294 SEX IN MICROORGANISMS 



types, which they designate pins (even-numbered) and minus (odd- 

 numbered). 



In contrast to the group A varieties the three group B varieties 

 (Sonneborn's even-numbered varieties 2, 4, and 6) show no cross 

 reactions among themselves. However, Sonneborn and Dippell 

 (1946b) have described one race (race 31), which conjugates readily 

 with variety 4 animals of group B and surprisingly also gives a weak 

 mating reaction, but never complete conjugation, with variety 3 of 

 group A. On the basis of this cross reaction and certain genetic 

 properties, Sonneborn and Dippell (1946b) regard race 31 as con- 

 stituting a distinct variety, variety 8, which links the group A and B 

 varieties. Finally, this series of cross reactions admits the mating types 

 of the intermediate variety 8 and the group B variety 4 to positions 

 in the two series of general mating types of group A (Sonneborn, 

 1950). Therefore, eleven of the fifteen mating types in P. aurelia may 

 be assigned general mating-type designations. Types II, VI, X, XVI, 

 and VIII are plus types; types I, V, IX, XIII, XV, and VII are minus 

 types (Table I). 



From the foregoing account it is evident that a very high order 

 of specificity obtains in the mating reaction in P. aurelia; an order 

 of specificity that compares very favorably with that of antigen-anti- 

 body reactions, fertilizin-antifertilizin reactions, and fertilization in 

 other organisms. Since this specificity is determined by surface sub- 

 stances, the mating-type substances, these must be highly complex 

 substances or molecular configurations. 



According to the simplest scheme (Metz, 1948) the mating reac- 

 tion within a variety in P. aurelia should result from interaction of a 

 pair of such substances, A and a, which are complementary in struc- 

 ture and combine in antigen-antibody-like fashion (Fig. 3a). To in- 

 clude the two general mating-type systems (plus and minus) in this 

 concept it is only necessary to assume that the mating substances in a 

 homologous series are structurally similar but not identical. Thus the 

 plus series should consist of five structurally similar but not identical 

 substances A\ A\ A'', A\ A^ (superscripts designate the variety). 

 Correspondingly the minus series of general types should contain six 

 similar but distinct substances ^\ a\ a'', a\ a'', a^, which are all comple- 

 mentary in structure to the members of the plus series. In the "lock 

 and key" terminology of immunology, near perfect complementari- 

 ness obtains only between the two mating types (A^ and a^) in a 



