198 The Nature of Biological Diversity 



isms differ greatly among themselves and are in some ways unique. 

 We would helieve conclusions to he fundamental only if they were 

 found to he applicable to multicellular as well as to unicellular organ- 

 isms. Such a search for the fundamental is the ohject of this discussion 

 section. Previous pertinent work on doublets and other cortical vari- 

 ants in ciliates will he considered first; then related work on multi- 

 cellular organisms; and finally, the general conclusions to he drawn 

 on the role of cortical structure. 



it. Doublets and other cortical variants 

 in ciliates 



The inheritance and morphogenesis of doublets have been studied 

 in a number of genera of ciliates in the course of the last 40-odd years. 

 Most of this work has been critically and thoughtfully reviewed from 

 several points of view in a series of papers by Faure-Fremiet (1945; 

 1948a, b; 1950; 1954). All studies show that doublets reproduce true 

 to type during fissions; but to the question "How long can they do 

 this?" the answer is not so simple. Many workers found that cultures 

 begun with doublets eventually end up with only singlets present ( for 

 example, Chatton, 1921, on Glaucoma; Sonneborn, 1932, on Col- 

 pidium; Faure-Fremiet, 1948a, on Leucophrys; Tartar, 1954a, on 

 Stentor; Hanson, 1962, on Paramecium). Does this mean that the 

 "inheritance" of this cortical variation is limited and that eventually 

 the nuclear genotype prevails by restoring the normal singlet condi- 

 tion? While even limited inheritance would be of some significance, 

 the full genetic import of cortical variations depends in part upon 

 the degree to which they persist during reproduction. What then is the 

 genetic significance of the reversion of cultures of doublets to singlets? 



Doublets have been reported to give rise to singlets in several ways. 

 A commonly observed way is for a depression to arise between the 

 two sets of organelles at the anterior end and gradually to deepen in 

 the course of successive fissions until it reaches the fission plane, 

 which then cuts off two singlets and a doublet (Margolin, 1954). 

 There are also some minor variations of this process. Another very 

 different process is asymmetric reduction of the number of kineties 

 and the distance on one side between two corresponding meridians 

 such as the two oral meridians (Faure-Fremiet, 1948a). This leads to 

 eventual interaction between, and loss of one of, the two sets of struc- 

 tures (see page 193 above). Once this process starts in a subline, it 

 cannot be reversed by selection. A third process is resorption of the 



