A strategy for fine-structure functional analysis of a 6- to 11-centimorgan region of mouse chromosome 7 by high-efficiency mutagenesis.

A refined functional map of a 6- to 11-centimorgan region surrounding the albino (c) locus in mouse chromosome 7 is being generated by N-ethyl-N-nitrosourea (EtNU) "saturation" mutagenesis of stem-cell spermatogonia. In the first phase of an experiment that will eventually test at least 3000 gametes, we screened 972 mutagenized gametes for the induction of both lethal and visible mutations with a two-cross breeding protocol. Thirteen mutations mapping within the limits of a segment corresponding to the cytologically visible Df(c Mod-2 sh-1)26DVT deletion were recovered. They represented three phenotypic groups: prenatal lethality (six mutations); a fitness/runting syndrome (three mutations, provisionally designated as fit variants); and a neurological/balance-defect abnormality (four mutations). Complementation analysis provided evidence for a true repeat mutation at the sh-1 (shaker-1) locus (for the neurological mutations) and another at the here defined fit-1 (fitness-1) locus. In addition, four complementation groups were defined by induced lethal mutations; the two other lethal mutations were each part of a cluster. The recovery of the repeat mutations suggests that the EtNU-induced mutation rate, estimated from specific-locus tests, should make it possible to achieve saturation mutagenesis of a chromosomal region. This experiment is providing basic logistical and statistical information on which to base strategies for expanding the functional map of larger segments of the mouse genome by experimental mutagenesis. It is also yielding additional mutations useful in dissecting the functional and molecular complexity of this segment of chromosome 7.