Sexual reproduction in the pennate diatoms Pseudo-nitzschia multiseries and P. pseudodelicatissima (Bacillariophyceae)

Clones of the domoic-acid-producing pennate diatom Pseudo-nitzschia multiseries (Hasle) Hasle and of the potentially toxic P. pseudodelicatissima (Hasle) Hasle normally decrease in cell size in culture until they eventually die without undergoing sexual reproduction to regain the largest cell size. However, we induced sexual reproduction by mixing individual exponentially growing clonal cultures of the appropriate minimal cell size under the same conditions that are normal for vegetative growth. We observed pairing of parent cells (gametangiogamy); production of four morphologically isogamous, nonflagellated gametes per gametangial pair; rearrangement of the gametes and their fusion to form zygotes, revealing physiological anisogamy; enlargement of auxospores; and formation of long initial cells. Our observations of allogamous reproduction are consistent with those reported for other dioecious pennate diatoms. Clones of P. pseudodelicatissima from the Black Sea and from the CCMP culture collection failed to auxosporulate when mixed together, although they are the same species according to scanning electron microscopy. The range in apical length of P. multiseries was broader than that reported in the literature for field samples, necessitating a modification of the species description. Knowledge of the pattern and timing of sexual reproduction in Pseudo-nitzschia spp. may provide insights into their bloom dynamics.