Spontaneous reversion of the white-ivory mutant of Drosophila melanogaster.

EVERAL studies have shown that certain mutants revert more frequently when homozygous than when hemizygous, a phenomenon termed “selfing” by DEMEREC (1 962). Reversion of serial tandem duplications by asymmetric pairing and crossing over has been demonstrated for several mutants following the classical study of Bar by STURTEVANT (1925). For Salmonella typhimurium, DEMEREC (1 962, 1963) has reported an increased reversion rate for certain mutants when infected with homologous transducing phage. His first explanation ( DEMEREC 1962) invoked unequal crossing over. Further experimentation led him to suggest (DEMEREC 1963) that the phenomenon is the result of a position effect, i.e., that the presence of the exogenous genetic material induces an undefined instability within the gene. In a study of several biochemical mutants of Saccharomyces cereuisiae, MAGNI and VON BORSTEL (1962) found that meiotic reversion rates are several times higher than the corresponding mitotic rates in the same homozygous diploid strains. The reversion event is correlated with exchange of outside markers (MAGNI 1963). More recent experiments have shown that the same properties apply to forward mutation but not to the reversion of base-analogue induced changes (MAGNI 1964). The interpretation is that a shift in the reading frame caused by base deletion or insertion may be corrected by unequal crossing over as postulated by CRICK, BARNETT, BRENNER and WATTS-TOBIN ( 1961). A similar phenomenon in Neurospora crassa has been reported by BAUSUM and WAGNER (1965). The ivory ( wz) mutant occurred spontaneously and was recovered as a cluster of males among the progeny of a single-pair mating (MULLER 1920). It behaves phenotypically as an allelomorph of other white mutants and does not show dosage compensation. GREEN (1959a) found that with the exception of wz the noncompensating alleles of white (w) are enhanced by enhancer of white-eosin and are dominant suppressors of the zeste mutant. The site of w$ is to the left of w (MACKENDRICK 1953; LEWIS 1959), a location shared only with compensating mutants (GREEN 1959a). Recombination is less frequent between wz and either w or white-cherry (tuch) than between white-apricot (w“) and the same two mutants (LEWIS 1959). No crossing over between wz and U@ has been observed. ’ This work was completed while the author was a predcctoral trainee under Public Health Service Genetics Research