Protist kinetids: structural conservatism, kinetid structure, and ancestral states.

The protist kinetid is considered a conservative indicator of phylogenetic relationship. This conclusion is supported by the apparently inverse relationship between the conservation of structure and its level in the organizational hierarchy. A comparative examination of features of protist kinetids indicates that pattern is strongly conserved; within and among flagellate taxa there is more variability in pattern than within the ciliates. It is concluded that the ancestral flagellate kinetid is a pair of orthogonally oriented kinetosomes having one striated rootlet and at least one microtubular ribbon. The ancestral kinetid of the ciliates is also a dikinetid structure, though represented by a paired set of parallel kinetosomes, probably similar to that found today in karyorelictid ciliates, presumptive descendants of the ancestral stock.