Unusual extrusive organelles in karyorelictid ciliates: an argument for the ancient origin of this group

The karyorelictid ciliates never possess extrusomes that are typical of most other ciliates, i.e. trichocysts, mucocysts, and toxicysts, but instead present unusual types of extrusive organelles, most existing nowhere else. These organelles are: (1) Nematocysts with a filament making only 2-3 coils in the longitudinal plane, in Remanella multinucleata; (2) 'Orthonematocysts' with a short straight internal filament, in Remanella rugosa and R. brunnea; (3) Tiny bottle-shaped organelles somewhat resembling haptocysts, in Remanella granulosa; (4) Rhabdocysts, arrow-shaped extrusomes somewhat resembling certain trichocysts but undergoing no strong elongation during extrusion, in species of Tracheloraphis and in Kentrophoros latum; (5) Ampullocysts, complex vesicular organelles with hyaline secretion, occurring in Kentrophoros latum; (6) pigmentocysts or extrusible pigment granules, often with some internal structure, in almost all karyorelictids (Trachelocerca, Tracheloraphis, Trachelonema, Loxodes, Remanella, Geleia). This is the only type of cortical organelles the karyorelictids share with other ciliates, namely, the Heterotrichida (Stentor, Blepharisma). This highly aberrant set of extrusomes in karyorelictids argues that they are a very ancient branch of ciliates which separated from the main trunk early in evolution, conserving or developing an unusual set of extrusomes independently from the rest of ciliates. There is also some evidence for the relatedness of the Karyorelictida to Heterotrichida, already supposed from studies of the ciliary fibre systems and sequencing of ribosomal RNAs.