Symmetry and Cell Division in Raphid Diatoms

The development and final morphology of the valve in raphe-bearing diatoms exhibit a cryptic lateral polarity, and hence two types of frustule can be distinguished. In the cis type both valves have the same orientation; in the trans type they have opposite orientations. Examination of a variety of taxa suggests that in all dividing raphid diatoms, both new valves have the same orientation and so only three types of division are possible: cis;→ cis + cis, cis → trans + trans, trans;→ cis+trans. The possession of different combinations of these explains the observed ratios of cis: trans in different taxa, viz all cis; 1: 2 cis: trans; and roughly 1: 1 cis: trans. The implications of the results for diatom systematics are examined, with special reference to Navicula Bory.