TABLE 4

Evidence from random ascospores and from tetrads that new cya-8 occurrences originate preferentially or exclusively in the nucleus that contains easUCLA

Unordered asci that contain four cya-8 progeny
Parental genotypes (all cya+)cya niccya niccya niccya nicNo.Ascus typea% cya-8–nic-3 recombination
A. Asci
easUCLA; nic-3+ × eas+; nic-3− (+)− (+)+ −+ −10PD12
− (+)− (−)+ ++ −3T
− (−)− (−)+ ++ +0NPD
easUCLA; nic-3 × eas+; nic-3+− (−)− (−)+ ++ −32PD25
− (+)− (−)+ ++ −21T
− (+)− (+)+ −+ −1NPD
Parent genotypes (all cya+)No. and genotype of cya-8 progenyb% cya-8–nic-3 recombinationc
B. Random isolates
easUCLA; nic-3+ × eas+; nic-341 nic+, 13 nic-325
easUCLA; nic-3 × eas+; nic-3+8 nic+, 35 nic-319
Pooled22

  • The genotype of each ascospore pair is given. The nic-3 marker was not scored in the slow-growing cytochrome-deficient progeny. However, the inferred allele is shown in parentheses. The percentage of asci in which a new cya mutation was segregating was 15% in the first cross and 55% (54/98) in the second. eas and cya-8 were unlinked (3 PD, 46 T, and 5 NPD in the second cross). PD, parental ditype; T, tetratype; NPD, nonparental ditype.

  • ↵a PD and NPD would be equally frequent if mutation to cya-8 were equally likely to occur in easUCLA and eas+ nuclei. The linkage observed (PD > NPD) would be expected if the mutant cya-8 allele originated in the eas nucleus.

  • ↵b cya-8 progeny were rescued by co-inoculating with nic-3 arg-10 A or nic-3 arg-10 a on slants containing nicotinamide but not arginine. The resulting vigorous heterokaryons were then tested on minimal medium to determine whether the cya-8 component was nic-3 or nic-3+.

  • ↵c Assuming that cya-8 originated in the easUCLA nucleus. Recombination between cya-8 and nic-3 is 26% in the conventional crosses in Table 1.