Chapter 5

Grey-Lethal, Grizzled, Mocha, Pallid, Muted, Misty, and Pearl

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VII. Pearl (pe)

For the pe allele:
pe Allele (MGI) Gene (MGI) All Alleles (MGI)

A. Origin and Influence on Pigmentation

Pearl (pe) is a simple autosomal (chromosome 13) recessive which arose spontaneously in the C3H/He strain. The mutation was described originally by Sarvella ( 1954), what follows is based on her description. On the agouti black ( A/A;B/B) background on which pearl occurred the pigmentation of the entire hair is diluted with the base of the hair being affected to a disproportionately greater extent that the tip. The snout, ears, feet, and tail of pearl agouti mice are considerably lighter than normal and the yellowish mask about the eyes, characteristic of some C3H strains, is diluted to near-white. Pearl agouti mice can be distinguished from their typical agouti littermates at birth by the smaller amount of pigment in their eyes. This distinction, however, is not evident in adult animals.

Nonagouti black pearl ( a/a;B/—;pe/pe) mice bear a striking resemblance to nonagouti black ruby-eye ( a/a;B/—;ru/ru) animals. In fact the two phenotypes can be distinguished readily only by blowing the fur so that the much lighter base of the pearl hair is revealed. The coat color of a/a;b/b;pe/pe mice resembles that of a/a;b/b;cch/c heterozygotes. The effects of pe/pe and b/b on eye color are additive so that the eyes of adult brown pearl mice have a definite ruby cast. On a yellow background ( Ay/a;B/B) the effect of pearl depends on the amount of sootiness. When sootiness is present the coat color appears pale grey even though the bottom quarter of the hair is cream-colored. On the other hand, in the absence of sootiness, or when yellow is on a brown background ( Ay/a;b/b), the action of pearl is similar to that of chinchilla ( cch/cch), producing a cream-colored fur. As might be expected lethal yellow pearls show the same tendency toward obesity as "normal" lethal yellows ( Sarvella, 1954). 12

B. Stability

An interesting feature of pearl is its stability. While on certain backgrounds, such as its strain of origin, it is stable; on another background, strain 201 ( Ay/a;pe/pe), somatic reversions to full-color are frequent ( L.B. Russell and Major, 1956). Thus, L. Russell ( 1964) reported in this latter strain the frequency of reversion of pe/pe to full color was about 6% in the 800 or so animals observed. Moreover, the incidence was not affected by whether one bred from mosaic or nonmosaic mice, indicating that the degree of instability was characteristic of the stock as a whole rather than being transmitted in sublines ( L. Russell, 1964). While in most cases the intensely colored spots occupied from 0.1 to 1% of the coat and did not affect the germinal tissue, six animals of a total of 61 had from 50 to 100% of their coat full-colored, and one had a full-color spot covering about 5% of the surface ( L. Russell, 1964). Five of the animals in which the proportions of full-colored fur were about 5, 50, 80, 100, and 100%, produced respectively 18.2, 52.7. 43.6, 38.5 and 36.2% full-colored young ( L. Russell and Major, 1956). Since the two completely full-colored mice produced significantly less than 50% full-colored offspring, it was concluded that they, too, were most probably the result of somatic rather than germinal events.

Although the basis for this mottling is not known, L. Russell and Major ( 1956) believe that spontaneous somatic reverse mutations at the pe-locus is the most likely cause and that these can occur either very early or moderately late in embryonic development. If they occur early, mutant cells participate in the fur and in the gonad and if they occur late they usually involve only the fur. Animals mosaic in other tissues would, of course, not be detected ( L. Russell, 1964) 13 (see also Chapter 6, note 11).

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