Structure of P. anserina asci.

(A) S×S or T×T crosses yield 100% four binucleated ascospores per ascus. In S×T cross, 2-spored asci are indicative of spore killing. The Sk locus is linked to the centromere since only 10% of the asci have four ascospores (arrow in S×T). (B) Schematic representation of P. anserina FDS and SDS asci with Sk. FDS: no crossover between the Sk locus and the centromere results in First Division Segregation of Sk, triggering death of the two ascospores lacking the Sk locus. SDS: a crossover between the Sk locus and the centromere results in Second Division Segregation of Sk, generating four surviving heterokaryotic ascospores. Proportion of FDS and SDS asci depends upon the frequency of crossover and thus upon the genetic distance between Sk and the centromere. The 90% of 2-spored asci in S×T cross (A) is indicative of a close linkage of the Sk with the centromere.