160 ADRIANO A. BUZZATI-TRAVERSO 



mutations with X-rays, and whether an increase in the mutation rate may 

 speed up the evolutionary rate under selection pressure. 



For this purpose I have set up four artificial populations starting from an 

 isogenic stock of Drosophila melanogaster. One of these is being kept as con- 

 trol while the other three get, every two weeks, 500, 1000, and 2000 r-units 

 respectively. At the start, and at various intervals, I am measuring fecun- 

 dity, fertility, and longevity of the flies. The few data so far collected show 

 clearly that in the irradiated populations the percentage of eggs that do not 

 develop is much higher than in the control. This is due to the effect of 

 dominant and recessive lethals. But the startling result is that the fecundity, 

 measured by the number of eggs laid per day by single females of the irradi- 

 ated populations, is higher than in the control series. Probably X-rays have 

 produced a number of mutations for higher fecundity which have been ac- 

 cumulated by natural selection in the course of the experiment. But, are spe- 

 cific mutations for higher fecundity being produced, or am I dealing with 

 heterosis phenomena dependent upon nonspecific mutants? 



These few examples from my own experience with population-genet ical 

 studies show, I think, how important the heterosis phenomenon can be in our 

 field of work. Both in natural and artificial populations, heterosis seems to 

 be at work, making our analysis rather difficult, but stimulating as well. 

 Closer contacts between students of selection and heterosis in plant and 

 animal breeding and students of evolutionary problems are to be wished. 

 Let us hope that a higher level of hybridization between various lines of 

 investigation might become permanent, since it surely will make our studies 

 more vigorous and better adapted to the requirements of a rapidly growing 

 science. 



