'radial centric diatoms'

Some molecular phylogenies indicate that the radial centric diatoms are paraphyletic with respect to a monophyletic group containing the polar centric diatoms and pennate diatoms. There is little support for monophyly or paraphyly. The radial centrics are regarded as a class or (Coscinodiscophyceae) or subdivision/subphylum (Coscinodiscophytina) by some authors

Introduction

The radial centric diatoms have circular valves and lack obvious signs of bi- or multipolarity in the arrangement of special pores or processes (there are very rare exceptions). They are probably a paraphyletic group of lineages that split from each other before the evolution of more complex (non-circular) valve shapes in the ancestors of the 'polar centric diatoms'. Current molecular data do not give a clear picture of the relationship between the radial centrics and the polar centrics, nor of the relationships among major groups within the radial centrics.

The definition of radial centrics is in fact wholly negative. They are those lineages of diatoms that (1) have circular valves because they lack the special siliceous bands that force the auxospore to expand anisometrically, (2) lack fultoportulae and so are not circular-valved members of the Thalassiosirales, and (3) do not possess the synapomorphies of the few other groups of diatoms that have circular valves.

Even if the radial centrics do prove to be monophyletic (e.g. as in the diagram of Sims et al. 2006, fig. 2), it is highly unlikely that any morphological features will be consistently useful for diagnosis.

If they are paraphyletic, we can hope that a classification will eventually be developed that truly reflects evolutionary relationships. In the meantime, the name 'radial centrics' is simply a shorter and more descriptive quick-reference than 'lineages that probably diverged from each other earlier than all except one of them from the polar centrics (including pennates), that lack a sternum, that have an annulus but lack fultoportulae and discrete areas of porelli, and that lack elongate or multipolar shapes through having a simple auxospore wall'.

The radial centric diatoms are mostly planktonic and marine, but there are interesting exceptions. Ellerbeckia arenaria is often semi-terrestrial, occurring on damp shady rocks. Some Melosira species occur in freshwater periphyton, and Aulacoseira is an important component of freshwater phytoplankton, usually in well-mixed water columns.

Characteristics

• chain-forming or solitary
• valves circular (very rare exceptions with elliptical or hemielliptical valves)
•  distance between valves (pervalvar depth of the girdle) sometimes short, sometimes so long that the cell becomes spicular (Proboscia and Rhizosolenia: see right-hand title illustration)
• valve pattern subtended by a circular annulus
• auxospore wall scaly, without a properizonium or perizonium
• rimoportulae or tube processes present, but no fultoportulae
• lacking well-defined areas of small pores (porelli) for secretion of pads or stalks

Discussion of Phylogenetic Relationships

The relationships between the major groups of radial centrics are unclear and many groups of species and genera have not been studied in detail, nor included in molecular phylogenies. Ellerbeckia sol (also called 'Paralia sol') sometimes appears as sister to all other diatoms (e.g. Medlin & Kaczmarska 2004, Alverson et al. 2006), and Leptocylindrus also sometimes splits off very early (e.g. Alverson et al. 2006, Sorhannus 2007).

The rich Lower Cretaceous flora described by Gersonde & Harwood (1990) contains several genera that possess the characteristics of radial centrics, i.e. with circular valves but without features that might place them among the polar centric lineages (discussed by Medlin & Kaczmarska 2004, Sims et al. 2006). Some of these show no obvious links to any of the extant groups of radial centrics.

Medlin & Kaczmarska (2004) refer to the radial centric diatoms as the subdivision Coscinodiscophytina.

Other Names for 'radial centric diatoms'

• Coscinodiscophytina Medlin & Kaczmarska 2004

References

Alverson, A.J., Cannone, J.J., Gutell, R.R. & Theriot, E.C. (2006). The evolution of elongate shape in diatoms. Journal of Phycology 42: 655-668.

Gersonde, R. & Harwood, D.M. (1990). Lower Cretaceous diatoms from ODP Leg 113 site 693 (Weddell Sea) Part 1: vegetative cells. Proceedings of the Ocean Drilling Program, Scientific Results 113: 365–402.

Medlin, L.K. & Kaczmarska, I. (2004). Evolution of the diatoms: V. Morphological and cytological support for the major clades and a taxonomic revision. Phycologia 43: 245–270.

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

Sims, P.A., Mann, D.G. & Medlin, L.K. (2006). Evolution of the diatoms: insights from fossil, biological and molecular data. Phycologia 45: 361-402.

Sorhannus, U. (2007). A nuclear-encoded small-subunit ribosomal RNA timescale for diatom evolution. Marine Micropaleontology 65: 1-12.