TABLE 4.

Frequency of viable interspecific hybrids

Crossa	No. of putative hybrids on SDb	Frequency of putative viable hybridd
A
Y185 × Y170	∼104	32 × 10−2
Y184 × Y245	1,000	3.8 × 10−2
Y185 × Y244	1,500	4.5 × 10−2
Y169 × Y244	1,500	4.5 × 10−2
Y170 × Y245	1,000	3.3 × 10−2
B
Y185 × Y339	6c	6.7 × 10−6
Y184 × Y344	50c	10−4
Y185 × Y344	70c	4.7 × 10−5
C
Y345 × Y257	∼103	∼3.3 × 10−3
Y345 × Y344	∼103	∼6.7 × 10−4
Y257 × Y344	5 + 50e	7.8 × 10−5

^aA, crosses within the sensu stricto group; B, crosses between yeasts belonging to the sensu stricto and sensu lato groups; C, crosses between yeasts belonging to the sensu lato group. Note that only interspecific crosses, which gave viable hybrids, are shown in this table. 

^bAll of the parental strains were tested for revertants on SD, and the reversion rate was shown to be lower than 10⁻⁷ for each of them. Therefore, when a colony appeared on SD, it was likely to be a hybrid because only approximately 10⁷ parental cells, including zygotes, were plated on the selective plate. 

^cSmall colonies. 

^dFrequencies of putative viable hybrids are the ratio between the number of colonies on SD plates and the presumed number of hybrids plated out. 

^eFive large and 50 small colonies.