Inheritance in Ciliate Protozoa

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Abstract
A critical review (including some unpublished work) on ciliate genetics, emphasizing observations not readily interpreted by orthodox genetic principles. Nearly all observations on hereditary diversities within a clone are excluded as (a) not clearly within a clone, (b) not clearly hereditary, or (c) not indicative of constitutional diversity. The following appear unobjectionable: (1) Middleton''s isolation of diverse lines by selection in clones of Stylonychia; (2) Kim-ball''s observation of genetically diverse lines in unstable clones of Paramecium aurelia; (3) Sonneborn''s observation of the origin of a whole complex of new characters within a clone of P. aurelia after regeneration of new macronuclei from single fragments of an old macronucleus. Especially in cases 2 and 3, genic interpretations seem excluded. Recent studies on genic inheritance in P. aurelia show that hereditary diversities between clones arising at conjugation and other nuclear reorganization processes, as reported in many older papers, apparently were not due to recombination of genes, because the stocks investigated should have been homozygous as a result of the special type of autogamy that occurs. Hereditary clonal differences in mating type in certain stocks appear not to be due to genic differences in the micronuclei or macronuclei, though these characters are determined in some way by macronuclei (influenced by temp. at the time of their origin from micronuclei). The Dauermodifikationen reported by Jollos and interpreted by him as non-genic in origin are contrasted with the preceding. The foregoing independent lines of evidence all suggest that most hereditary differences in ciliates are due either to mutations occurring with unprecedented frequency and in most improbable ways, or to at present unrecognized non-genic factors.