Mi-spotted: a mutation in the mouse

Abstract

Mi -spotted ( mi sp ) is a new mutation in the mouse at the microphthalmia ( mi ) locus. It has no obvious visible effect in either the heterozygote ( mi sp /+) or homozygote ( mi sp / mi sp ) and was discovered by virtue of its interaction with white ( mi wh ). Mice of genotype mi wh / mi sp are pale yellow with white spots; yellow areas later become ‘sooty’ at the first moult. Though mice of genotypes a / a ; mi sp /+ and a / a ; mi sp / mi sp cannot be visibly distinguished from those of a / a ; +/+, the amount of tyrosine-2-C 14 incorporated in melanin in skins of 5-day-old mice of these genotypes differed. Assays of tyrosine incorporation were extended to include other non-agouti genotypes differing only at the microphthalmia locus. The amount of tyrosine incorporated was greatest in +/+ followed in order by mi sp /+, mi sp /+, mi sp / mi sp , mi wh /+, mi wh / mi sp , and mi wh / mi wh . Pigment granules were examined in club hairs of these same genotypes for different regions of the hair shaft. Hairs of mi sp /+, mi sp / mi sp , and mi /+ could not be distinguished from +/+ either in number, kind or arrangement of granules. Hairs of mi wh /+ showed reduced cortical and medullary pigment, especially distally, which was even more pronounced in hairs of mi wh / mi sp . Medullary granules of mi wh / mi sp varied greatly in size with a few large yellow-brown granules.