom genus Schizostauron was described by Grunow with four marine species included. The major feature of Schizostauron was the bifurcate central area of the raphid valve, whereas the sternum valve was characterized by robust perpendicular ribs connected by a complex, rather randomly arranged network of siliceous elements. Likely due to the scarcity of observations, the species belonging in Schizostauron have not received much attention. According to investigations by Robert Ross the generic name is validly published in accordance with ICBN, and the typus generis is S. reichardtianum Grunow. The number of established taxa in Schizostauron amounts to less than a dozen ; however, our recent examination of marine littoral material have revealed numerous unknown species, all more or less resembling Schizostauron fimbriatum Grunow [=Achnanthes fimbriata (Grunow) Ross] and Achnanthes danica (Flögel) Cleve. Here we present results of our morphological (LM & SEM) and genetic research on several Schizostauron taxa from the coastal waters of the tropical Indian Ocean. They have been cultured successfully and subject to genomic DNA extraction. Three genes --nuclear- encoded SSU rDNA and chloroplast-encoded rbcL and psbC--have been amplified and sequenced. Our sequencing data, together with additional monoraphid and raphid pennate diatom sequences, either from our own data base or downloaded from GenBank, have been used to estimate a phylogenetic tree. To our surprise, Schizostauron taxa within the phylogenetic tree clustered with taxa belonging in the Stauroneidaceae, i.e. Stauroneis, Craticula and Fistulifera. Sister to Schizostauron within the Stauroneidaceae was the monoraphid taxon Astartiella sp. (isolated from the Gulf of Mexico in Texas) and another unnamed monoraphid diatom isolated from Texas. Identify features, either cytological or morphological, common to the monoraphids and other Stauroneidaceae (Stauroneis and Craticula) is difficult. Schizostauron seems to have chloroplasts similar to Stauroneis and Craticula, whereas Astartiella and the unnamed taxon possess a single chloroplast, similar to Cocconeis and Planothidium. All three monoraphid taxa, however, share the coaxial central raphe internal endings and strongly-hooked external apical endings. It seems that Schizostauron and the other monoraphid genera provide evidence that monoraphy in diatoms may have evolved in several independent evolutionary events. Here we provide examples which are molecularly distant from the monoraphid diatoms of the Achnanthales.More research is required to uncover cytological or morphological synapomorphies that link the monoraphid forms with biraphid Stauroneidaceae.

Phylogeny; Monoraphid diatoms; SSU rDNA; rbcL; psbC; Schizostauron; Stauroneidaceae

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