Chapter 2

The Agouti and Extension series of Alleles, Umbrous and Sable

III. "Umbrous" and "Sable" Determinants

Although we have already considered sombre ( E^so) and tobacco darkening ( E^tob), alleles of the extension locus which cause darkening of the coat, there are a number of other unrelated determinants which also produce similar effects. Thus, as mentioned previously, yellow ( A^y/—;B/—) mice are often characterized, especially in outbred populations, by variable degrees of sootiness. In some animals the sootiness is confined to a mid-dorsal streak, in others this streak is wider, covering the entire back and sometimes the flanks, so that only the belly is phenotypically "yellow." This situation is due to the admixture of hairs possessing significant amounts of eumelanin to the yellow fur. If these hairs are relatively few, the animal appears to be dingy yellow; with increasing numbers, however, the dorsum becomes darker until, at the extreme, an animal with a dark sable back and yellow belly is produced. ³⁹ As in the case of E^so and E^tob, these modifications are not limited to animals of yellow genotype as the amount of phaeomelanin may also be drastically reduced in black agouti ( A/—;B/—) and even in nonagouti animals. This dark form of agouti has been termed "umbrous" ⁴⁰ ( Barrows, 1934; Mather and North, 1940).

A. Polygenic Factors

The initial studies on the inheritance of factors which darken the coat color of mice were conducted more than 60 years ago. In 1916 Little attempted to alter the agouti pattern by crossing the darkest heterozygous agouti animals selected from a "sooty yellow" stock to nonagouti mice from the same stock. Both the agouti and nonagouti animals became darker as the selected crossing progressed ( Little, 1916). These findings indicated that the darkly pigmented phenotypes resulted from the cumulative action of a number of genes and that high concentrations of these genes could even make nonagouti mice look darker. This was subsequently substantiated by Dunn ( 1920a) who demonstrated that the responsible factors were inherited independently of the agouti series. Dunn ( 1920a) also firmly established that both the sable and umbrous phenotypes had an essentially similar hereditary basis. If the genes responsible for producing the sable phenotype in A^y/— mice were incorporated into A/— animals, their backs were almost black, and agouti hairs with yellow tips were confined to the flanks and belly. Dunn's investigations also revealed that the expression of both umbrous and sable varied between wide limits and that the darker grades produced the more variable offspring. This indicated that the factors responsible for the darkening were either largely dominant in their effects or very strongly cumulative ( Robinson, 1959). ⁴¹

Barrows ( 1934) too studied the umbrous phenotype. Unlike Dunn, he concluded that it depended upon the presence of a in the heterozygous condition along with one or more dominant or semidominant modifying "umbrous genes" whose effect was to make nonagouti partially dominant to agouti.

B. Mendelian Mutations

1. Umbrous (U)

Although, as the work above testifies, sable and umbrous phenotypes often result from the cumulative effect of an unknown number of genes, as already noted ( Section II) they can also be produced by single Mendelian factors. The first of these was described by Mather and North( 1940) and was appropriately designated "umbrous" ( U). Umbrous (linkage not known) is a semidominant which appears to interact with a.

Contrary to usual belief, A is not invariably fully dominant to a, even in the absence of U; the A/a;u/u phenotype displays a slight darkening along the medial line as compared with A/A;u/u. The difference is difficult to perceive in living mice, but becomes more obvious from comparisons of skins, although some overlapping occurs ( Mather and North, 1940). This slight darkening effect which a has when heterozygous with A is further augmented by the presence of U. Thus as a consequence of the interaction of U and a the following six agoutis may be ranked for increasing umbrous:
A/A;u/u < A/a;u/u = A/A;U/u < A/A;U/U = A/a;U/u < A/a;U/U.

While the basis for this interaction is not known, Mather and North ( 1940), like Barrows ( 1934), stress that U may modify the dominance of agouti over nonagouti. In fact a significant part of their paper is devoted to discussing this possibility with regard to various theories of dominance. Although they may be correct, the alternative and simpler explanation, that a is itself an "umbrous gene" acting in concert with U, should not be overlooked. This possibility is attractive especially since, as noted above, even in the absence of U, A/a mice may be slightly darker than A/A animals.

Mather and North ( 1940; Grüneberg, 1952) noted that a sable color of low intensity occurred in A^y/a;U/U mice, but reported that umbrous had no effect upon nonagouti individuals. This, however, is not the case as a/a;U/U animals are significantly darker than a/a;u/u mice with a phenotype bearing a close resemblance to extreme nonagouti ( a^e/a^e) and sombre ( E^so/—) ( Poole and Silvers, unpublished). Indeed, the difference between a/a;U/U and a/a;u/u animals is so striking that it is difficult to understand how it was overlooked.

More recently Robinson ( 1959) described a similar "umbrous" gene segregating in a stock of fancy origin but no tests were made to determine whether this mutation was the same as, or allelic with, Mather and North's. This is unfortunate since it is not known whether Robinson's failure to detect any obvious differences between umbrous animals of A^y/a^t versus A^y/A^w or A^w/a^t versus A^w/A^w genotypes was due to the fact that his mutant was different from the one previously described, or to the fact that umbrous may not interact with a^t the way it does with a, or to the fact that prepared skins were not critically matched.

2. Mahogany (mg)

In 1960 Lane and M.C. Green described a recessive umbrous gene which they named mahogany ( mg). The mutation appeared in agouti mice of unknown origin and the animals fit the descriptions of umbrous as given by Barrows and by Mather and North. "the central dorsal hairs were considerably darker than those of normal agouti mice, the yellow ticking characteristic of agouti being considerably reduced. The darkening was present but less intense in the lateral hairs. The ventral hairs were almost solid grey with no yellow ticking. In addition the ears and tail appeared more deeply pigmented than those of normal wild-type mice" ( Lane and M.C. Green, 1960). Mahogany proved to be detectable also in nonagouti mice; a/a;mg/mg mice are coal black with no lightly pigmented hairs evident behind the ears or around the perineum. The tail, feet, and ears are also darker than in normal nonagouti animals ( Lane and M.C. Green, 1960). In fact, in general appearance nonagouti mahogany mice fit the description of the nonagouti mice selected for dark modifiers by Little ( 1916), mice homozygous for extreme nonagouti, and nonagouti mice homozygous for U. Mahogany proved to be on chromosome 2, about 12 cM from agouti. ⁴²

3. Mahoganoid (md), Nonagouti Curly (nc) and Dark (da)

Three other umbrous mutations have been reported and briefly described. Mahoganoid ( md), another recessive (chromosome 16), ⁴³ occurred in the C3H/HeJ strain and is identical to mahogany ( mg) on both agouti and nonagouti backgrounds ( Lane, 1960a). Nonagouti curly ( nc), which is probably an allele of md. ⁴⁴ is likewise recessive and was recognized in an F₂ animal from a mutagenesis experiment using caffeine. The deviant was nonagouti with black pinna hairs and curly whiskers. nc, like E^so, is epistatic to A^w so that nc/nc;A^w/A^w mice look like a^e/a^e animals but have curly whiskers and a slightly plush coat (R.J.S. Phillips, personal communication). ⁴⁵ nc homozygotes also have a reduced viability (R.J.S. Phillips, 1963, 1971).

Finally, dark (da) arose in the CBA/Fa strain. Animals homozygous for this recessive mutation are smaller than normal with reduced fertility. When combined with A or A^y, dark produces a phenotype in which the yellow pigment on the back is replaced by black so that both look nonagouti except on the flanks. The darkening of the back of A/—da/da decreases as the animal becomes older. This umbrous gene, which was reported by Falconer ( 1956a), has been assigned to chromosome 7. Unfortunately, it may be extinct.

C. "Reds"

Before concluding this section it should be mentioned that there are certain yellows of the fancy which are known under the name of "reds" ( Grüneberg, 1952). Such animals are genetically brown ( A^y/—;b/b) and the intensity of their "red" coat is probably dependent on intensifiers, analogous to, but distinct from, the "umbrous" genes ( Dunn, 1916). A detailed analysis of this phenotype has not yet been made.