Nuclear characteristics and phylogeny in the protistan phylum Ciliophora.

Ciliates possess a number of nuclear characteristics which, in combination, are unique among the Protista. Nevertheless, attempts to understand the origin - presumably from a flagellate ancestry - of the Ciliophora as a phylum must be made, as well as efforts to elucidate phylogenetic pathways within the large and diverse assemblage represented by its present-day forms. The macronucleus may provide an important clue to early ciliate phylogeny, since we still have, among extant species, groups of distinct "karyological relicts" exhibiting the very features expected in hypothetical forms corresponding to postulated stages in macronuclear origin and evolution. The relationship of the groups of "relict" species to the predominant polyploid-macronucleate forms, with a direct impact on the classification system as well as ciliate evolution and phylogeny in general, is discussed in some detail. Arguments are presented for taxonomic separation of the relatively primitive homokaryotic and diploid-macronucleate forms, which also share other features in common related to their being members of the interstitial fauna, from the more advanced ciliates. The problem is complicated by the non-nuclear structural complexities of these sand-dwelling forms, apparently secondarily-derived specializations which, by convergence, have come to resemble certain non-homologous features of the allegedly more highly evolved groups.