THE PnVSK)1.0(;V OF 1 1:111 ILIZAIION IN CILIATF.S 291 



witli an animal of opposite type. This suggests the transfer of surface 

 substances from one animal to another at the contact. Studies of 

 the effect of various agents on tlie mating behavior of Varainechnn 

 lend further support to this view . Paramecia that have been killed 

 by appropriate treatment with a wide variety of agents will give 

 specific mating reactions with living animals of opposite mating type. 

 Furthermore this reactivity can be blocked by treatment with certain 

 mild agents such as antiserum and protein group reagents. Although 

 no extract with mating substance activity (inhibition of the mating 

 reaction, action on animals of opposite type) has yet been prepared, 

 the evidence presented above and other characteristics of the mating 

 reaction to be outlined below leave no reason to question the view 

 that the initial adhesion depends upon reactions at the molecular level. 

 Therefore the initial adhesion, the mating reaction, can be attributed 

 to an interaction of substances or at least molecular configurations 

 attached to or built into the surface structure of the paramecium. For 

 convenience these are called the viating-type substances. The follow- 

 ing sections will deal mainly with the role of these mating-type sub- 

 stances in other aspects of fertilization. 



Specificity of the Mating-Type Substances 

 Breeding Systems in Paramecium 



The initial adhesion, the mating reaction, is the first step in con- 

 jugation. If the mating reaction does not occur, the succeeding events 

 are not observed. Conversely, completion of conjugation rarely fails 

 to follow the mating reaction under optimal conditions. In view 

 of this relationship the specificity of the mating reaction is a limiting 

 factor in conjugantion, and, since the mating reaction results from 

 interaction of the mating-type substances, the primary specificity in 

 conjugation will be determined by these mating-type substances. It 

 will be evident from the account to follow that this is a very high 

 order of specificity. 



Most species of Faraiiieciwn have been examined for mating 

 reactions, and breeding systems have been worked out in nearly all 

 of these. Almost from the time of the discovery of mating types in 

 Faraineciumy two distinct breeding systems were recognized. In one of 

 these the morphological species consists of a number of sexually iso- 

 lated varieties each of which contains two interbreeding mating types. 



