Fine structure of the gamete, auxospore and initial cell in the pennate diatom Pseudo-nitzschia multiseries (Bacillariophyta)

The fine structure and ontogeny of gametangia, gametes, auxospores and initial cells were examined by scanning electron microscopy from cultures of the domoic-acid-producing dioecious pennate diatom Pseudo-nitzschia multiseries (Hasle) Hasle. The structure of gametangia, position of the gametes relative to the gametangia, and their sequential maturation observed previously using light microscopy were confirmed in our material. The general structure of the auxospore conforms to that seen in many previously investigated pennate diatoms. In addition, we observed scales in the gamete wall as well as in the internal lining of the primary wall of the auxospore. Scales in gamete walls have not previously been reported in any diatom, while in the primary auxospore wall they are known only in three other species. The presence of scales in sexual stages of P. multiseries supports the monophyletic origin of diatoms and their relationship to scale-bearing ancestors. The lack of similar reports from other diatom species may be attributed to the scarcity of investigations into the fine structure of sexual cells. Scales may be more commonly found when other species are carefully examined. Then, gamete and auxospore structure may prove useful in defining higher rank taxa in the natural phylogeny of diatoms. We also discovered a correlation between the molecular (ssu rRNA) phylogeny of diatoms (Medlin et al. 1996, Molec. Biol. Evol. 13: 67-75) and the basic types of auxospores: isodiametric auxospores are grouped with Diatom Clade I; anisodiametric auxospores are grouped loosely with Diatom sub-Clade 2a; and auxospores with perizonia as seen in pennate diatoms form the tightly grouped Diatom sub-Clade 2b. Pseudo-nitzschia multiseries auxospore structure and development are typically pennate and place this diatom within DsC2b, assuming a close phylogenetic relationship of this genus to other nitzschioid genera that have been examined using ssu rRNA sequencing.