The Journal of Heredity 2002:93(3)
© 2002 The American Genetic Association 93:209-210
Brief Communication |
A Blond Coat Color Variation in Meadow Vole (Microtus pennsylvanicus)
From the Neuroscience Program, Department of Psychology, Florida State University, 209 Copeland Ave., Tallahassee, FL 32306.
Address correspondence to J. Thomas Curtis at the address above or e-mail: tcurtis{at}psy.fsu.edu.
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Abstract |
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Color mutations occur frequently among rodents. Here we describe a blond coat color mutation in the meadow vole (Microtus pennsylvanicus) that arose in a captive breeding colony established from wild-caught animals from southern Illinois. The blond coat coloration results from changes in the color and distribution of pigments in the hair. The mutation is monogenic autosomal recessive.
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Introduction |
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Voles are small (
35–60 g) mouselike mammals that occupy a variety of habitats throughout the Holarctic. Many species of voles can be reliably bred and housed in captivity, and over the past two decades, voles increasingly have become common subjects for laboratory studies of social behaviors and reproductive physiology. Color mutations have been described in many animal species and are common in rodents (Gaines 1985; Silvers 1979). These mutations often involve the loss or replacement of melanin pigmentation (cf., Roth and Dawson 1996). Here we describe a blond color mutation that arose in a captive breeding colony of meadow voles (Microtus pennsylvanicus).
The voles used in this study were individuals of the F3, F4, and F5 generations of a laboratory breeding colony of meadow voles originating from a population in southern Illinois. Animals were maintained on a 14 h light:10 h dark photoperiod (lights on at 0700) to enhance reproductive success. Breeder pairs were housed in plastic, shoebox-style cages (47 cm x 25 cm x 20 cm) with pine chips as bedding, and straw was provided for nesting material. Food (Purina rabbit chow) and water were provided ad libitum. Breeding pairs were checked daily and the phenotypes of any neonates were noted. Pups were weaned at 21 days and housed in same-sex sibling pairs in 29 cm x 18 cm x 13 cm cages. Test crosses were made after the animals had reached sexual maturity (60–70 days of age).
An inadvertent mating between an F3 male and his daughter produced a litter containing two females with a blond coloration. All individuals from this litter were retained for subsequent breeding. As both of the blond voles from the original litter were female, these individuals were bred back to the original F3 male to provide further breeder stock. After sufficient breeding stock was available, a series of test crosses was performed to assess the genetics of the mutation. In crosses involving suspected homozygous dominant individuals, two litters with a minimum total of eight pups, none of which displayed the blond phenotype, were assumed to verify the suspected genetic status. To minimize disturbance to the breeder pairs, pups were not weighed at birth, nor was sex ascertained until weaning. Since some pups were lost prior to weaning, this resulted in the loss of data on sex for those pups. At the end of the experiments, specimens were placed with the Harvard Museum of Comparative Zoology (Cambridge, MA; catalogue numbers MCZ 63146–63167).
Four test crosses between the original, wild-type (B) F3 male and female descendents displaying the blond (b) phenotype produced a total of 17 pups with a phenotype ratio of 9 B:8 b. This ratio is consistent with our original hypothesis that the father was heterozygous while the blond individuals were homozygous recessive. In addition to the two blond females, the initial father-daughter mating also produced three wild-type males. These individuals were mated with blond females. Two of the three males sired litters containing both wild-type and blond pups (three litters, 3 B:7 b). The remaining male from the original litter sired two litters, neither of which contained a blond pup (8 B:0 b). Thus the phenotype ratio of the original father x daughter litter was 1 BB:2 Bb:2 bb. Blond males were crossed with blond females in four testcrosses. In all cases, only blond pups were produced (n = 16). When four blond females were crossed with unrelated wild-type males (n = 8 litters), no blond pups were produced (33 B:0 b).
The mean (±SE) litter size for females displaying the blond phenotype (n = 20) was 3.9 ± 0.3 pups. The mean litter size for an equal number of wild-type females chosen at random from the breeding colony as a whole was 4.0 ± 0.5 pups. The sex ratio was 0.8 male:1 female (n = 33).
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Phenotype |
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Wild-type voles of the common North American species are very similar in coloration (Carlton 1985). The dorsal and lateral fur is a uniform dark brown color, while the fur on the ventral surface is generally lighter. Both the ears and tail are covered with fur. Blond voles display a cream-colored fur, although the underside was still slightly lighter in color, as displayed in wild-type voles. Differences in eye color are commonly seen in other rodents displaying blond color mutations (cf., Owen and Shackleford 1942; Roth and Dawson 1996). Eye color in the blond voles was similar to that seen in wild-type voles. Pups with the blond phenotype were recognizable at birth. At postnatal day 1, wild-type vole neonates display a variable pattern of dark pigmentation on the back and head (Nadeau 1985). This pattern was absent in blond neonates.
Microscopic examination of the fur from wild-type and blond voles showed that the blond coloration was the result of at least two major differences in the type and distribution of pigmentation. The fur of wild-type voles contained black pigment that was distributed throughout the length of each hair shaft. In contrast, hair from blond voles contained brown pigment and the pigment was not present in the distal portion of the hair.
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Discussion |
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Color mutations have been identified in several small rodent species. Mutations similar to that found in meadow voles in the present study have been observed in Peromyscus (Hance 1969; Roth and Dawson 1996), Microtus (Gaines 1985; Owen and Shackleford 1942), and Mus (Silvers 1979). In most cases these mutations have been monogenic and autosomal. The gene responsible for the blond color mutation in our voles also appears to be autosomal, since the blond phenotype was expressed in individuals of both sexes. Further, the results of testcrosses suggest that a single gene is responsible for coat coloration in meadow voles. Heterozygous individuals were indistinguishable from homozygous dominant individuals.
The original pair from which the mother of the blond line descended had previously produced in excess of 30 pups, all of which displayed wild-type phenotypes. Since a mating involving the male from this pair and a daughter produced pups with the blond phenotype, the most likely explanation is that the original male was heterozygous for the color gene while the original female was homozygous wild type and that wild type is dominant. The origin of the blond mutation is unknown. The blond mutation may have arisen de novo within our colony. Our breeding colony has produced several hundred individuals, none of which displayed the blond coloration. Crosses of randomly chosen wild-type males from our colony with blond females produced only wild-type pups, suggesting that the blond allele is not common.
Alternatively, the blond allele may be present in the general population, but at such a low frequency that it is rarely found in the homozygous condition. Descriptions of meadow voles variously described as "yellow," "cream," and "buffy" occur in the literature (Clark 1935; Gaines 1985; Owen and Shackleford 1942). However, in most cases, enough differences in phenotype (i.e., fur color, eye color, pigment color and distribution, etc.) exist to suggest that the various yellow voles are the result of different mutations. The color variation most similar to that found in the present report was a description by Clark (1935) of meadow voles captured in southern Michigan. It is possible that the voles found by Clark and the voles in the present report are from the same breeding population. Although separated by several hundred miles, these voles are considered to be of the same subspecies (Hoffman and Koeppi 1985), and it is possible that the mutation may spread longitudinally through a contiguous population. The possibility of longitudinal spread is enhanced by the facts that vole littermates often disperse as a group (Hilborn 1975), both male and female voles are likely to disperse (Lidicker 1985), and individuals heterozygous for the blond mutation are indistinguishable from wild-type voles (present report).
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Acknowledgments |
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I would like to thank Dr. Z. X. Wang for generously allowing access to his vole colony. This work was supported by NIH grants NIMH 58616 (to Z.X.W.) and HD 40722 (to J.T.C.).
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Footnotes |
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Corresponding Editor: Roger H. Reeves
Received March 2, 2001
Accepted March 29, 2002
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References |
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Nadeau JH, 1985. Ontogeny. In: Biology of New World Microtus (Tamarin RH, ed). Special publication no. 8. Lawrence, KS: American Society of Mammalogists; 254–285.
Owen RD and Shackleford RM, 1942. Color aberrations in Microtus and Pitymys. J Mamm 23:306–314.[CrossRef]
Roth VL and Dawson WD, 1996. Coat color genetics of Peromyscus: V. California Blond, a new recessive mutation in the deer mouse. J Hered 87:403–406.
Silvers WK, 1979. The coat colors of mice. New York: Springer-Verlag.
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