Functional and phylogenetic aspects of the ciliary membrane: a comparative freeze-fracture study.

This paper illustrates the structural diversity of the ciliary membrane of the somatic ciliature in 68 ciliate genera and describes the interior architecture of several presumptive sensory cilia. Freeze-fracture technique reveals a variety of intramembrane particle arrays known as ciliary necklace, ciliary plaques, and ciliary rosettes. While the function of these arrays is still largely unknown, their distribution among ciliates suggests a phylogenetic correlation. Whereas current systems of ciliate classification are primarily based on the character of the ciliature which surrounds the cytostome as well as on the morphogenesis of this oral ciliature, the data presented in this paper are well suited to emphasize the significance of the somatic ciliature for the reconstruction of ciliate phylogeny. This is particularly evident when the freeze-fracture data are mapped in parallel with a simplified phylogenetic tree based on the appearance of both the oral and the somatic ciliature. The freeze-fracture data are in agreement with hypothesis of a non-hymenostome origin of the spirotrichs and suggest a close relationship with respect to the origin of the Hymenostomatida from the Colpodida instead of from the Nassulida. The presumptive sensory cilia show a significantly greater particle density than most somatic cilia. While the particles may represent receptor proteins or ion channels, with the freeze-fracture technique it is at present impossible to specify the modalities of such "sensory" cilia.