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Ruby-eye ( ru; chromosome 19) was found by Dunn ( 1945) in a silver piebald stock. The mutation is easily recognized at birth, the eyes of the newborn lacking the iris pigment ring of the wild type and resembling the eyes of the mutants pink-eyed dilute, pallid, and albino. The eyes darken during the first week after birth and when opened are a dark ruby color.
This mutation also influences the pigmentation of the coat. Ruby-eyed, black ( a/a;B/B;ru/ru) mice are a dull dark sepia and ruby-eyed black agouti ( A/A;B/B;ru/ru) mice superficially resemble brown agouti animals since the black base of the fur is reduced to dark slate, while the yellow tip is only slightly reduced in intensity ( Dunn, 1945). Ruby-eye does not have as drastic an effect on reducing the intensity of black pigment as does pink-eyed dilution ( pp) but has a somewhat greater effect than p/p in reducing the intensity of phaeomelanin. When p/p and ru/ru are incorporated into the same stock, the phenotype more closely resembles that of p/p ( Dunn, 1945).
The most distinctive feature of the pigment granules of the normal retina is their great diversity of size and shape ( Figure 4-8). Their shapes vary from spherical, through all gradations, to thin elongated rods with the largest granules being at least 500 times the volume of the smallest granules ( Markert and Silvers, 1956). While the retinal granules of ru/ru mice also vary in size, their average size, as well as their range of diversity in morphology, is significantly reduced and there are not as many of them. Moreover, their color in animals of black genotype ( a/a;B/B;ru/ru) is only slightly darker than typical brown granules ( Markert and Silvers, 1956).
The granules found in the melanocytes of the harderian gland of ruby-eyed black mice are also dark brown and their morphology is altered here too. Thus although the sizes and shapes of these melanocyte granules, unlike those of the retina, are more or less uniform, the spheroidal granules of a/a;B/B;ru/ru melanocytes resemble those of brown melanocytes more than they resemble the oval or long-oval-like granules of "normal" black pigment cell ( Markert and Silvers, 1956).
Ultrastructural studies of ru/ru pigment granules have disclosed that in spite of the fact that those of a/a;B/B;ru/ru mice resemble the granules of a/a;b/b animals, the quality of pigment deposited on the granule nevertheless is determined by the b-locus ( Moyer, 1966 (see Figure 3-1)). In fact, according to Moyer the only unique feature of ru/ru retinal melanosomes is that their development is delayed and they do not become melanized until after birth. 20
With the limited amount of evidence available, it is difficult to pinpoint the specific action of the ru gene. Since it does not appear to alter significantly the ontogeny or fine structure of the melanosome, its only major influence seems to be in delaying the onset of granule synthesis ( Moyer, 1966). It is conceivable that during the course of this delay, the concentrations of substrates or cofactors required for protein synthesis are altered in such a way as to influence the number and size of the granules produced ( Moyer, 1966). It is known that newborn ru/ru skin incorporates about 14% less tyrosine than +/+ skin ( Coleman, 1962), but this reduction in activity could well be due to the reduced number of granules in ruby-eyed animals than to any change in the structure of matrix protein ( Moyer, 1966). Indeed, as Moyer suggests perhaps the major effect of the ru-locus is to regulate the activation of loci that direct the synthesis of matrix protein 21 (see also Chapter 6, note 11).
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