clade I araphid pennate diatoms

Introduction

These diatoms were never grouped together previously, in classifications based on morphology, but in molecular phylogenies (e.g. Kooistra et al. 2004, 2006, Medlin & Kaczmarska 2004, Sorhannus 2007) they consistently appear as a monophyletic group, although very few species have yet been grown and sequenced. No morphological synapomorphies have been identified for the group and there may be none. However, the cytological characteristics of most members have not been recorded and absolutely nothing is known about the reproductive biology of clade I araphid pennates, which is particularly unfortunate because it is conceivable that they might provide the key to understanding how the oogamy of centric diatoms was replaced by isogamy (rarely anisogamy).

All species known to belong to clade I araphid pennates are marine. The Plagiogrammaceae and Rhaphoneidaceae grow attached to rocks, sand grains and seaweeds (though like other benthic marine species, they are sometimes swept up into coastal plankton), living as solitary cells or forming chains or clusters. The exception is Neodelphineis, which is habitually planktonic and grows in zig-zag colonies (Round et al. 1990). Asterionellopsis and Asteroplanus are are both planktonic, forming helical or stellate colonies.

Clade I is discussed by Kooistra et al. (2004, 2006).

Clade I araphid pennates are apparently the first pennate diatoms to appear in the fossil record although, because the clade is characterized by molecular genetic data, any statements about its fossil record are rather speculative.

Characteristics

Clade I is based solely on molecular evidence (principally from SSU rDNA). No morphological synapomorphies have been identified. None of the features mentioned below is restricted to clade I araphids, and none is possessed by all clade II taxa.

• colonial (stellate or chain-forming) or solitary 
• valves linear, lanceolate, square, triangular, consisting of a swollen base and hair like extension, elliptical, or circular!
• valve pattern organized with respect to a sternum, which can be wide (some Rhaphoneidaceae) or very narrow (some Plagiogrammaceae, Asterionellopsis). There is sometimes (e.g. Glyphodesmis) a circular boss at the centre of the valve, mimicking the annulus of centric diatoms. In Psammodiscus, if it belongs here, the sternum is reduced almost to nothing, and a lesser reduction is present in other Rhaphoneidaceae.
• pore fields sometimes present at the valve apices.
• rimoportulae sometimes present (Rhaphoneidaceae, Asterionellopsis), but lost in Plagiogrammaceae.
• areolae usually occluded by rotae or cribra (but not in Asterionellopsis).

References

Kooistra, W.H.C.F., Forlani, G., Sterrenburg, F.A.S. & De Stefano, M. (2004). Molecular phylogeny and morphology of the marine diatom Talaroneis posidoniae gen. et sp. nov. (Bacillariophyta) advocate the return of the Plagiogrammaceae to the pennate diatoms. Phycologia 43: 58-67.

Kooistra, W.C.H.F., Chepurnov, V., Medlin, L.K., De Stefano, M., Sabbe, K. & Mann, D.G. (2006). Evolution of the diatoms. In: Plant Genome: Biodiversity and Evolution. Vol. 2B: Lower Groups (Ed. by A.K. Sharma & A. Sharma), pp. 117-178. Oxford & IBH Publishing, New Delhi , India and Science Publishers, USA.

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.