444 Genetics of Sex Determination 



much inferior to FF, as the greater rarity of subgynoecious plants 

 shows. In the transition from monoecism to dioecism, the relatively 

 unstable A(M) A(M)/X(F) condition, alone capable of producing 

 anthers, but not of suppressing ovaries, is shifted by the appearance 

 of a modifying action in the Y-chromosome which so strongly sup- 

 presses the female anlagen that it overrules the F/M balance in all 

 genotypes possessing X(F), XX (FF), or XXX (FFF) chromosomes, 

 but not 4X(4F). This alone suffices for the explanation of all the facts, 

 and no male determiners in the Y are needed. 



This simple solution, which takes from the Melandrium case all 

 the features that seem to require a genetic system different from all 

 other well-explored ones, is in harmony with important facts known 

 in regard to the Y-chromosome in Drosophila outside the sphere of sex 

 determination. We found that the heterochromatin in the Y-chromo- 

 some has a considerable modifying action (probably proportional to 

 quantity) upon other genically controlled developmental processes, 

 which are sometimes enhanced, sometimes inhibited, as the case may 

 be. We refer especially to the extreme case of podoptera K (see I 2 

 C d aa), in which a remarkable and extreme mutant action is realized, 

 not in the normal female but in XXY females. Hence it is in no way 

 unusual to find that Y heterochromatin has a strong effect upon other 

 genie actions. In Melandrium this means that in the transition from 

 monoecism to dioecism via dropping one female-determining X-chromo- 

 some, an additional modifier has appeared, namely, a mass of hetero- 

 chromatin suppressing still further female action. Melandrium thus 

 has the same sex determination as Drosophila plus a powerful anti- 

 female modifying action of Y heterochromatin. 



d. Dioecism derived from monoecism in other coses 



We might expect to find a comparable situation in other cases 

 where dioecism is derived from a monoecious condition. This is indeed 

 so. The classic example is sex determination in frogs with its great 

 modifiability and lability from a more or less monoecious basic condi- 

 tion. Witschi ( 1914 ff . ) showed in classic papers that the basic condi- 

 tion is the F/M balance system, but, in addition, some action of the 

 Y-chromosome must be introduced in order to explain all the detailed 

 facts. (See full discussion in Goldschmidt, 1931; and Witschi, 1939a.) 

 This condition is assumed to be a weak female factor in the Y-chromo- 



