A STUDY OF rDNA MAGNIFICATION PHENOMENON IN A REPAIR-RECOMBINATION DEFICIENT MUTANT OF DROSOPHILA MELANOGASTER

Abstract

The rDNA magnification process consists of a rapid and inheritable rDNA increase occurring in bobbed males: in a few generations the bb loci acquire the wild-type rDNA value and reach a bb+ phenotype.—We have analyzed the rDNA magnification process in the repair-recombination-deficient mutant mei9a, both at the phenotypical and rDNA content levels. In mei9a bb double mutants the recovery of bb+ phenotype is strongly disturbed and the rDNA redundancy value fails to reach the wild-type level. The strong effect of this meiotic mutation on rDNA magnification suggests a close relationship between this phenomenon and the repair-recombination processes.

Mutant alleles of the meiotic locus,mei-9, differ in degree of effects on rod chromosome magnification and ring chromosome transmission inDrosophila

“…In summary, our results confirm the observation by Polito et al (1982) that mei-9 a inhibits magnification and demonstrate that mei-9" has a similar, although less severe inhibitory effect. They show that two reduction-of-function alleles at a given locus may not have the same effect on a given process, in this case, magnification.…”

“…Hawley & Tartof (1983) and Hawley et al (1985) further observed that mei-9", a repair replication-defective mutation (Nguyen & Boyd, 1977), had little or no effect on magnification. This observation was in apparent conflict with a previous report that mei-9 a inhibits magnification (Polito et al 1982).…”

“…Our results show that there is a difference between the two mei-9 alleles with respect both to rod chromosome magnification and ring chromsome transmission under magnifying conditions: mei-9" strongly inhibits both rod X*" chromosome magnification and ring X"" chromosome transmission under magnifying conditions, while mei-9" has a smaller inhibitory effect on rod chromosome magnification and on the transmission of ring chromosomes under magnifying conditions. Our results confirm the report by Polito et al (1982) that mei-9 a inhibits magnification and demonstrate that mei-9" has a small, but detectable, inhibitory effect on magnification. The observation that mei-9", which has been characterized as normal in post-replication repair but defective in excision repair (Boyd, Golino & Setlow, 19766) and repair replication (Nguyen & Boyd, 1977), and mei-9", which is defective in repair replication (Nguyen & Boyd, 1977), inhibit magnifying events suggests that a component of the excision repair pathway is involved in rDNA magnification.…”

“…The observation that one allele of mei-9 has a marked effect on magnification, while a second allele has a much smaller effect, explains the conflicting conclusions regarding the role of mei-9 in magnification reached by investigators who examined only one mutant allele (Polito et al 1982;Hawley & Tartof, 1983;Hawley et al 1985). Polito et al (1982) observed that approximately 50 % of mei-9 a males were bb after eight magnifying generations, when the process of magnification is normally complete. This indicated a strong inhibitory effect of the meiotic mutation on magnification.…”