Physiological Reorganization and Post-Traumatic Regeneration in Stylonychia mytilus: Reflections on Developmental Constraints and the Evolutionary Origin of Alternative Modes of Asexual Morphogenesis

We investigated development of cortical ciliature in Stylonychia mytilus during starvation-induced physiological reorganization, and during regeneration following amputation of the anterior part of the cell. Cortical reorganization in the two processes is generally similar. The posterior part of the adoral zone of membranelles is resorbed and replaced with newly assembled membranelles. The pre-existing set of ventral cirri and dorsal bristles is entirely resorbed and replaced with new ones. Regenerants exhibit posterior displacement of the frontal-ventral-transverse cirri primordium and the undulating membrane primordium, and recruit basal bodies from ectopic locations for the development of these ciliature. This illustrates flexibility in the initiation site of ciliary primordia, and opportunism in utilizing building blocks. Such morphogenetic versatility of hypotrichs provides the basis for the operation of a global control of pattern formation, which governs cortical reorganization in dividers, and additionally, in the absence of the prerequisites for binary fission, alternative modes of cortical development such as physiological reorganization or regeneration. These considerations suggest that the three processes are homologous and that physiological reorganization and regeneration have evolved from binary fission. In physiological reorganization and regeneration, the micro- and macronuclei reorganize to resemble that in binary fission; these nuclear events are considered evolutionary relics of the nuclear development of binary fission. Tetrahymena also exhibits such morphogenetic flexibility; stomatogenesis is under global control, so that asexual cells can replace its oral apparatus without undergoing binary fission. Paramecium , on the other hand, adopts a more rigid strategy in relying heavily on pre-existing structures for morphogenetic cues; this could have imposed constraints in the exploration of alternative modes of asexual development.