PROTOZOA 



to variety 2, 4, 6, or 8; if it is a mate-killer, probably it belongs 

 to variety 8. If a nonkiller strain is extraordinarily resistant to a 

 known killer, it almost surely belongs to variety 2. 



Serotypes. As is well known, serologic techniques have been 

 used in the study of evolutionary relationships, and they are cur- 

 rently used in routine identification of certain microorganisms. 

 The possibility of using such techniques in identification of the 

 varieties of P. aurelia therefore needs to be considered, particu- 

 larly since serological traits have been much studied in several 

 of the varieties. These studies have exposed a serologic situation 

 which is far from simple and not yet completely known; but 

 enough is known to indicate that serologic identification of the 

 varieties of P. aurelia may become possible. 



Present knowledge is confined almost exclusively to antigens 

 which are detected by immobilization of the paramecia in the 

 presence of appropriate antisera obtained from injected rabbits. 

 Any one individual seems to manifest, as an almost invariable 

 rule, one and only one immobilization antigen at a time. This 

 antigen defines its serotype in the sense in which that term is 

 here used. 



The serotype of an individual commonly persists among its 

 progeny; but, as in phase variation of bacteria, changes of sero- 

 type may occur within a single line of descent. However, there 

 are more phases or serotypes expressible in a line of P. aurelia 

 than in a line of bacteria. As many as 14 different serotypes have 

 been brought to expression in different individuals of a single line 

 of descent (Margolin, 1956b), and nearly as many serotypes have 

 been found in other lines by others. Change of serotype occurs 

 without genetic recombination or gene mutation. The whole ar- 

 ray of serotypes characteristic of a line of descent appears in 

 individuals of identical — and even entirely homozygous — geno- 



pe. The changes of serotype are reversible and can be experi- 

 mentally controlled and directed by appropriate manipulation of 

 environmental conditions. In order to discover the array of sero- 

 type!; producible by any line or strain, prolonged study with a 

 wide variety ol cultural conditions is required. 



Different strains of the same variety show similarities in their 



