27-11 THE YEAST CELL 



prevented the development of stamens but which is incapable of in- 

 fecting either MS ms or MS MS plants, and which cannot be trans- 

 mitted through the pollen but is capable of transmission through the 

 egg, in contrast to the general rule for plant viruses. Male sterility 

 would serve to establish a virus in a population because it would 

 force out-crossing and insure the fertilization of the infected female 

 by a non-infected male. Female -sterility would only bring an end 

 to both parasite and host, so the only viruses which could be egg 

 transmitted and survive with their hosts would be those controlling 

 male -sterility. 



On the basis of this discussion, we may conclude that there are 

 at least three types of plant viruses: 



(1) Viruses transmitted by infection, principally through insect 

 vectors. They are not transmitted through the egg. Infected plants 

 are usually characterized by the familiar chlorotic symptoms. 



(2) Viruses transmitted only by grafting and not by infection. 

 They are not transmitted through the egg and present no clear-cut 

 symptons on their adapted host. 



(3) Viruses transmitted through the egg, producing male steril- 

 ity in a homozygous recessive stock and persisting, but without 

 symptom in the heterozygote. 



RECIPROCAL U A TINGS 



^ Ms as X &" MS MS ^ MS MS X <y Ms MS 



Male fertile Male sterile 



ale fertile 

 roM Male 

 terile Mother 



Male fertile 



Progeny Progeny 

 s ms <■ ► Male fertile Ms ms <• ► Male fertile 



S MS 



Male sterile ms ms * ► Male fertile 



Fig. 27-4 Reciprocal matings in male-sterile pedigrees 



B. The Kappa Substance 



Sonneborn has proposed to call the Kappa substance in the killer 

 Paramecium a plasmagene rather than a simple virus. I (1946) have 

 objected to this view since it implies that the Kappa substance is 

 part of the hereditary apparatus and therefore it involves concepts 

 more complex than the simple inter -relationship of host and parasite. 



