Chapter 5

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

V. Muted (mu)

A. Origin and Influence on Pigmentation

Another coat-color mutation affecting otolith development is muted ( mu) which arose spontaneously in a stock segregating for t-alleles ( Lyon and Meredith, 1965a). This autosomal recessive on chromosome 13 ( Lyon, 1966) slightly dilutes both eumelanin and phaeomelanin ( Lyon and Meredith, 1969). Accordingly black becomes dark grey, yellow and brown become a somewhat lighter shade, and many animals have white underfur. The color of the eyes also is diluted, so that the iris ring appears very light in newborns, and the adult eye is a dark red color. When combined with other genes affecting eye color the eyes may be colorless at birth and pink in the adult. In general, the color change produced by muted ( mu/mu) resembles that produces by ruby-eye ( ru/ru) and beige ( bg/bg) ( Chapter 6, Section I), and is much less severe than that produced by pallid ( pa/pa) ( Lyon and Meredith, 1969).

B. Behavior and Its Association with Otolith Defects

As well as affecting pigment, muted causes changes in postural reflexes very similar to those described above for pallid mice. "Some muted homozygotes hold the head tilted to one side; some flex the spine and tuck the head under when held by the tail, and give no normal landing reaction when dropped head downwards towards a table" ( Lyon and Meredith, 1969). As in the case of pallid these behavioral disorders are associated with the absence of otoliths from the sacculus and utriculus of one or both ears as well as by an absence of pigmentation in the membranous labyrinth ( Erway et al., 1966). According to Lyon and Meredith ( 1969) of 17 mu/mu animals examined "12 lacked all otoliths, 3 had one otolith in each ear, one had two otoliths in the left ear and one in the right, and the remaining one had otoliths in both ears. The bony labyrinth appeared completely normal in all the animals, and none was deaf or showed circling behavior."

Although the effect of muted parallels very closely that of pallid, especially insofar as otolith development is concerned, only future studies can reveal whether these two genes act similarly. Of particular interest will be whether supplementing the mother's diet during pregnancy with manganese can alleviate the otolith defect in muted offspring, as it does in pallid mice ( Erway et al., 1971).