200 The Nature of Biological Diversity 



One report of limited persistence of doublets, not due to reversions 

 at all, requires special mention. Calkins (1925) noted that a doublet 

 clone of Uroleptus mobilis lost the capacity to conjugate. The singlets 

 that arose in the clone regained this capacity. Because Uroleptus 

 exhibits a clonal life cycle ending eventually in death in the absence 

 of conjugation, the doublet Uroleptus was doomed to extinction. No 

 other case of this kind has ever been found. Doublets do conjugate 

 and this has made possible study of the inheritance of the doublet 

 condition through fertilization, especially in Euplotes and Para- 

 mecium. 



Both in Euplotes patella (Kimball, 1941) and in Paramecium 

 aurelia (Sonneborn, 1942, and the present paper) doublets can con- 

 jugate with doublets; doublets of P. aurelia also undergo autogamy. 

 The doublet condition persists in both the exautogamous and excon- 

 jugant progeny. In both genera, doublets were also crossed to singlets 

 (Kimball, 1941; Powers, 1943; Sonneborn, 1942), with the same 

 results as reported in the present paper. Genie markers showed that 

 the doublets had been fertilized by their singlet mates (Powers, 1943) . 

 Although genetic analysis of the basis of the difference between 

 singlets and doublets was carried no further than this, the results — as 

 far as they go — are in complete harmony with ours. Both singlet and 

 doublet exconjugants reproduced true to type in spite of having 

 identical genotypes. 



That no genotypic difference distinguishes singlets from doublets is 

 also indicated by the modes of origin of doublets. In some genera, the 

 two products of fission of a normal singlet reunite to form a doublet, 

 as was first noted by Chatton (1921) for Glaucoma. In this case, the 

 fission is abortive; the two daughter cells remain united by a cyto- 

 plasmic bridge and the opisthe moves up alongside of the proter in 

 homopolar orientation. In other genera, such as Colpidium (Sonne- 

 born, 1932), the abortive fission may by further replication of parts 

 form a multiple monster from which eventually both homopolar 

 doublets and normal singlets may pinch off. In Paramecium (Sonne- 

 born, 1942) doublets arise from a pair of conjugants by the formation 

 of a cytoplasmic bridge which spreads in the main posteriorly. The 

 first fission then yields two singlets from the anterior part of the fused 

 exconjugants and a doublet from the posterior part. The details of 

 origin show minor variations from pair to pair. The two conjugants 

 that yield a doublet are of identical genotypes (as a result of the 

 mating, if not prior to it) . Thus, in all three of these modes of doublet 

 origin, the two components of a doublet are genotypically identical 



