Epigenetic Control of a Transposon-Inactivated Gene in Neurospora is Dependent on DNA Methylation

An unstable allele of the Neurospora am (GDH) gene resulting from integration of the retrotransposon Tad3-2 into 5′ noncoding sequences was found in previous work. We report that reversion to Am+ depends on DNA methylation within and upstream of Tad. Levels of methylation were correlated with the proportion of Am+ conidia, whether the cultures were derived from Am− or Am+ isolates. Reversion to Am+ did not occur when conidia were plated on 5-azacytidine, which reduces DNA methylation. The mutation dim-2, which appears to abolish DNA methylation, also prevented reversion to Am+. The native am allele, in a strain that lacked Tad elements, was replaced with am:: Tad3-2 or with a deletion derivative that prevents transposition of Tad. Transformants of both classes showed instability comparable with that of the original isolates, which contain multiple Tad elements. Deletion of the upstream enhancer-like sequences, URSamα and β, did not prevent the instability of am:: Tad3-2. The results suggest that am expression is dependent on DNA methylation but not on proliferation or transposition of the Tad element and that the instability does not require the upstream sequences of am.

COUPLED INSTABILITY OF TWO X-LINKED GENES IN DROSOPHILA MAURITIANA: GERMINAL AND SOMATIC MUTABILITY

ABSTRACT A highly unstable allele has been isolated at the white locus of Drosophila mauritiana, a sibling species of D. melanogaster. This allele, white-peach (wpch), mutates spontaneously in males and females to give both wild-type and bleached-white derivatives. The mutation frequency is about 10-3 mutations/generation. There is no evidence for clustering among mutant progeny, and phenotypically wpch flies with mosaic patches of wild-type tissue in the eyes are frequently recovered. Another X-linked locus, plum, is destabilized when wpch is on the same X chromosome.

AN INCREASE IN THE X-LINKED LETHAL MUTATION RATE ASSOCIATED WITH AN UNSTABLE LOCUS IN DROSOPHILA MELANOGASTER

ABSTRACT The behavior of an unstable allele of the singed-bristle locus on the X chromosome was studied in connection with the occurrence of lethal mutations on that same chromosome. The unstable allele, weak singed (snw), is under the control of the P-M system of hybrid dysgenesis and, in the M cytotype, mutates secondarily to extreme singed (sne) and to wild type (sn +) at high rates. Chromosomes whose snw allele had mutated in this fashion sustained lethal mutations at a rate of 3%; whereas, those whose snw allele had apparently remained unchanged, acquired lethals at a lower rate, 1.3%. The significant difference between these values indicates a statistical coincidence between the phenomena of snw instability and X-linked lethal mutation induction. This coincidence can be explained by postulating that mutations at the singed locus sometimes release a genetic element capable of reinserting elsewhere in the chromosome. Alternately, snw instability and lethal induction might be associated because they are the effects of a common cause, perhaps some mutation-inducing substance present in various amounts in the germ cells of dysgenic flies.—The lethals that occurred on chromosomes whose snw allele had mutated to sne mapped preferentially close to singed. The lethals on the snw and sn + chromosomes did not show this concentration on the map. Cytological analysis of samples of all three types of lethal chromosomes indicated that, with one exception, there was no detectable breakage at the singed locus itself. The single instance of breakage at singed was not associated with any change in the singed phenotype. Thus, the instability of snw apparently does not involve detectable breakage of the singed locus, or if it does, this breakage is not a common event.