11-17 THE YEAST CELL 



MATING TYPE SPECIFICITY 



Mating type specificity is determined by observing the appear- 

 ance of copulations in 0.5 cc. of standard broth in a 6 x 3/4 -inch 

 test tube. Cells from each of the four spores from a single ascus 

 are transferred into four separate tubes each containing 0.5 cc. of 

 broth, by inoculating heavily with a needle from an agar slant of 

 the haplophase culture. Each of the four tubes receives a similar 

 amount of the cells of our standard mating -type tester. The mixed 

 cultures are placed in an incubator at 16°C. overnight and are ob- 

 served in the morning. When a critical test is required eight tubes 

 are used, one set of four is mated to a standard a type culture, and 

 the other set to an a type culture. We routinely use only an a type 

 culture. In some asci all four spores fail to produce copulations 

 when mated to the standard a strain. This is probably due to in- 

 fertility. There are always a few copulations even in the haplo- 

 phase cultures obtained from a single ascospore and a clone mated 

 to a standard a strain is only classified as a if from 25 to 50 per 

 cent of the cells have copulated in the test tube. This character 

 has proved extraordinarily stable in inbred pedigrees. Exceptions, 

 although encountered, are not frequent enough to be troublesome; 

 if fact, they sometimes prove very useful as in the analysis of 

 ascus 167, 168, 169, and 170; in which 169, originally classified as 

 a , was subsequently capable of mating with another a type culture 

 from the same ascus; and in which culture 170, originally classi- 

 fied as a, finally produced some illegitimate progeny. 



SUPPRESSORS 



This designation was given by Bridges (1932) to recessive genes 

 which transform a recessive character at another locus into its 

 normal wild-type form. For example, the wild-type color of Dro- 

 sophila eyes is red; a recessive gene in homozygous form is capa- 

 ble of transforming this red color to purple, which may be looked 

 upon as a reduced capacity for color production. The suppressor 

 of purple (when homozygous) changes the purple (deficient) color 

 to red (normal) color. The purple gene interferes with the com- 

 plete normal color production. When a purple fly is made homo- 

 zygous for the suppressor of purple, the wild-type color appears; 

 the fly achieves complete ability to produce the normal wild-type 

 pigment in spite of the presence of the purple genes which nor- 

 mally reduce its ability. The suppressor gene makes a relatively 

 weak organism capable of producing the full quantity of pigmenta- 

 tion. Suppressors do not actually suppress a character, but enable 

 a recessive gene incapable of completing a synthesis to bring the 

 synthesis to completion, or to bring into function an alternative 

 route of synthesis. 



