-insensitive, Recombination-deficient Alleles in Drosophila

The procedure for the selection of a temperature-sensitive recombination mutant in Drosophila is described. Use of this procedure has led to the recovery of three alleles at a new recombination locus called rec-Z, located within the region of chromosome 3 circumscribed by Deficiency(3R)~bd'~~. One allele, rec-Z26, is temperature sensitive, and the other two alleles, rec-Z6 and rec-1I6, are temperature insensitive. Gene dosage studies reveal rec-lZ6 to be a leaky mutant with greater recombination activity in two doses than in one. The other two alleles show no dose response, implying that they may be null mutants. The temperature response curves of rec-1" as a homozygote and in heteroallelic combination with rec-Z " suggest that the sharp decrease in recombination between 28" and 31" indicates temperature denaturation of an enzyme or other protein specified by the mutant and associated with the recombination process. The ability of small changes in temperature to reverse or abolish polarity in recombination along the X chromosome arm in rec-ZZ6/rec-Zl6 females brings into question the use of the "polarity" criterion to partition mutants into two functional types, i.e., precondition mutants that display polarity and exchange mutants that do not. Evidence that rec-1 may be part of a complex locus residing in a chromosome segment harboring a variety of recombination-related genes is presented. EMPERATURE-sensitive mutants are valuable tools for localizing the time T of gene expression. They have been used to determine the time of action of a particular gene, even when the specific nature of the gene product is unknown (EDGAR and EPSTEIN 1965). By applying the restrictive temperature at sequential time points during development, in temperature shift experiments, the period during which wild-type activity is blocked identifies the active phase of the gene product. The alteration in the biological activity of the protein at the restrictive temperature has been shown to be the consequence in most cases of a single amino acid substitution in a polypeptide UOCKUSCH 1966). In earlier studies with Drosophila, oocytes were exposed to elevated temperature at sequential time points during their development to mark the senGenetics 108: 425-443 October, 1984.