Sellaphora Mereschkowsky 1902

Note: this tree is still under construction. It does not yet contain all known Sellaphora subgroups.

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).

Characteristics

• freshwater epipelic diatoms
• plastid single, H-shaped, lying with its centre against the epivalve.
• pyrenoid ± tetrahedral during interphase, invaginated; one per plastid.
• plastid translocated onto the girdle before mitosis.
• division of plastid beginning before cytokinesis and before the previous division is complete, proceeding asymmetrically, and arresting before completion to produce the H-shape of interphase.
• 90º rotation of the plastid onto the epivalve occurring at the end of valve formation.
• frustule generally wider whan deep and usually lying in valve view 
valve outline elliptical, linear to lanceolate, with rounded, rostrate or capitate poles.
striae uniseriate (rarely biseriate), not interrupted by lyre-shaped plain areas
poroids small, round, containing hymenes.
a narrow plain conopeum sometimes present on either side of the valve, extending out from the raphe-sternum; conopea sometimes continuous from pole to pole, elsewhere only from pole to centre.
internal central raphe endings deflected towards the primary side of the valve.
gametangia not surrounded by an obvious mucilage sheath; closely associated throughout meiosis and plasmogamy; opening only partially to allow gamete migration via a copulation aperture.
one functional gamete and one apochlorotic cell formed per gametangium.
gametes physiologically anisogamous: auxospore formed within one gametangium and expanding parallel to its apical axis.

References

Evans, K.M., Wortley, A.H. & Mann, D.G. (2007). An assessment of potential diatom “barcode” genes (cox1, rbcL, 18S and ITS rDNA) and their effectiveness in determining relationships in Sellaphora (Bacillariophyta). Protist 158: 349–364.

Evans, K.M., Wortley, A.H., Simpson, G.E., Chepurnov, V.A. & Mann, D.G. (2008). A molecular systematic approach to explore diversity within the Sellaphora pupula species complex (Bacillariophyta). Journal of Phycology 44: 215–231.

Mann, D.G. (1989). The diatom genus Sellaphora: separation from Navicula. British Phycological Journal 24: 1–20.

Mann, D.G. (1999). The species concept in diatoms (Phycological Reviews 18). Phycologia 38: 437–495.

Mann, D.G., Thomas, S.J. & Evans, K.M. (2008). Revision of the diatom genus Sellaphora: a first account of the larger species in the British Isles. Fottea 8: 15–78.

Mereschkowsky, C. (1902). On Sellaphora, a new genus of diatoms. Annals and Magazine of Natural History, series 7, 9: 185–195.

Round, F.E., Crawford, R.M. & Mann, D.G. (1990). The diatoms. Biology and morphology of the genera. Cambridge University Press, Cambridge, 747 pp.