Chapter 5

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

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III. Mocha (mh)

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

Mocha (mh), like grizzled, with which it is very closely linked on chromosome 10 6, also dilutes yellow pigment but affects eumelanin as well ( Plate 2-G). The mutation occurred in 1963 in the C57BL/6J- pi (pirouette) stock and was named as a result of the phenotype it produced on this nonagouti background ( Lane and Deol, 1974). Mocha, which is a recessive, can be recognized at birth by the absence of visible eye pigment. The mutants are generally smaller than their normal littermates and some do not survive the first few days of life — a situation akin to that in grizzled.

A. Influence of Pigmentation

On a nonagouti background mocha ( a/a;mh/mh) mice are slightly lighter than the mutants ruby-eye ( a/a;ru/ru) and white ( a/a;Miwh/+) and a little darker than pallid ( a/a;pa/pa). Moreover, because phaeomelanin as well as eumelanin is diluted, the hair in the pinna of the ear and around the genitalia appears to be white instead of yellow. The eyes of adult mocha mice are deep red ( Lane and Deol, 1974).

When whole mounts of a/a;mh/mh and a/a;mh/+ hairs were examined microscopically and compared, those of the mutant-type contained fewer and smaller eumelanin granules, particularly in the medullary cells ( Lane and Deol, 1974).

B. Influence on Behavior

Mocha also affects behavior. According to Lane and Deol the abnormal behavior of mocha mice is similar in some respects to pallid ( pa/pa) animals ( Section IV). At weaning some mocha mice hold their head tilted to one side and some of these cannot swim on the surface. All mocha mice seem to be more active or nervous than their normal littermates and when disturbed they react quickly and violently, jumping and scampering wildly around their cage. When those mice which do not tilt their head are picked up by the tail they tend to jerk constantly, waving their front legs rapidly while quickly flexing and extending or kicking their hind legs. When animals which do tilt their head are picked up they tend to twist their body as well as constantly jerk and kick.

While there is evidence that mocha mice can hear at weaning some, and perhaps all, eventually become deaf. Mocha mice are poor breeders; most males will not breed and the few females which produce litters usually destroy them.

C. Inner Ear Abnormalities and their Significance

Studies of the inner ear of mocha mice revealed the cochlea to be abnormal. Degenerative changes were observed in the organ of Corti, the stria vascularis, and the spiral ganglion. These changes were mild in young animals but became progressively more severe so that "in animals over 100 days old there was generally a heavy loss of hair cells in the organ of Corti, and the various types of supporting cells, although present, had largely lost their characteristic form and organization on which their identification normally depends. The tectorial membrane was frequently curled back. There was a severe loss of cells in the spiral ganglion. The stria vascularis was nearly always reduced" ( Lane and Deol, 1974). Abnormalities of the otoliths in both the saccule and utricle also were observed in most of the mutants.

The causal connection between these ear abnormalities and the pigmentary changes is not evident. While all of the features of the "mocha syndrome" have been separately reported in other animals, in no other single mutant do they occur together. Thus, although the coat-color mutants pallid ( Section IV) and muted ( Section V) also possess abnormal otoliths and share some behavioral traits with mocha, cochlear abnormalities do not occur in these animals. On the other hand, such cochlear abnormalities do occur in the neurological mutants of the shaker-waltzer-deaf type but only one of these, varitint-waddler ( Va) (see Chapter 11, Section IV), affects pigment (Deol, 1954, 1968). The cochlea is known to be severely affected by a number of genes associated with white spotting, e.g., piebald lethal ( sl), microphthalmia ( mi), white ( Miwh) ( Deol, 1970b), some of which also cause general dilution, but the abnormalities produced by these genes follow a characteristic pattern that is quite distinct from that in mocha ( Lane and Deol, 1974). Furthermore, while the spotting and inner ear abnormalities associated with these spotting genes are best explained by a defect in the differentiation of neural crest cells ( Deol, 1970b), such an explanation is difficult to accept for the fully pigmented mocha phenotype.

Clearly the primary defect responsible for the various abnormalities associated with the mocha mutation remains to be elucidated.

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