158 ADRIANO A. BUZZATI-TRAVERSO 



experimental conditions. The exceptional occurrence of many mutant indi- 

 viduals in a free living population can be accounted for by assuming that 

 they have a higher selective value. 



BASIS FOR SUPERIORITY OF THE HETEROZYGOTE 



It would be interesting to try to find out how selection discriminates 

 against both normal and mutant homozygotes. I am just beginning to attack 

 this problem. 



Dr. E. Caspar! has some interesting results on a similar problem, and I 

 wish to thank him for permission to quote them (Caspari, 1950). In free 

 living populations of the moth Ephestia kuhniella, this author has observed 

 a balanced polymorphism, whereby individuals having brown colored and 

 red colored testes occur in various numbers. The character brown behaves as 

 a complete dominant with respect to red. The polymorphism seems to be 

 determined by a higher selective value of the heterozygote. It has been pos- 

 sible to show that the heterozygote is equal or superior to the homozygous 

 recessive and the latter is superior to the homozygous dominant with respect 

 to viability. It was found that, while the heterozygote is equal or superior to 

 the homozygous dominant, the homozygous brown is superior to the homozy- 

 gous red with respect to mating activity. The dominance relationships of such 

 two physiological characters are therefore reversed. 



There is no decisive evidence for heterosis for any of the characters 

 studied. The recessive for the testis color acts as dominant with respect to 

 viability, and the dominant testis color acts as dominant with respect to 

 mating behavior. The net result is a selective advantage of the heterozygote 

 that can account for the observed polymorphism. This seems a good ex- 

 ample of how a heterosis mechanism can be determined by the behavior of 

 two visible alleles in heterozygous condition. It is hoped that similar analyses 

 will be developed for other cases of balanced polymorphism. 



The search for clear-cut examples of heterosis depending on single genes 

 seems to me the most promising line of attack on the general problem under 

 discussion. If I could find another gene behaving in a way similar to the one 

 I have studied in Drosophila melanogaster, and could study the interaction of 

 the two, it would be possible to go a step further in the analysis of heterosis 

 mechanisms. The evidence derived from such single genes being favored in 

 heterozygous condition is likely to be very useful in more complex condi- 

 tions where the action of several genes is involved. 



When we come to consider the selective advantage of polygenic charac- 

 ters, even in such an easy experimental object as Drosophila, the problem 

 becomes very entangled indeed. In recent years I have been studying, for 

 example, a number of quantitative characters being favored by natural 

 selection in artificial populations in numerical equilibrium, such as the ones 

 I have been speaking about. I have set in competition at the beginning of one 



