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Pennate Diatom Structure

Pennate diatoms have a diversity of symmetry, from entirely bilaterally symmetric in all axes, to asymmetric in all axes.

Symmetric frustules:  Bilateral symmetry in all three frustule planes: 

1. apical plane – in the plane of the girdle, or parallel to it. traversing the long axis of the frustules from end to end (apex to apex).  Includes the pervalvar axis that lies in the center of its shorter dimension, from side to side, and the apical axis along its longer dimension, from end to end of the frustule.

2. transapical plane – lies perpendicular to the apical plane, traversing the frustules from side to side along the shorter dimension of the frustule.  The transapical axis lies along its central region from side to side of the frustules, and the pervalvar axis described above.

3. valvar plane – lies parallel to the valves, with an outline dependent on the shape of the valves in “valve view,” and includes the apical axis and the transapical axis.

 [From: Hustedt 1930, figures 2 and 3.]

Entirely bilateral symmetry in all three planes and axes is found in what may be considered the morphologically least modified genera, and therefore the ancestral forms.

Asymmetry is a modification from bilateral symmetry along any one or more of the frustules axes and can include bending of the axis, change in outline of the valve, or displacement of the raphe toward one edge of the valve.  Monoraphes are asymmetric because their two valves are different (one with and the other without a raphe).

Examples of asymmetry that come to mind:  Meridion has a wedge-shaped girdle, thus is asymmetric on the long axis of the girdle.  Gomphonema and its family members have valves with different outlines on either end, thus an asymmetric short axis of the valve, and a single puncta offset to one side of the long axis of the valve.  In addition they also have a wedge-shaped girdle making the short axis of the girdle asymmetric..

Perhaps the most extreme asymmetry is found in the “keeled” genera, with the keel lying along one margin (e.g. Nitzschia and Hantzschia ). Another type of asymmetry is internal, with asymmetric septa (e.g. Achnanthes and Tabellaria).

Rings of silica often develop on the walls (cingula) of each valve, called “intercalary bands” – increasing the width of the girdle as well as the cell volume.

In various genera, edges of silica inside the margins of the valve and/or intercalary bands form “septa” that partially compartmentalize the cell. The septa are diagnostic of genera that have them. A common example is in the genus Tabellaria that often has a stack of many intercalary bands with septa of various lengths, some nearly completely separating the cell into two compartments (which wouldn't be viable if it happened).

Summary

• Pennate diatom frustules are elongate. Their valves typically have bilateral or approximately bilateral symmetry.
• The frustule patterns of ornamentation of pennate diatoms are also often symmetrically arranged on either side of a central axis running from one pole of the frustule to the other (apical plane).

Valve view of a Navicula with bilateral symmetry

• Type of symmetry is a diagnostic trait.
• Frustules may be entirely bilaterally symmetric in both the long and short axes of both the valve and girdle, as is the case of Navicula .
• Asymmetry in any of the axes can be found in many genera.

References

Hustedt F. 1930.  Die Susswasser-flora Mitteleuropas – Heft 10:Bacillariophyta (Diatomeae). Jena. Verlag Von Gustav Fischer

Graham L. and L. Wilcox 2000. Algae. Prentice-Hall