SEGREGATION, VARIATION AND RECOMBINATION 13-7 



and their specific colonial character becomes apparent, distinguish- 

 ing them from the original segregant. Whether this is a process 

 involving several genetical steps has not been determined. Varia- 

 tions occurring in the haplophase can be selected and propagated. 

 In spite of the wide variety of types that occur, the existing geno- 

 type of any cell limits its potentialities and the range of its pos- 

 sible variations. This fact has been especially brought out in ex- 

 periments aimed at adapting haplophases by selection. S. cere- 

 visiae is unable to ferment melibiose, and prolonged exposure of 

 haplophase cultures of S. cerevisiae to melibiose failed to produce 

 any mutants capable of fermenting this sugar. A haplophase vari- 

 ant of S. cerevisiae incapable of fermenting galactose, could not be 

 induced through a four -month period to produce mutants capable of 

 fermenting galactose, although this strain produced an abundance 

 of colonial variants during the same period. Although "losses" 

 occur easily, "gains" as in the case of specific fermentative or syn- 

 thetic abilities, apparently do not occur at all under certain con- 

 ditions. 



RECOMBINATION 



Segregation and mutation produce a great variety of haploid g 

 gametes. Copulations between these gametes selected at random 

 produce recombinations different in a number of characters from 

 the diploid from which they originated. Gtenetical variation in 

 yeasts is produced by (1) segregation, (2) variation, and (3) recom- 

 bination. The large ellipsiodal cells which produce a smooth colo- 

 ny on solid medium are usually heterozygous for a number of char- 

 acters which are segregated at the reduction division and trans- 

 mitted to four different ascospores. Each of these four spores gen- 

 erally produces a characteristically different colony; the original 

 segregant. The haploid cells are usually rough and much smaller 

 than the original diploid cell. When colonies are grown from the 

 haploid cells, a variety of colonial types appears, indicating that 

 stable gene mutations or depletion mutations have occurred. Oc- 

 casionally diploid cells appear in some of the single ascospore cul- 

 tures, revealing that copulations had produced an illegitimate dip- 

 lophase. The illegitimate diploid cells are usually somewhat small- 

 er than the legitimate diploids. 



Legitimate diploids; generally produce an abundance of viable 

 ascospores (fig. 1-7), while illegitimate diploids generally sporu- 

 late less vigorously and produce fewer and less viable ascospores. 

 The asci of illegitimates often fail to produce 4-spored asci so 

 necessary for genetical analysis and if they sporulate at all may 

 produce mostly 2-spored asci. 



