CYTOPLASMIC INHERITANCE 27-10 



though the hybrid was capable of infecting scions grafted on to it. 

 Thus, the paracrinkle virus is clearly not a plasmagene but a true 

 virus (having a common characteristic of plant viruses, namely, 

 the inability to pass through the egg), in spite of the fact that it can- 

 not be transmitted except by graft. It is obviously not a plasmagene, 

 since plasmagenes are a priori maternally inherited. 



There are many plant viruses which have nearly reached a sym- 

 biotic relationship with their hosts or do them so little harm that 

 they persist with little effect on survival of their hosts. Minchin 

 first proposed the view that the chloroplasts are evolved from in- 

 dependent green algae which have become so completely adapted to 

 the cytoplasm of green plants that they are incapable of independent 

 existence, and have in turn made their hosts completely dependent 

 upon them. Woods and duBuy have proposed a theory of the origin 

 of plant viruses which I prefer over one which I had proposed earlier 

 (1938). They have presented abundant evidence indicating that the 

 viruses of green plants are derived from chloroplasts and their argu- 

 ments based on the wide distribution of viruses and their spread 

 through the plant by cell division rather than invasion support their 

 thesis. The principal advantage of their view over Darlington's, in 

 my opinion, is that it does not require the acquisition of "autonomy" 

 by cytoplasmic proteins. This characteristic seems too complex to 

 be quickly attained and that view is more acceptable to me which 

 makes a virus the product of the longer evolutionary period. At any 

 rate, the paracrinkle virus which Darlington chose as an example 

 has clearly been identified as a true virus and his whole argument 

 was based on the contrary opinion. 



One of the phenomena which Darlington claims as support for the 

 plasmagene hypothesis is the maternal inheritance of male sterility 

 in plants, especially in Allium. A male sterile plant is incapable of 

 producing pollen, and therefore, can only be used as a female parent. 

 The male sterile plants are homozygous recessives, ms ms. When 

 such a plant is fertilized by the pollen from a dominant homozygote, 

 MS MS, a hybrid MS ms is produced. The hybrid is male fertile. 



The original homozygous recessive was a single unusual plant. 

 Most of the ms ms individuals found in Nature are male fertile. 

 The original ms ms male sterile plant was an exception discovered 

 in the field. When reciprocal crosses were made between the heter- 

 ozygous MS ms hybrid and a male -fertile ms ms plant, the results 

 indicated in fig. 27-4 are obtained. The different results of these 

 reciprocal matings have been considered evidence of the transmis- 

 sion through the cytoplasm of a factor controlling sterility counter- 

 balanced by the ability of the MS gene to produce fertility under any 

 circumstances. However, the fact that both male -fertile and male- 

 sterile ms ms plants are known can be interpreted as follows: the 

 original male -sterile ms ms plant was infected by a virus which 



