Algae World: Sellaphora

Diversity: the 'pupula group'

Sellaphora: pupula group

Description

In the 'pupula group', the valves are very variable in outline, from linear to elliptical, and have capitate, subcapitate, rostrate, acute or rounded poles. All have polar bars. The axial area is narrow, and the central area is generally ill-defined and ± rectangular or bow-tie-shaped, rarely rounded. The axial area and adjacent parts of the striae may or may not be depressed below the rest of the valve face, with or without a conopeum; if present, the conopea do not traverse the centre of the valve. The effect in the LM is to create two grey longitudinal areas running parallel to the raphe slits, from near the centre to the poles (see the right-hand image), or no areas or lines.

The narrow axial area and ill-defined central areas and presence of polar bars separate the 'pupula group' from the 'americana group'. The presence of polar bars separates the 'pupula group' from the 'laevissima group'. The ill-defined central area and absence of conopea from the central part of the valve separate the 'pupula group' from the 'bacillum group'.

Species list

Overview of the  Sellaphora pupula complex

There are a lot of “S. pupula” species!  The exact number we don’t yet know, but the more ponds we study, the more we find.  We are continually surprised.  For example, our most recent sampling was in Australia (Victoria) where we chose ponds that at least superficially resembled Blackford Pond in Edinburgh, i.e. they were urban duck ponds.  When we used a microscope to look at the S. pupulas that were present we were confident that on the whole, we had sampled a similar S. pupula flora to Blackford Pond.  We turned out to be right and wrong!

Our current approach to assess diversity is to use a combination of microscopic and DNA-based techniques.  When we sample a pond or lake, we make a microscope slide of the natural diatom assemblage.  Using a light microscope we work our way through the slide, looking for S. pupula.  We take lots of photos and for each distinct S. pupula we find, we give it a provisional identity.  From the same material we also attempt to collect live Sellaphoras, grow them up in culture and extract their DNA so that we can also make DNA-based identifications (“DNA barcoding”).  We routinely use part of a mitochondrial gene called cytochrome oxidase (or cox1) as an identification tool.  If the cox1 DNA sequence suggests that we have found a new S. pupula species, we also obtain chloroplast (rbcL and psaA) and nuclear (18S and ITS) rDNA sequences. 

Using our current approach, we find that we rarely manage to collect live S. pupula species that represent all the S. pupula diversity in a pond or lake.  This may be because some are much less common than others or because the isolation methods we use are better suited for the survival of some species over others.  We are working on finding better ways to sample diversity.  Despite the present shortcomings, we have still managed to identify at least 40 different "S. pupula" species. 

Even the nature of Sellaphora diversity is not straightforward.  For example, species that look the most similar are not necessarily the most closely related to each other; closest relatives are not usually found in the same ponds; and some of the species that are traditionally recognised as S. pupula are actually more closely related to S. bacillum, a diatom whose appearance is very different to S. pupula (Evans et al. 2008).  We estimate that S. pupula has been diversifying for at least 12 million years.

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, in press). A phylogenetic approach to explore the nature of cryptic diversity within the model species complex Sellaphora pupula agg. (Bacillariophyta). Journal of Phycology

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