1Grüneberg ( 1966b, 1966c, 1967a, 1967b) has raised a number of objections to this hypothesis and his arguments, as well as Lyon's ( 1968) retorts, are concisely summarized and discussed by Eicher ( 1970b).
2It should be noted, however, that Cattanach et al. ( 1972) observed that there was a tendency for allophenics to display "phenotypes that were at the extremes or possibly beyond the range" of those found in the equivalent heterozygotes. Indeed, a detailed analysis of the variability displayed by allophenics and X-inactivated mosaics by Falconer and Avery ( 1978) has confirmed this observation. These investigators found that although allophenics and mosaics are alike in appearance, there is a significant difference in the percentage of marker cells likely to be found in their coats. Whereas mosaics display a binomial (bell-shaped) distribution when the percentage of animals is plotted against the percentage of marker cells in the coat, allophenics show a flat distribution, with all percentages of marker cells being equally likely, including pure classes. Falconer and Avery believe the greater variability in allophenic patterns is caused by two sampling events which occur when cellular heterogeneity is present in allophenics but not in mosaics. These are (1) the separation of the inner-cell mass from the trophectoderm and (2) the division of the inner cell mass into primary ectoderm and primary endoderm. This paper also includes an ingenious mathematical analysis of the variation in allophenic pigment patterns, a treatment which although based on certain assumptions, some of which remain to be confirmed, nevertheless leads to results in accord with Mintz's hypothesis of 34 progenitor cells. (see J. Embryol. Exp. Morphol. 43:195-219, 1978).
3At least one other X-linked deviant has been reported which also could represent a mutation to Mo. It occurred at Oak Ridge and is known as 26K ( Welshons and L. Russell, 1959; L. Russell, 1960).
4When sections of skin from 7-day-old heterozygous females were examined microscopically they revealed that all the hair follicles possessed either pigment cells or amelanotic melanocytes. The follicles which contained pigment displayed a range of variation from completely normal amounts of melanin, to almost none. This histological picture therefore paralleled the gross appearance of the coat.
5Tabby (Ta) is an X-linked mutation which when homozygous or hemizygous eliminates both guard hairs and zigzags from the coat. The fibers which are present have been regarded as awls by Falconer ( 1953) though many of them are atypical ( Grüneberg, 1969). Ta/Ta females and Ta/Y males also possess a bald patch behind each ear, bald tail with a few kinks near the tip, reduced aperture of the eyelids, a respiratory disorder, and, in combination with agouti, a modified agouti pattern. Ta/+;A/ females display transverse dark stripes. These darks stripes, which according to Mintz ( 1970, 1971a) are the result of hair follicle phenoclones, are deficient in zigzags and are composed mainly of normal and abnormal awls which lack the yellow band. The mutation was described originally by Falconer ( 1953) who demonstrated that it was situated about 4 cM from both Mo and Mobr. Tabby has been described in detail by Grüneberg ( 1966b, 1966c, 1969, 1971a, 1971b) and his colleagues ( 1972), by Kindred ( 1967) and by McLaren et al. ( 1973).
|For the Ta allele:|
|Ta Allele (MGI)||Gene (MGI)||All Alleles (MGI)|
6D. Hunt and D. Johnson ( 1972a) have shown that the neurological involvement of Mobr is associated with significant differences in brain tyrosine and tryptophan levels which cannot be accounted for by the observed changes in the systems of synthesis and degradation of these amino acids. They suggest that the accumulation of tryptophan may be a contributory factor in the lethality of Mobr since elevated levels of brain tyrosine but not of tryptophan occur in viable-brindled ( Movbr) males ( D. Hunt and D. Johnson, 1972b). These investigators also have provided evidence that the neurological impairment of both Mobr and Movbr males is associated with decreased central norepinephrine levels (D. Hunt and D. Johnson, 1972a, 1972b).
7Falconer and Isaacson ( 1972) also observed that brindled males which originated from mothers of the light line were not as viable as those from mothers of the dark line and suggest that autosomal modifiers are the cause of this difference. They state that "these autosomal differences might be in the viability of the Mobr males themselves, or in a maternal effect." Styrna ( 1975) too found that male mutants from the Ms dark line were clearly more viable than those from the light line. This difference was, however, obscured when Ms/+ mothers were replaced by +/+ foster mothers from either the light or dark line. It thus appears, at least in this latter situation, that a maternal effect(s) does not play an important role in the survival of mutant males (see note 18).
8There is no significant correlation between the color of the mother's coat and that of the offspring, and attempts to select for color intensity have failed ( R.J.S. Phillips, 1961b).
9This effect does not occur in Mobr/+ ( Cattanach et al., 1972) or in Movbr/+ heterozygotes (Cattanach, personal communication) and, as already noted, the major influence of Mobr and Movbr seems to be in the hair follicle.
10Although the basis for the variegated pattern produced by ep remains to be determined there is some evidence which suggests that, although autosomal, it may be due to a mechanism similar to that responsible for X-linked variegation. Thus not only does white spotting have a similar influence on the ep/ep genotype as it has on dappled and on the X-linked tortoiseshell pattern of the domestic cat, but in both situations there is considerable variation in the amount or normal and mutant hair (for further information, including possible mechanisms, see Searle, 1968a).
11Phillips notes that death at this stage of gestation is unusual in the mouse as most prenatal deaths occur prior to the fifteenth day of development.
12In 1972 Eicher reported a new sex-linked mutation affecting coat color and hair which she called "silver-grey." This mutation resembles blotchy and could be a remutation to this allele.
13According to Grahn et al. ( 1969b), Moblo/Mobr is sublethal postnatally and Moblo/Modp is essentially a postnatal lethal.
14About half of To/Y males die before implantation ( Grahn et al., 1969b).
15One Moblo/To female survived for 104 days. A faint mosaic pattern was visible in the light gray coat (Grahn et al., 1969b, 1971).
16Rowe and his colleagues ( 1977) recently reported that lysyl oxidase activity is markedly reduced in Movbr/Y and Moblo/Y skin and that its activity in Moblo/+ skin is intermediate between Moblo/Y and normal (C3H) skin. On the other hand, the lysyl oxidase activity in Mobr/Y skin is only moderately reduced from that of C3H skin and the activity in Mobr/+ skin is normal. These findings correlate with the fact that whereas viable-brindled and blotchy males have a connective tissue defect which results in weakness of skin and blood vessels, brindled males succumb to a neurological disorder rather than to connective tissue weakness.
17In contrast, in vitro assays of dopamine-beta-hydroxylase in the presence of exogenous copper resulted in increased activity, suggesting that, in vivo, there is an increased synthesis of enzyme protein but a reduced availability of copper cofactor ( D. Hunt, 1974b)
18The main defect in mosaic (Ms) ( Section J) males may likewise be related to a deficient absorption of copper from the intestines. Styrna ( 1976, 1977b) found a higher than normal content of copper in the intestine and in the feces, and a lowered content in the brain and liver of Ms/Y males. He also found the mutants to possess a lower level of ceruloplasmin, which is one of the main cuproproteins in the body, and to have reduced levels of o-diphenol oxidase, an enzyme involved in the synthesis of melanin which requires the presence of copper ions for its activity. Stryna likewise found higher amounts of cysteine in the brains and liver of mosaic males. Furthermore, he has reported that the life of these males can be prolonged if beginning at 4 days of age they are innoculated with an aqueous solution of CuCl2. Following this treatment some darkening of the coat occurs too ( Styrna, 1977a). Finally it should be noted that Styrna ( 1977b) found that the mammary glands of Ms/+ females of the light line (see Section C, 4) contained less copper than the mammary glands of Ms/+ females of the dark line and of +/+ females. This is especially interesting since it could explain the maternal effect noted previously (see note 7). Thus the milk of Ms/+ mothers of the light line may supply less copper than Ms/+ females of the dark line.
19Because these symptoms, namely, light pigmentation of hair and neurologic disorders (characteristic bending of extremities on lifting Ms/Y males by the tail), suggested that they might result from changes in the metabolism or aromatic amino acids which are precursors of melanins and of cerebral amines necessary for the normal function of the central nervous system, Klein and his colleagues ( A. Klein and Sitarz, 1971; A. Klein and Styrna, 1971) analyzed the occurrence of free amino acids in the blood and urine of these males. They found that the blood of mutants contained no tryptophan, and smaller amounts of tyrosine and phenylalanine than normals. In the urine no differences in the composition of aromatic amino acids were noted but (particularly in adult Ms males) the levels of glycine were nearly twice as high as in normal males. Nevertheless, since there are a number of nonlethal genes which exert an influence on the content and levels of amino acids excreted in the urine ( Hrubant, 1963), Styrna ( 1977b) believes these observed changes are probably secondary effects of the mutation.