Sellaphora pupula (Kütz.) Mereschk. has become a model system for studying the species concept in diatoms, following the demonstration of reproductive incompatibility between sympatric genodemes in the early 1980s (Mann 1984, 1988, 1989a, b, 1999, 2001, Mann and Droop 1996, Mann et al. 1999). It has long been known to be morphologically variable (e.g. Hustedt 1927-66) and the species complex as a whole is cosmopolitan, occurring in meso- to eutrophic lakes and rivers.

In several lakes, but particularly in Blackford Pond, Edinburgh, it has been shown that slight differences in valve shape and size, striation pattern, and stria density, are markers for populations that cannot interbreed, and may differ in reproductive biology and susceptibility to chytrid and oomycete parasites. The stability of these differences has been demonstrated in culture (e.g. Mann et al. 1999) and appears to be corroborated by molecular data (Mann 1999 and unpublished data).

In Blackford Pond, seven demes of S. pupula have been discovered (Mann 1989a, 1999) and they are fairly easily and consistently identifiable by an experienced diatomist. However, in other lakes, such as Threipmuir Reservoir, near Edinburgh, we have discovered a much more complex variation pattern, probably involving 12 or more demes. The Threipmuir S. pupula population seemed particularly appropriate for ADIAC, (1) to test feature extraction, which could then be used as a basis for analyzing the variation pattern and circumscribing demes; and (2) to evaluate the performance of ADIAC identification algorithms in identifying valves when the differences among the valves are so subtle as to be confusing even for experts (see Fig.1).

After a preliminary analysis of 200 randomly sampled valves we selected a subset of 120 images representing the six most frequent demes. The taxonomic classification of valves had been carried out using morphometric analysis (size measurements, rectangularity, striation frequency, Fourier descriptors), followed by multivariate analyses (cluster analysis, principal components), as well as Analysis of Variance of individual characters. Each of the demes used was represented by 20 valves.

References:

Hustedt, F. 1927-66. Die Kieselalgen Deutschlands, Österreichs und der Schweiz unter Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete. In Dr L. Rabenhorsts Kryptogamenflora von Deutschland, Österreich und der Schweiz, Vol. 7. Akademische Verlagsgesellschaft, Leipzig.

Mann, D.G. 1984. Observations on copulation in Navicula pupula and Amphora ovalis in relation to the nature of diatom species. Ann. Bot. 54: 429-38.

1988. The nature of diatom species: analysis of sympatric populations. In F.E. Round [Ed.] Proceedings of the 9th International Diatom Symposium. Biopress, Bristol and O. Koeltz, Koenigstein, pp. 317-27.

1989a. The species concept in diatoms: evidence for morphologically distinct, sympatric gamodemes in four epipelic species. Plant Syst. Evol. 164: 215-37.

1989b. The diatom genus Sellaphora: separation from Navicula. Br. Phycol. J. 24: 1-20.

1999. The species concept in diatoms (Phycological Reviews 18). Phycologia 38: 437-95.

2001. The systematics of the Sellaphora pupula complex: typification of S. pupula. In Jahn, R., J.P. Kociolek, A. Witkowski & P. Compère [Eds.]: Lange-Bertalot Festschrift. Gantner, Ruggell,

Mann, D.G. & Droop, S.J.M. 1996. Biodiversity, biogeography and conservation of diatoms. Hydrobiologia 336: 19-32.

Mann, D.G., Chepurnov, V.A. & Droop, S.J.M. 1999. Sexuality, incompatibility, size variation, and preferential polyandry in natural populations and clones of Sellaphora pupula (Bacillariophyceae). J. Phycol. 35: 152-70.