27-3 



THE YEAST CELL 



ability to ferment sucrose on the Warburg apparatus. Only those 

 matings which occurred between unrelated strains were tested; for 

 example, hybrids between L and HD, L and M, L and FLD, L and 

 D, L and B; and hybrids produced by mating (800 x L) x L with HD, 

 M, FLD, D, and B, as well as some produced by matings of M x D, 

 and (800 x L) x M were used in the study. None of the inbred cul- 

 tures (L X L, M X M, etc.) was tested. A standard baker's yeast 

 which produced around 330 mm^ of CO2 per mgm. of dry weight per 

 hour and consumed about 80 mm^ of O2 per mgm. of dry weight of 

 yeast per hour, was used as a control. The number of mm^ of CO2 

 produced in the presence of O2 per mgm. of dry weight of yeast per 

 hour is designated the Qro? (Warburg's notation); the amount of O2 

 consumed under the same condition is called Qo2- The ratio of CO2 

 produced to O2 consumed is called the respiratory quotient, or R. Q. 

 The agar slants on which the hybrids were stored were plated 

 out in petri dishes and single colonies selected and planted in sepa- 

 rate tubes of broth. The obviously inferior cultures and the hap- 

 loids were discarded and the apparently superior diploids were tested 

 in the Warburg apparatus, suspended in 4 per cent sucrose and M/15 

 KH2PO4 solution. The tests made between August 21 and September 

 5 are shown in Table 27-1. Nine of the hybrids had R. Q's of 1 with 

 QC02'^ between 313 and 1730. It is rather unusual to find yeasts 

 which act on external substrate with an R. Q. of 1. Twelve of the hy- 

 brids had Qc02's f^^^^ 1650 to 8500. The standard control baker's 

 yeast had a Qfoo °^ ^^^' '^^^ metabolic activities of these freshly- 

 formed hybrids were enoT*mously higher than normal. 



Table 27-1 

 Heterosis and "Running Out" of Yeast Hybrids 



