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Introduction

Species of Sellaphora have been known since c. 1840 but were classified in the genus Navicula for most of time up to 1990. 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. There is nothing particularly unusual about the structure of the frustule in Sellaphora and so the original classification of Sellaphora species in Navicula (then a catch-all for undistinguished boat-shaped diatoms) is understandable. However, if chloroplast morphology is taken into account, differences between Sellaphora and Navicula species are immediately apparent. This was first noticed by Mereschkowsky (1902), who gave the following description:

‘Valve small, symmetrical, linear to elliptical, with obtuse ends, terminal nodules distant; striae usually fine, connecting-zone simple. Endochrome composed of one plate, resting with its narrow median part on the surface of one of the valves, with four long prolongations along the connecting zones. Pyrenoid absent. A few elaeoplasts, sometimes represented by two libroplasts.’

The key feature was the shape of the chloroplast (Mereschkowsky’s ‘endochrome’), which is a single H-shaped plate; this is what gives the genus its name (Sellaphora = ‘saddle-bearer’). Most 20th century floras reverted to the 19th century practice of including Sellaphora in Navicula and the species now assigned to Sellaphora (see below) were not always put in the same group within Navicula. However, following SEM studies of the frustule and new observations of the living cell and sexual reproduction, Sellaphora was reinstated as a separate genus (Mann 1989, Round et al. 1990). Molecular genetic data have thus far supported the monophyly of Sellaphora.

The history and taxonomy of Sellaphora are discussed by Mann (1989), Mann et al. (2008) and Evans et al. (2008). The genus has been developed as a model system for studying and documenting speciation in freshwater diatoms, including DNA barcoding (Evans et al. 2007). More is known about the mating systems of Sellaphora species than in any other diatom genus (e.g. Mann 1999, Mann et al. 1999).

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