17-8 THE YEAST CELL 



The development of a new progressive characteristic (like the 

 ability to ferment lactose) is in a different category from these 

 minor "loss" mutations. The Drosophila evidence indicates that 

 the evolution of a new character probably begins with the produc- 

 tion of a "repeat." The "repeat" produces an extra gene which is 

 available for mutation. After the "repeat" has occurred the or- 

 ganism may continue for an indefinitely long time with this extra 

 gene unchanged. When a maltose -fermenting yeast drops into milk, 

 the maltose gene is quite useless since milk contains no maltose. 

 The previous discussion suggests that some time within the last 

 twenty thousand years a maltose -fermenting gene achieved the 

 ability to ferment lactose. This information, however, is not en- 

 couraging from the standpoint of developing new progressive mu- 

 tations in the laboratory. Apparently no "repeat" has occurred in 

 Saccharomyces producing a strain able to ferment both maltose 

 and lactose (Brettanomyces ferments both sugars slowly). 



It seems probable that duplication and mutation have led to the 

 development of an organism capable of fermenting galactose, from 

 one originally able to ferment only glucose, but himdreds of mil- 

 lenia may have been required for this progressive step. The high 

 degree of stability of the ability to ferment glucose suggests that 

 this gene was the original mutation and the galactose gene developed 

 from it by a repeat and a mutation. The hydrolytic mechanism 

 capable of breaking down disac char ides may have developed inde- 

 pendently. The sexual mechanism is capable of throwing off a varie- 

 ty of haplophases, such as Zygosaccharomyces lactis, which can 

 become adapted to milk and then could be reincorporated into the 

 wild-type organism. Judging from the failure of the available 

 a.pparatus to incorporate the lactose fermenting ability into the 

 standard organism over a period of approximately fifty thousand 

 years, other similar progressive changes may have required ex- 

 traordinarily long periods of time. 



These considerations have led to the conclusion that mutations 

 involving an ability to ferment galactose or other sugars do not oc- 

 cur in the laboratory. When an experiment seems to indicate that 

 a mutation from nonfermenter to fermenter has occurred, I prefer 

 to interpret the result as indicating that the fermentative ability 

 had never been lost, but that the gene controlling it had been parti- 

 ally degraded and some component of the medium had supplied the 

 essential element necessary for bringing the gene into full function- 

 al activity. 



A HYBRID BETWEEN S. CEREVISIAE AND S. FRAGILIS 

 We have attempted to hybridize lactose and maltose fermenters 

 numerous times, but generally found the spores of lactose fermen- 

 ters to be nonviable. One hybrid between the lactose fermenter, 



