CELL-DIVISION IN GLOEOTILOPSIS-PLANCTONICA, A NEWLY IDENTIFIED ULVOPHYCEAN ALGA (CHLOROPHYTA) STUDIED BY FREEZE FIXATION AND FREEZE SUBSTITUTION

Gloeotilopsis planctonica Iyengar & Philipose, a soil isolate, was cultured for the first time since its description in 1956. Cell division was investigated ultrastructurally by using rapid freeze fixation and freeze subsitution. Vegetative interphase cells contain a persisting basal apparatus (centrioles) in which the absolute configuration of the basal body pair is identical to that in ulvophycean flagellated cells. Basal apparatus replication and segregation occurs shortly before prophase. Also before mitosis, a shallow plasma membrane indentation is formed, subtended by 2-3 cortical microtubules, in the forthcoming division plane. Daughter basal body pairs are located lateral to the mitotic spindle poles. A Golgi stack occupies each polar position of the mitotic spindle. During mitosis, which is structurally identical to that described previously for members of the Ulotrichales sensu Sluiman (Ulvophyceae), an extensive network of tubular membranes (trans Golgi reticulum, TGR) develops from each polar Golgi stack to the incipient cleavage furrow. A microtubular system, possibly organized by the basal apparatus, is associated with both the TGR and the cytokinetic septum. The TGR appears to be involved in the further development of the furrow after mitosis is completed. I conclude on the basis of this ultrastructural study that the genus Gloeotilopsis, which includes G. planctonica Iyengar & Philipose (type species) and G. sterilis Deason, should be classified in the Ulotrichales sensu Sluiman, class Ulvophyceae.