The effects of the pink-eyed unstable gene on the retinal pigment epithelium of the mouse.

Pink-eyed unstable (pun), an autosomal gene in the mouse, causes variegation of the coat. In some melanocytes it functions as the normal allele p+, producing dark pigment, and in others as the mutant p, producing light pigment. As a study of another unstable gene at a different locus had shown that the instability was strongly influenced by the tissue environment, it seemed desirable to find out whether this also applied to pun. An examination of the retinal pigment epithelium, the only structure in mammals in which it is practicable to determine the position of individual melanocytes, showed that the distribution of dark and light cells in pun pun animals was not random. The dark cells increased in frequency with the distance from the optic nerve, suggesting that the tissue environment was a factor in the instability of the gene (i.e. in its rate of mutation), although the increase was less striking than in the other mutant. It is generally assumed that when pun behaves as p+ it is a case of reversion, and that reversion can also occur in germ cells, the revertant p+ allele subsequently behaving as a stable gene. It is here argued that it is unlikely to be a case of reversion, and that the evidence for the involvement of the germ line is inconclusive. Further, it is suggested that the phenotype of pun pun animals is probably an instance of Position Effect variegation, the instability resulting from some chromosomal alteration, which is too small to be cytologically detectable.