N-Ethyl-N-Nitrosourea Mutagenesis of a 6- to 11-cM Subregion of the Fah–Hbb Interval of Mouse Chromosome 7: Completed Testing of 4557 Gametes and Deletion Mapping and Complementation Analysis of 31 Mutations

An interval of mouse chromosome (Chr) 7 surrounding the albino (Tyr; c) locus, and corresponding to a long 6- to 11-cM Tyr deletion, has been the target of a large-scale mutagenesis screen with the chemical supermutagen N-ethyl-N-nitrosourea (ENU). A segment of Chr 7, from a mutagenized genome bred from ENU-treated males, was made hemizygous opposite the long deletion for recognition and recovery of new recessive mutations that map within the albino deletion complex. Over 6000 pedigrees were analyzed, and 4557 of these were completely tested for mutations specifying both lethal and gross visible phenotypes. Thirty-one nonclustered mutations were identified and assigned to 10 complementation groups by pairwise trans-complementation crosses. Deletion-mapping analyses, using the extensive series of radiation-induced Tyr deletions, placed the loci defined by each of these complementation groups into defined intervals of the Tyr-region deletion map, which facilitates the identification of each locus on physical and transcription maps of the region. These mutations identified seven new loci and provided new ENU-induced alleles at three previously defined loci. Interestingly, no mutations were recovered that recapitulated three phenotypes defined by analysis of homozygous or partially complementing albino deletions. On the basis of our experience with this screen, we discuss a number of issues (e.g., locus mutability, failure to saturate, number of gametes to screen, allelic series) of concern when application of chemical mutagenesis screens to megabase regions of the mouse genome is considered.