Mutagenesis and chromosome structure

Abstract

The spectra of forward-mutations occurring spontaneously (SP) or induced by X rays (X), nitrous acid (NA), or 2-aminopurine (AP) at the ad-3A and ad-3B loci of Neurospora crassa have been compared by means of tests for allelic complementation and leakiness. Such tests showed a marked correlation between the percentages of leaky mutants and mutagenic origin. High percentages were found among the NA and AP samples and low percentages among the X and SP samples. The same trend was found with regard to allelic complementation among the ad-3B mutants. A set of testers derived from a 17-complon map of the ad-3B cistron also showed that complementing NA and AP mutants have predominantly nonpolarized complementation patterns with an average complon coverage of 4.7, whereas complementing X and SP mutants have predominantly polarized patterns with an average complon coverage of 13.6. These data provide striking evidence for mutagen specificity in Neurospora and suggest, furthermore, that there is a correlation between the type of genetic alteration and the type of complementation pattern.