The Relation of Mitotic Recombination to Dna Replication in Yeast Pedigrees

ALLELIC recombinants occur among mitotically dividing diploid yeast cells at frequencies significantly lower than those commonly observed subsequent to meiosis (ROMAN 1956a). Consequently, information about such events derives mainly from studies employing selective techniques (reviewed by PRITCHARD 1963). Typically, prototrophic revertants among heteroallelic diploid cells are detected as clones on minimal medium, some 20 to 25 cell generations after the initial recombinational event. Since any auxotrophic segregants of the prototrophic cell are not recovered with this procedure, each genetic strand present at the time of reversion is not necessarily available for analysis. Thus the data obtained from diploid prototrophs are considerably less detailed than from meiotic tetrads. The present experiment represents part of a study designed to elucidate the genetic characteristics of mitotic intragenic recombination. This paper is concerned with the relationship of DNA replication to X-ray induced prototroph formation and outside marker recombination in synchronous, prereplicative cultures of Saccharomyces cerevisiae. The recovery of all of the viable cell products of each prototrophic cell is operationally analogous to tetrad isolation. It is found that both reciprocal and nonreciprocal products of recombination can originate at a two-strand stage in mitotic cells. Further, evidence is presented for the occurrence of events that are formally similar to those reported for phage heterozygotes (HERSHEY and CHASE 1951) and the postmeiotic segregants observed in yeast ( FOGEL and HURST, in preparation) and other fungi (KITANI, OLIVE and EL-ANI 1962; LISSOUBA, MOUSSEAU, RIZET and ROSSIGNOL 1962).