EFFECTS OF THE WHITE ALLELE OF THE MI-LOCUS ON COAT PIGMENTATION IN CHIMERIC MICE

To investigate the cellular action of the Mi(wh) allele in the mouse w ith regard to its effects upon coat color patterns, we generated a ser ies of aggregation chimeras, using embryos that differ in their mi loc us genotype. We have obtained 11 chimeras Mi(wh)/+ 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 analys is of GPI-1 isoenzymes. In Mi(wh)/+ C/C <-> +/+ c/c mice white coat co lor prevailed due to the higher percentage of unpigmented areas and th e higher percentage of unpigmented hairs in pigmented areas. Our data indicate that a single Mi(wh) gene dose decreases the melanoblast prol iferative activity, causing the lightening of coat pigmentation. In Mi (wh)/+ C/C <-> +/+ c/c mice a few pigmented hairs were often detected on the belly where Mi(wh) / + mice always had a white spot. This sugge sts that in the chimeras the presence of some non-Mi(wh) cells in the skin of the belly allows pigment cells to develop. Using embryos of tw o substrains of Mi(wh)/Mi(wh) mice that differ in their Gpi-1 locus ge notype we have produced 8 Mi(wh)/+ <-> Mi(wh)/Mi(wh) chimeras. In all these chimeras coat color patterns resembled those of Mi(wh)/+ heteroz ygotes despite the higher percentage of the Mi(wh)/Mi(wh) component in three chimeras. Mosaic hairs were absent in the chimeras. This shows that the chimeras have only one Mi(wh)/+ melanoblast population which actively proliferates and colonizes almost all hair follicles. Thus th e Mi)(wh)/Mi(wh) dermis and epidermis do not suppress proliferation an d differentiation of the Mi(wh)/+ melanoblasts except the certain area on the belly.