Effects of the white allele of the mi locus on coat pigmentation in chimeric mice.

To investigate the cellular action of the Miwh allele in the mouse with regard to its effects upon coat color patterns, we generated a series of aggregation chimeras, using embryos that differ in their mi locus genotype. We have obtained 11 chimeras Miwh/+C/C<==>+/+ c/c and 8 chimeras +/+ C/C<==>+/+ c/c. Chimerism was determined by coat and retinal pigment epithelium mosaicism and by the electrophoretic analysis of GPI-1 isoenzymes. In Miwh/+ C/C<==>+/+ c/c mice white coat color prevailed due to the higher percentage of unpigmented areas and the higher percentage of unpigmented hairs in pigmented areas. Our data indicate that a single Miwh gene dose decreases the melanoblast proliferative activity, causing the lightening of coat pigmentation. In Miwh/+ C/C<==>+/+ c/c mice a few pigmented hairs were often detected on the belly where Miwh/+ mice always had a white spot. This suggests that in the chimeras the presence of some non-Miwh cells in the skin of the belly allows pigment cells to develop. Using embryos of two substrains of Miwh/Miwh mice that differ in their Gpi-1 locus genotype we have produced 8 Miwh/+<==>Miwh/Miwh chimeras. In all these chimeras coat color patterns resembled those of Miwh/+ heterozygotes despite the higher percentage of the Miwh/Miwh component in three chimeras. Mosaic hairs were absent in the chimeras. This shows that the chimeras have only one Miwh/+ melanoblast population which actively proliferates and colonizes almost all hair follicles. Thus the Miwh/Miwh dermis and epidermis do not suppress proliferation and differentiation of the Miwh/+ melanoblasts except the certain area on the belly.