Ultrastructural changes during reticulopod withdrawal in the foraminiferan protozoan Allogromia sp., strain NF

Allogromia sp. is a benthic foraminiferan protozoan which extends and withdraws a dynamic network of branching and anastomosing pseudopodia, i.e., reticulopods. Each reticulopod contains an elongate cytoskeleton composed primarily of microtubules (MT). When withdrawal was induced with artificial seawater supplemented with MgCl₂, we found a time-dependent decrease in the number of reticulopodial MTs and a concomitant increase in 5-nm-diameter helical filaments. During the initial stages of withdrawal these helical filaments associated laterally to form loose aggregates. Later they formed dense paracrystalline aggregates, which appeared similar to those seen in the cell bodies of untreated Allogromia juveniles prior to network extension. Similar results were obtained when withdrawal was induced by using seawater supplemented with other salts (NaCl, KCl). Treatment with an isotonic seawater substitute with an altered Na⁺:K⁺ ratio induced a momentary withdrawal, after which the organism recovered and reextended a network. During the withdrawal phase of this response, MTs became less abundant and aggregates of helical filaments more conspicuous. Together with earlier observations these findings suggest that helical filaments and paracrystalline material are an alternative or intermediate assembly form of MT proteins.