456 Genetics of Sex Determination 



combination of both; or by the existence of weak and strong races, 

 meaning different potencies of F and M which may be recombined 

 in an abnormal way after crossing. I had formerly assumed that the 

 Streptocarpus case follows the last alternative (Goldschmidt, 1938fo). 

 But the new results of Oehlkers with polyanthus, giving a 3:1 Fo 

 segregation, requires an autosomal action, a modifier of the type found 

 in the Drosophila cases mentioned. However, this alone does not 

 suflBce for the explanation of the R X W crosses; an additional inter- 

 action between a plasmon and the autosomal modifiers is required. 

 This puts the modifier system on the same plane as that discussed 

 above for male sterility, which then might be a beginning intersexu- 

 ality superimposed on monoecism (though this is improbable, as 

 discussed in IV 3 B c). The modifier in Drosophila repleta worked 

 by means of the cytoplasm as did also the Bd M intersexuality (see 

 II 2 A), though no separate plasmon existed. However, there still 

 remains this riddle: Why does the change in F(X) (plus the Y-chro- 

 mosome modifying action) in Melandrmm produce only a shift in 

 the development of the flower anlagen in the way of suppression- 

 enhancement, while the modifier in Streptocarpus affects both anlagen 

 in their actual sex differentiation, in the presence of the proper 

 plasmon? The nature of monoecism cannot be involved in this dif- 

 ference. It must therefore be the presence of different plasmons 

 interacting with the genetic modifiers of F/M balance which is 

 responsible for the genuine intersexual effect in Streptocarpus. A 

 more specific meaning will be given at once to this conclusion. 



Facts resembling those in Streptocarpus have been described in 

 other plants after species crosses, though they could not be analyzed 

 genetically as well as in Streptocarpus. In these cases even the Fi 

 showed the intersexuality, as in Ltjmantria. This might suggest that 

 the genetic basis is different, more as in Lynmntria. But it is of no 

 use to speculate on this, in view of the meager information. Good 

 examples are the Salix crosses of Rainio (1927) with complete series 

 of female and male genuine intersexuality, and the comparable 

 Solarium crosses of Koopmans (1952). It is remarkable that in the 

 latter, also, similar anomalies of the flowers themselves (petals) were 

 found, like those studied by Oehlkers in Streptocarpus, and explained 

 by a recessive mutant collaborating with a definite plasmon. In view 

 of the fact that certain color mutants in fishes act as modifiers of sex 

 determination, one could expect the intersexuality modifier in Strepto- 

 carpus (and also in Solarium) to be identical with the flower modifier 

 for choripetaly. If this were so, an explanation of the dilemma 



