Symmetry and Cell Division in Raphid Diatoms

The development and final morphology of the valve in raphe-bearingdiatoms exhibit a cryptic lateral polarity, and hence two typesof frustule can be distinguished. In the cis type both valveshave the same orientation; in the trans type they have oppositeorientations. Examination of a variety of taxa suggests thatin all dividing raphid diatoms, both new valves have the sameorientation and so only three types of division are possible:cis; cis + cis, cis trans + trans, trans; cis+trans. The possessionof different combinations of these explains the observed ratiosof cis: trans in different taxa, viz all cis; 1: 2 cis: trans;and roughly 1: 1 cis: trans. The implications of the resultsfor diatom systematics are examined, with special referenceto Navicula Bory. Diatom systematic, diatom valve morphogenesis, cell symmetry, raphe structure