278 AN INTRODUCTION TO MODERN GENETICS 



particular structure which carries them, except in plants in which chloro- 

 phyll diseases or deficiencies may be inherited through the plastids, and 

 even here it is not entirely clear whether the plastids themselves are de- 

 ficient in some way or whether the abnormaUties in the production of 

 chlorophyll are caused by interaction between normal plastids and a 

 modified cytoplasm. Even the unlocalized cytoplasmic faaors seem to be 

 quite self-perpetuating and constant. Wettstein has made a cross between 

 Physcotnitrium piriforme {Pi) $ X Funaria hygrometrica (Hy) ^. In 

 this, an intergeneric cross, the cytoplasmic influences are even more 

 effective than before; the Hy chromosomes cannot survive when 

 isolated in the [pi] cytoplasm, so that no [pF] Hy gametophytes are 

 formed from the Fi. But, by various regenerations, etc., polyploid 

 types can be made including ^pU PiHy, [pn PiHyHy, [pT] PiHyHyHy, 

 which were viable. Wettstein showed that even after six generations in 

 which the [pi] cytoplasm was exposed to one, two, or three doses of the 

 Hy genotype, the characteristics of the [pT] cytoplasm were still present 

 and the [pi] Hy type was still not able to survive. Thus the cytoplasmic 

 factors, in this case at least, are only very slowly, if at all, affected by the 

 chromosomes. This cannot be an entirely general rule, however, since 

 in Limnea we know a case in which the cytoplasm is altered in a single 

 generation by the genes which it contains (p. 1 43). 



In some pairs of species, the cytoplasm of one is poisonous to the 

 chromosomes of another. Thus a whole hnkage group of factors from 

 Vicia faba major is eliminated when in V. faba minor cytoplasm.^ In 

 crosses between different species of echinoderms, many of the paternal 

 chromosomes may be eliminated during cleavage or later, and in some 

 cases the reciprocal effect, a morphological influence of the chromo- 

 somes on the cytoplasm, has been observed.^ 



2. Persistent Modifications^ 



In some organisms, belonging to very different groups, experimental 

 treatment has produced changes which are maternally inherited for 

 some generations and then gradually lost. They seem to be caused by 

 persistent, but not unalterable, changes in the cytoplasm. They were 

 discovered in Protozoa,^ where they were produced by heat and chemi- 

 cals; and it was found that repeated treatments increased the effects 



^ Sirks 1932. 2 cf Schleip 1929. ^ Rev. Jollos 1938. 



* Jollos 1 92 1. For an interpretation in terms of gene mutations, see Raff el 

 1932. 



