Brown (b = brn)
ALLELES: b+ (= brn+) (black-tip, wild-type), bot (orange-tan), b (brown), bpo (brown in P. polionotus).
PHENOTYPE: (brown). Dorsal coat color is yellowish brown. Eumelanin granules in the hairs are sepia. Basal portions of hairs are chocolate or brown. Sepia granules are smaller and less compact than in wild type. There is a loose arrangement of eumelanin granules. Phaeomelanin granules are lemon yellow. Ears, tail stripe and soles of feet are brown.
PHENOTYPE: (orange-tan). Adult coat color is yellow-tan to orange-tan, richer on the sides and head than mid-dorsally. Proximal portions of the hairs are slightly paler than in the wild type. Prior to weaning the coat is pale diluted brown, but the orange intensity increases with the age of the animal.
In the original stocks female orange-tan deer mice showed increased spontaneous tumor rate, usually about 15%. Aggressive behavior accompanied the trait in the founding stocks, as did reduced litter size and fertility
LINKAGE: Linkage Group II (McIntosh, 1956a).
INTERACTIONS: Brown interacts with silver to produce beige "silver-brown" which is superficially similar to the pink-eyed dilution coat color, but is readily distinguished by the near white basal coat and dark eyes of silver brown animals. Interacts with gray to produce pale brown coat color. The dominant allele, black, is virtually epistatic to red-eye.
SOURCE: About 1932. University of Michigan. There was a recurrence of this mutation among laboratory stocks of P.m. blandus originating from deer mice collected near Tularosa NM.
1966. Savannah River Reservation SC. The brown mutant appeared in natural populations of P. polionotus lucubrans and in laboratory offspring derived from populations in this area. This mutant is identical or allelic with the original brown.
About 1955. University of Utah Dugway Ecological Laboratory. Orange-tan appeared among wild-caught P.m. sonoriensis from Toole County UT.
COMMENT: Allelism tests at the University of South Carolina between brown derived from the Huestis line and orange-tan showed these two traits to be allelic and virtually indistinguishable. The traits can not be separated with certainty among the segregants, however, some "brown" deer mice are "oranger" than others, indicating that orange-tan may be a distinct allele and not an independent occurrence of brown. "Recessive buff" (Egoscue, pers. comm.), a variant reported from Egoscue's Utah-Dugway colony, also appears to be a separate recurrence of the brown mutation.
A discussion of the possible adaptive significance Of the b allele is considered in Dawson et al. (1969) Horner and Dawson (1993) and Roth and Dawson (1996). Homology between the b locus of P. maniculatus and the b (Tyrp1 = tyrosinase-related protein-1) locus in Mus is discussed in the latter two references. The "golden nugget" trait in P. leucopus is likely another recurrence of its brown mutation.
REFERENCE: Brown: Huestis and Barto (1934), Blair (1947), McIntosh (1956); orange-tan: Egoscue and Day (1958); brown in polionotus: Dawson et al. (1969), Smith et al. (1972).