Relationships between growth rate, cell size, and induction of spermatogenesis in the centric diatom Thalassiosira weissflogii (Bacillariophyta)

Patterns of changes in cell size, growth rate, and the inducibility of spermatogenesis were followed in eight sub-clones of two isolates of the centric diatom Thalassiosira weissflogii (Grunow) Fryxell & Hasle grown at saturating light. One isolate originated from Long Island Sound, New York, USA and the other originated from Jakarta Harbor, Indonesia. As expected from previous studies, oscillations between intervals of cell size reduction and cell size enlargement were observed for each sub-clone. For both isolates, sperm were easily detected, but cells resembling eggs and auxospores were rarely observed and fertilization was not confirmed, suggesting that the observed cell size increases may have resulted from a combination of asexual cell enlargement and rare auxosporulation. The two isolates differed in their minimum and maximum sizes, and the threshold size for the induction of sperm formation. However, the two sets of isolated sub-clones displayed comparable relationships between growth rate, sperm inducibility, and cell size relative to the minimum, maximum, and threshold sizes. Growth rate increased as cell size decreased during vegetative divisions until the threshold for sperm inducibility was crossed. Below the size threshold for sperm inducibility, growth rate declined as cell size continued to decrease. Smaller cells were susceptible to failure of normal cytokinesis and valve deposition, resulting in the formation of abnormally long and often multinucleate cells. Culture conditions may select against restoration of cell size via auxosporulation due to the relationship between growth rate and cell size.