HYBRID VIGOR IN DROSOPHILA 479 



Ames II, in two similar experiments, has two females laying 3168 and 3108 

 eggs and thirty-two others laying between 2000 and 3000 eggs. The other 

 pure races have no individuals laying more than 2000 eggs. Hybrid vigor con- 

 tributes consistently high performance to all individuals rather than excep- 

 tional performances to a few. It is the consistency of high performance which 

 calls for explanation. 



MEASURES OF HETEROSIS 



As lifetime egg production is a difficult character with which to work, a 

 less tedious measure of productivity was sought. The character chosen was 

 daily egg yield 5, 6, 7, 8, and 9 days after the female emerged from the pupa. 

 These records are at the general maximum of the female's productive life. 

 The correlation with lifetime production is high. 



Chromosomal and Cytoplasmic Basis for Hybrid Vigor 



The possibility of creating homozygous races of Drosophila through out- 

 crossing offers a unique opportunity for analyzing the causative agents be- 

 hind hybrid vigor. Hybrid vigor has been postulated as due to differences in 

 allelic genes and to differences in the cytoplasms which combine at fertiliza- 

 tion. The reduction in yield of inbred races is accompanied by increasing 

 identity in both the combining alleles and the cytoplasms which combine to 

 form succeeding generations. Both these factors have been invoked to explain 

 the low yield of such inbred races. Production of homozygous tya^ through 

 outcrossing furnishes a contrast between these two possible causes of low pro- 

 ductivity. The allelic genes are made homozygous so that any undesirable re- 

 cessive gene would have full expression in the different races and thus lower 

 the yield. The cytoplasms which combine are diverse and as such should 

 give high yield to the individuals if hybrid vigor is an expression of differences 

 in combining cytoplasm. This contrast is shown as follows: 



Genes tend Cytoplasms tend 



toward toward 



Inbreeding 



(^ Gametes 

 9 Gametes 



d^ Gametes 

 9 Gametes 



identity identity 



Homozygous by outcrossing 

 identity diversity 



The effects of genes as contrasted with the effects of the cytoplasm may be 

 measured by comparing inbred performance with that of a race made homo- 

 zygous for the same genes. Table 29.2 shows this comparison. 



For Princeton 1, the inbred progeny resulting from brother-sister mating 

 for 28 generations had an egg production of 73.6 eggs per day over the test 

 period. The homozygous Princeton 1 race, coming from the outcross breeding 



