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 init ial cells were examined by scanning electron microscopy from cultures of th e domoic-acid-producing dioecious pennate diatom Pseudo-nitzschia multiseri es (Hasle) Hasle. The structure of gametangia, position of the gametes rela tive to the gametangia, and their sequential maturation observed previously using light microscopy were confirmed in our material. The general structu re of the auxospore conforms to that seen in many previously investigated p ennate 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 g amete walls have not previously been reported in any diatom, while in the p rimary auxospore wall they are known only in three other species. The prese nce of scales in sexual stages of P. multiseries supports the monophyletic origin of diatoms and their relationship to scale-bearing ancestors. The la ck of similar reports from other diatom species may be attributed to the sc arcity of investigations into the fine structure of sexual cells. Scales ma y be more commonly found when other species are carefully examined. Then, g amete and auxospore structure may prove useful in defining higher rank taxa in the natural phylogeny of diatoms. We also discovered a correlation betw een the molecular (ssu rRNA) phylogeny of diatoms (Medlin et al. 1996, Mole c. Biol. Evol. 13: 67-75) and the basic types of auxospores: isodiametric a uxospores are grouped with Diatom Clade I; anisodiametric auxospores are gr ouped loosely with Diatom sub-Clade 2a; and auxospores with perizonia as se en in pennate diatoms form the tightly grouped Diatom sub-Clade 2b. Pseudo- nitzschia multiseries auxospore structure and development are typically pen nate and place this diatom within DsC2b, assuming a close phylogenetic rela tionship of this genus to other nitzschioid genera that have been examined using ssu rRNA sequencing.