The use of correlated ultrastructural and morphogenetic characters in evolutionary taxonomy of hypotrich ciliates.

A new morphogenetic criterion, the "unit of turn-over" emerges from the dynamic analysis of hypotrich morphogenesis. The unit corresponds to the set of morphogenetically interdependent somatic structures whose turn-over is correlated to nuclear modifications. Hypotrichs are seen to conform to one of two very general modes of turn-over of their ciliary structures during a variety of morphogenetic processes, one in which all somatic cilia and their infraciliature are replaced, and the other in which only the cirral structures (not the dorsal dikinetids) undergo replacement. A strong correlation is observed between the type of turn-over occurring and ultrastructural features of the corresponding species. From an evolutionary point of view, these data lead to a new subdivision of hypotrichs, into the Euhypotrichina and the Pseudohypotrichina.