GENE CONVERSION 



26-42 



segregants from a tetrad is held in glucose broth together with a 

 standard a clone known to copulate vigorously with other haploids. 

 A positive. mating reaction identifies its mate as a mating type; in 

 the absence of copulations, the mate is diagnosed as a clone of like 

 mating type, i.e., as an a mating type. Copulations are generally 

 effected within a few hours. 



Mating type specificity segregated 2 a/a a in most asci. Occa- 

 sional disturbances in mating reactions were, however, encounter- 

 ed. For instance, tetrad M528 - M531 (W W W W) mated to the 

 standard a clone but no copulations were observed until the fifth 

 day, while the normal tetrads M536 - M539, and M548 - M551 (2 

 P: 2W) copulated vigorously within 12 hours. Other tetrads, none 

 of whose haplophases copulated with the standard a clone even on 

 the fifth day, were subsequently mated with a standard a clone and 

 two of the haplophases copulated vigorously. Still others, M540 - 

 M543 (Table 26-18) mated with neither the a nor the standard a 

 clones. Similar instances of W W W W tetrads failing altogether to 

 copulate are cited (chapter 25). 



The fact that mating strength is normal in 2 P: 2 W asci, but is 

 impaired or completely absent in some W W W W asci (obtained 

 from P X W crosses) suggests that the phenomenon is, in some way, 

 associated with disturbances at the AD/ad loci. 



Fig. 26-2 Melibiose Pour-Plates of Tetrad M532 - M535. 

 Three plates (left to right) have colonies each of which is 

 individually capable of fermenting melibiose. Inocula de- 

 rived from glucose slants of M535 has minyte, nonfermenter 

 colonies and the clone derived from the corresponding slant 

 cannot ferment melibiose. The medium-sized colonies in M535 

 do not result from crowding. 



