Different Genetic Possibilities within the Balance Theory 437 



expected that such a system is much more subject to variation than the 

 one in Lymantria and Drosopliila. The F/M balances in the two sexes 

 of such organisms may be very httle different and, therefore, additional 

 genetic and environmental features may shift the sexes more easily. 

 Thus indeed in fishes and amphibians, external conditions like tem- 

 perature and internal ones like age, hormones, and overripeness of 

 eggs may easily supersede the primary balance system; so also may 

 genetic modifiers located in autosomes and, in male heterogamety, 

 even in the Y-chromosome. These facts are probably responsible in 

 large part for the different, often complicated explanations of genetic 

 experiments, in which the only slightly different primary balance con- 

 dition in the two sexes was not or could not be clearly separated from 

 the additional modifying genetic properties, I may point to the dis- 

 cussion of sex determination in fishes by Kosswig and Breider (see 

 Kosswig, 1939) and the very different conclusions of Hammerling, 

 Goldschmidt, and Hartmann. The Kosswig group thinks that sex de- 

 termination is dependent upon multiple factors in the autosomes, with 

 little effect of the X-chromosomes and different types of Y-chromo- 

 somal action. I agree with Hartmann that the facts can be accommo- 

 dated in the general F/M scheme together with different modifying 

 actions, though Hartmann thinks only of the FF-FM scheme of Cor- 

 rens without clear application of the balance system, which is known 

 to work in Amphibia and therefore is supposed to work in fishes also. 

 There is the additional fact that different species of the same fish 

 genus may be male-heterogametic or female-heterogametic and that 

 some genera are actual hermaphrodites. A detailed discussion is 

 beyond the scope of this book, but the basic genetic principles in- 

 volved are apparent, I think, in this brief discussion. 



c. The Melandrium case 



This leads finally to the extremest variant known, extreme because 

 it seems at first glance to deviate farthest from the simple balance 

 scheme, the facts found by Ono, Blakeslee, Warmke, and Westergaard 

 in Melandrium (literature in Westergaard, 1948). Sex in Melandrium 

 involves both Y-chromosomal action and sex dimorphism derived from 

 hermaphroditism. Melandrium album is dioecious and has the classic 

 sex-determination scheme of XX = $ , XY = $ , the Y-chromosome 

 being very large. This plant, as is well known, was one of the first in 

 which not only the sex-chromosome mechanism but also the first case 

 of sex-linked heredity was demonstrated in plants. Only Melandrium 

 album and rubrum are dioecious; other related species are monoecious. 



