Alb 



JOHN W. GOWEN 



purebreds, 43.4 days, but does not exceed the range. The parents entering 

 into the cross for this hybrid laid eggs for an average of 38.4 and 17.2 days 

 respectively. Hybrid vigor is only 113 per cent for the length of the produc- 

 tive period. 



The physiological fitness of the purebred races, as measured by their dura- 

 tion of life, ranged from 22 days to 58 days. The average life of the hybrid 

 was 52.2 days. The hybrid's parents lived for 38.7 and 50.2 days respectively. 

 This character showed little hybrid vigor — 104 per cent. 



These results show that egg production is the most favorable of the 

 Drosophila characters analyzed for the study of hybrid vigor. The lifetime 



TABLE 29.1 



CHARACTERISTIC VARIATION OF DROSOPHILA RACES 

 IN EGG PRODUCTION AND DURATION OF LIFE 



distributions of egg productions for the inbred and hybrid races are shown in 

 Figure 29.2. 



Newly hatched females require a short period after emergence for maturing . 

 Heavy egg production begins on the fourth day and rises rapidly to a maxi- 

 mum in early life. From the high point, production gradually declines. The 

 rate of this decline varies with the different races. The average slope is shown 

 by straight lines. 



Drosophila egg production presents a single cycle as contrasted with the 

 series of cycles or egg clutches observed in the egg production in certain other 

 forms — the domestic fowl or the fungus fly, Sciara. This fact makes Drosoph- 

 ila egg production an easier character to study. The egg yield curve is deter- 

 mined by the initial high point in production and the rate of loss in produc- 

 tivity with age. 



The form of the egg production curve in Drosophila fits in with Ashby's 

 hypothesis of metabolic reserves being responsible for hybrid vigor. The hy- 



