Genic variation in the deer mouse,Peromyscus maniculatus,in a small geographic area

Genic variation was examined with starch and polyacrylamide gel electrophoresis at 14 loci in 12 populations of Peromyscus maniculatus nubiterrae from a small part of the Appalachians in eastern Tennessee and western North Carolina. Polymorphism was observed at eight loci with no significant correlations between frequency of common genotypes or alleles and altitude. Average individual heterozygosity (\-H) values were low for P. maniculatus whether using only slow evolving loci (mean = 3.0%) or both slow and fast evolving loci (mean = 4.9%). No significant correlation was present between altitude and \-H. Interlocality variation of \-H was as great in this study as previously reported for P. maniculatus over larger geographic areas. Rogers' coefficients of genetic similarity of paired combinations of populations based on slow evolving loci (range of 0.941 to 0.997) or based on slow and fast evolving loci (range of 0.858 to 0.974) showed all populations to be highly similar. Ranges of similarity values observed in the present study were as great as those previously reported for P. maniculatus over a larger geographic area.     

Persistent Sin Nombre virus infection in the deer mouse (Peromyscus maniculatus) model: sites of replication and strand-specific expression

To address Sin Nombre (SN) virus persistence in deer mice, we sacrificed experimentally infected deer mice at eight time points from day 21 to day 217 postinoculation (p.i.) and examined their tissues for viral nucleocapsid (N) antigen expression and both negative-strand (genomic) and positive-strand (replicative/mRNA) viral S segment RNA titers. All the animals that we inoculated developed persistent infections, and SN virus could be isolated from tissues throughout the course of infection. The transition from an acute to a persistent pattern of infection appeared to occur between days 60 and 90 p.i. Beginning on day 60 p.i., the heart, brown adipose tissue (BAT), and lung retained antigen expression and genomic viral RNA the most frequently. We found a statistically significant association among the presence of replicative RNA in the heart, lung, and BAT, widespread antigen expression (in > or =5 tissues), and RNA viremia. Of these three tissues, the heart retained negative-strand RNA and viral N antigen the most consistently (in 25 of 26 animals). During persistence, there were two distinct patterns of infection: restricted versus disseminated tissue involvement. Mice with the restricted pattern exhibited N antigen expression in < or =3 tissues, an absence of viral RNA in the blood, neutralizing antibody titers of < or =1:1,280 (P = 0.01), and no replicative RNA in the heart, lung, or BAT. Those with the "disseminated" pattern showed N antigen expression in > or =5 tissues, neutralizing antibody titers of 1:160 to 1:20,480, replicative RNA in the heart, lung, and BAT at a high frequency, and RNA viremia. Virus could be isolated consistently only from mice that demonstrated the disseminated pattern. The heart, lung, and BAT are important sites for the replication and maintenance of SN virus during persistent infection.     

Morphological analysis of hair in the hr-2 mutant deer mouse (Peromyscus maniculatus)

Skin from 36 hairless deer mice (Peromyscus maniculatus) homozygous for the recessive hr-2 mutation were analyzed for structural defects in hair and hair loss. Comparison of mutant to wild-type hairs demonstrated characteristic abnormalities in cellular organization, hair shape, length, and fragility. Matings between mutants homozygous for the hr-2 gene and for a second mutation producing hairlessness in deer mice, hr-1, showed that these two genes were nonallelic. Structural abnormalities in hairs associated with the expression of this gene suggest that its primary effect may be on the epidermis.     

Photoperiodic time measurement in the male deer mouse, Peromyscus maniculatus

Weanling male deer mice, Peromyscus maniculatus, were exposed for three weeks either to light-dark (LD) cycles with periods (T=L+D) ranging from T=23 (1L:22D) to T=25.16 (1L:24.16D) or to 24-h LD cycles with photoperiods ranging from 1 (1L:23D) to 19 (19L:5D) h. Both the circadian locomotor activity rhythms and the response of the reproductive system to these LD cycles were assessed. The results demonstrate that the photoperiodic effectiveness of light depends on the phase of the light relative to the animal's circadian system, as marked by the circadian activity rhythm. Light falling during the animal's subjective night, from activity onset to at least 11.8 h after activity onset, stimulates growth and maturation of the reproductive system, whereas light falling during the rest of the circadian cycle is nonstimulatory.     

Shedding and intracage transmission of Sin Nombre hantavirus in the deer mouse (Peromyscus maniculatus) model

The mechanism(s) by which Sin Nombre (SN) hantavirus is maintained in deer mouse populations is unclear. Field studies indicate that transmission occurs primarily if not exclusively via a horizontal mechanism. Using an experimental deer mouse infection model in an outdoor laboratory, we tested whether infected rodents shed SN virus in urine, feces, and saliva, whether infected mice transmit infection to na?ve cage mates, and whether infected dams are able to vertically transmit virus or antibody to offspring. Using pooled samples of urine, feces, and saliva collected from mice infected 8 to 120 days postinoculation (p.i.), we found that a subset of saliva samples, collected between 15 and 90 days p.i., contained viral RNA. Parallel studies conducted on wild-caught, naturally infected deer mice showed a similar pattern of intermittent positivity, also only in saliva samples. Attempts to isolate virus through inoculation of cells or na?ve deer mice with the secreta or excreta of infected mice were uniformly negative. Of 54 attempts to transmit infection by cohousing infected deer mice with seronegative cage mates, we observed only a single case of transmission, which occurred between 29 and 42 days p.i. Dams passively transferred antibodies to neonatal pups via milk, and those antibodies persisted for at least 2 months after weaning, but none transmitted infection to their pups. Compared to other hantavirus models, SN virus is shed less efficiently and transmits inefficiently among cage mates. Transmission of SN virus among reservoir rodents may require factors that are not required for other hantaviruses.     

Developmental plasticity in aerobic performance in deer mice (Peromyscus maniculatus)

While several studies have examined the abiotic effects of altitude (low ambient temperatures and hypoxia) on the aerobic performance of small mammals, few have explored the effects of development and maturation at different altitudes on aerobic performance as adults. We examined the basal metabolism and aerobic performance of deer mice (Peromyscus maniculatus) under four different developmental and testing regimes: (1) reared (gestation through weaning) and tested at high altitude; (2) reared and tested at low altitude; (3) reared at low altitude and tested at high altitude after acclimation; and (4) reared at low altitude and tested in hypoxia without acclimation. We found that mice that developed and were tested at low altitudes had a higher aerobic capacity (both aerobic performance and basal metabolic rate) than those that developed, or were acclimated as adults, at high altitudes. In addition, we found that mice that developed at high altitude did not have a higher aerobic capacity than those that developed at low altitude and were acclimated to high altitude as adults. Both groups tested at high altitudes had higher hematocrits (% red blood cells) and hemoglobin than mice tested at low altitudes. Surprisingly, mice acclimated to low altitudes and given an instantaneous exposure to hypoxia did not suffer a depression in aerobic performance.     

Fecal corticosteroids in agouti and non-agouti deer mice (Peromyscus maniculatus)

Total and per gram fecal corticosteroid concentrations were determined for agouti and non-agouti deer mice (Peromyscus maniculatus gracilis) over 24 h under normal caging conditions and after exposure to the stress of novel caging. Per gram corticosteroid concentrations, fecal output, and 24-h corticosteroid production were greater in stressed compared with unstressed deer mice of both color morphs, whereas stressed agoutis had a greater increase in per gram corticosteroid concentrations when compared with non-agoutis. However, due to increased fecal output, stressed non-agouti deer mice had greater 24-h corticosteroid production. Thus, agouti and non-agouti deer mice differ in their hormonal reaction to stress. This is the first demonstration of corticosteroid differences associated with the agouti locus.     

Prediction of Peromyscus maniculatus (deer mouse) population dynamics in Montana, USA, using satellite-driven vegetation productivity and weather data

Deer mice (Peromyscus maniculatus) are the main reservoir host for Sin Nombre virus, the primary etiologic agent of hantavirus pulmonary syndrome in North America. Sequential changes in weather and plant productivity (trophic cascades) have been noted as likely catalysts of deer mouse population irruptions, and monitoring and modeling of these phenomena may allow for development of early-warning systems for disease risk. Relationships among weather variables, satellite-derived vegetation productivity, and deer mouse populations were examined for a grassland site east of the Continental Divide and a sage-steppe site west of the Continental Divide in Montana, USA. We acquired monthly deer mouse population data for mid-1994 through 2007 from long-term study sites maintained for monitoring changes in hantavirus reservoir populations, and we compared these with monthly bioclimatology data from the same period and gross primary productivity data from the Moderate Resolution Imaging Spectroradiometer sensor for 2000-06. We used the Random Forests statistical learning technique to fit a series of predictive models based on temperature, precipitation, and vegetation productivity variables. Although we attempted several iterations of models, including incorporating lag effects and classifying rodent density by seasonal thresholds, our results showed no ability to predict rodent populations using vegetation productivity or weather data. We concluded that trophic cascade connections to rodent population levels may be weaker than originally supposed, may be specific to only certain climatic regions, or may not be detectable using remotely sensed vegetation productivity measures, although weather patterns and vegetation dynamics were positively correlated.     

Population delimitation across contrasting evolutionary clines in deer mice (Peromyscus maniculatus)

Despite current interest in population genetics, a concrete definition of a "population" remains elusive. Multiple ecologically and evolutionarily based definitions of population are in current use, which focus, respectively, on demographic and genetic interactions. Accurate population delimitation is crucial for not only evolutionary and ecological population biology, but also for conservation of threatened populations. Along the Pacific Coast of North America, two contrasting patterns of geographic variation in deer mice (Peromyscus maniculatus) converge within the state of Oregon. Populations of these mice diverge morphologically across an east-west axis, and they diverge in mitochondrial DNA haplotypes across a north-south axis. In this study, we investigate these geographically contrasting patterns of differentiation in the context of ecological and evolutionary definitions (paradigms) of populations. We investigate these patterns using a new and geographically expansive sample that integrates data on morphology, mitochondrial DNA, and nuclear DNA. We found no evidence of nuclear genetic differentiation between the morphologically and mitochondrially distinct populations, thus indicating the occurrence of gene flow across Oregon. Under the evolutionary paradigm, Oregon populations can be considered a single population, whereas morphological and mitochondrial differentiations do not indicate distinct populations. In contrast, under the ecological paradigm morphological differentiation indicates distinct populations based on the low likelihood of demographic interactions between geographically distant individuals. The two sympatric but mitochondrially distinct haplogroups form a single population under the ecological paradigm. Hence, we find that the difference between evolutionary and ecological paradigms is the time-scale of interest, and we believe that the more chronologically inclusive evolutionary paradigm may be preferable except in cases where only a single generation is of interest.     

Modoc-like virus isolated from wild deer mice (Peromyscus maniculatus) in Alberta

Small mammals were trapped in northeastern Alberta, Canada during 1976. Blood samples from these animals were tested for virus by inoculation of suckling mice. Blood clots from two deer mice yielded isolates of the same virus. The virus was related antigenically to a number of flaviviruses which have been isolated from mammals in Central America and North America and was related most closely to Modoc virus. Physical, chemical, and biological properties of the virus were similar also to those of Modoc virus. It did not produce illness or death in deer mice inoculated in the laboratory. Neutralization tests indicated that 1/38 (3%) red squirrels (Tamiasciurus hudsonicus), 3/35 (9%) least chipmunks (Eutamius minimus), 13/109 (12%) deer mice, and 3/50 (6%) humans were infected naturally. This is the first reported evidence of infection of red squirrels and chipmunks with a Modoc-like virus. These data extend the range of Modoc-like viruses northward by 1,500 km and comprise the first isolate from mammals in the boreal forest of Canada.     

The molecular organization of the beta-globin complex of the deer mouse, Peromyscus maniculatus

Recombinant DNA clones have been isolated that contain 80 kb of thebeta-globin complex from the deer mouse, Peromyscus maniculatus.Comparisons of this complex with that from the laboratory mouse, Musdomesticus (with an order 5'-Hbby, Hbb-bhO, Hbb-bhl, Hbb-bh2, Hbb-bh3,Hbb-bl, Hbb-b2 3') highlight organizational trends in the beta-globincomplex since the two species diverged. Unlike other mammals studied thusfar, the deer mouse possesses three adult genes. Partial sequence analysisindicates that each of the three adult genes is intact and hence may befunctional. Hybridization of one of the two Mus pseudogenes, Hbb-bh3, togenomic blots from Peromyscus reveals that it has a homologous counterpartin Peromyscus. Homologous genes to the two gamma-like Mus genes, Hbb-bhOand Hbb-bhl, are also found in Peromyscus. The strong hybridization betweenthe Hbb-bhl genes and significant nucleotide similarity between the Hbb-bhOgenes suggest that both pairs are important for the ontogeny of these micealthough no known product has been identified for the Hbb-bhO genes. Thepresence of Hbb-bhO and Hbb-bhl in Peromyscus suggests that the duplicationthat created this related gene set occurred before the two lineagesdiverged. A single gene for Hbb-y has been isolated from Peromyscus. Theadult region in Peromyscus has undergone significant divergence from thesame region in Mus, having three rather than two adult genes, theacquisition of at least 15 kb of extra DNA relative to Mus, and possiblythe loss of the Hbb-bh2 pseudogene. The nonadult region of the complex, incontrast, contains the same set of genes apparently distributed over thesame amount of DNA as in the Mus beta- globin complex. This observationsuggests that the embryonic region of the complex is more evolutionarilystable than the adult region.     

The biology and methodology of assisted reproduction in deer mice (Peromyscus maniculatus)

Although laboratory-reared species of the genus Peromyscus—including deer mice—are used as model animals in a wide range of research, routine manipulation of Peromyscus embryogenesis and reproduction has been lagging. The objective of the present study was to optimize conditions for oocyte and/or embryo retrieval and for in vitro culturing. On average, 6.4 oocytes per mouse were recovered when two doses of 15 IU of pregnant mare serum gonadotropin (PMSG) were given 24 h apart, followed by 15 IU of hCG 48 h later. Following this hormone priming, females mated overnight with a fertile male yielded an average of 9.1 two-cell stage embryos. Although two-cell stage embryos developed to 8-cell stage in Potassium Simplex Optimized Medium (KSOM; Millipore-Chemicon, Billerica, MA, USA) in vitro, but not further, embryos recovered at the 8- to 16-cell stages developed into fully expanded blastocysts when cultured in M16 media in vitro. These blastocysts had full potential to develop into late stage fetuses and possibly into live pups. As a result of the present work, all stages of Peromyscus preimplantation development are now obtainable in numbers sufficient for molecular or other analyses. These advances provide the opportunity for routine studies involving embryo transfer (e.g., chimeras, transgenics), and preservation of genetic lines by cryopreservation.     

Annual cyclicity of gall stone prevalence in deer mice (Peromyscus maniculatus gambelii)

The prevalence of gall stones (100% cholesterol) in a deer mouse (Peromyscus maniculatus) population located in Siskiyou County, California (USA) was studied on a monthly basis from February 1985 through January 1988. During each year we documented a pronounced annual cyclicity with peak prevalence (31 to 53%) during the winter and low prevalence (2 to 3%) during late summer. Gall stone prevalence was not related to sex or age of the animal. The earliest onset of gall stone production and the maximum prevalence achieved were associated with the greatest abundance of deer mice; lower levels of population abundance were associated with later onset of gall stone production and lower peak prevalences. This association of gall stone prevalence with both season and population abundance levels suggests that the causative factor(s) is/are related to the quality and availability of the diet.     

Differential opiate influences on food hoarding and intake in the deer mouse, Peromyscus maniculatus

The feeding behavior of the deer mouse, Peromyscus maniculatus, includes food hoarding as well as ingestion. In this animal the mu opiate agonist, morphine, and the kappa opiate agonist, U-50, 488H, selectively stimulate food hoarding and ingestion, respectively. This suggests that mu and kappa opiate systems may differentially mediate primary components of natural feeding behavior.     

Studies of pericopulatory pregnancy blockage and the gestation period in deer mice (Peromyscus maniculatus)

It has previously been shown that female deer mice in cycling estrus that copulate with or are exposed to more than one male at the time of mating have a lower probability of delivering a litter than when mating with and being exposed to just one male. In the present research the effect of post-mating exposure to the bedding of a strange versus a familiar male was studied in females in postpartum estrus. In four experiments females in postpartum estrus were protected from pericopulatory pregnancy blockage, presumably by suckling-induced elevated prolactin levels. Varying female experience, number of ejaculations, and the presence of suckling pups had no effect on this protection. Thus, both the pericopulatory block and the Bruce effect (a more delayed type of pregnancy blockage) occur in cycling, but not suckling, females. This suggests that the two may have a common mechanism and leads to a new perspective in the search for adaptive significance. In addition, the subsequent gestation periods were prolonged in females nursing young; the length was affected by the number of pups weaned from the previous litter and the number of pups being carried.