Morphology and phylogeny of the marine bipolar centric diatom Pseudoleyanella lunata (Cymatosiraceae) with special reference to the diatotepum

Abstract

Pseudoleyanella lunata Takano is a marine diatom with dorsiventral valves belonging to the family Cymatosiraceae. We studied the range of variation in valve morphology throughout the life cycle. We also observed the chloroplast division of this species. In large cells, the valves were narrowly lanceolate, slightly capitate at the apices, and asymmetrical with respect to the apical plane, i.e. with almost straight and convex margins on the ventral and dorsal sides, respectively. During cell size reduction, the valves gradually lost their dorsiventral nature, and eventually became almost circular. Although P. lunata was rectangular in girdle view in small cells, large vegetative cells, particularly those generated soon after auxosporulation, were slightly bent, as in Leyanella Hasle et al. However, in contrast to Leyanella, P. lunata lacked both pili and tubular processes at all stages of its life cycle. In phylogenetic analyses (SSU rDNA and rbcL), P. lunata was sister to Leyanella. We also observed the whole structure of the diatotepum which was a sheet-like structure underlying each theca. Under the transmission electron microscope, dot-like marks with high electron density were observed: their position and pattern corresponded to valve poroids. In addition, the diatotepum bore electron-dense lines corresponding to the internal sutures between the girdle bands.