A genetic map of Peromyscus with chromosomal assignment of linkage groups (a Peromyscus genetic map).

The rodent genus Peromyscus is the most numerous and species-rich mammalian group in North America. The naturally occurring diversity within this genus allows opportunities to investigate the genetic basis of adaptation, monogamy, behavioral and physiological phenotypes, growth control, genomic imprinting, and disease processes. Increased genomic resources including a high quality genetic map are needed to capitalize on these opportunities. We produced interspecific hybrids between the prairie deer mouse (P. maniculatus bairdii) and the oldfield mouse (P. polionotus) and scored meiotic recombination events in backcross progeny. A genetic map was constructed by genotyping of backcross progeny at 185 gene-based and 155 microsatellite markers representing all autosomes and the X-chromosome. Comparison of the constructed genetic map with the molecular maps of Mus and Rattus and consideration of previous results from interspecific reciprocal whole chromosome painting allowed most linkage groups to be unambiguously assigned to specific Peromyscus chromosomes. Based on genomic comparisons, this Peromyscus genetic map covers ~83% of the Rattus genome and 79% of the Mus genome. This map supports previous results that the Peromyscus genome is more similar to Rattus than Mus. For example, coverage of the 20 Rattus autosomes and the X-chromosome is accomplished with only 28 segments of the Peromyscus map, but coverage of the 19 Mus autosomes and the X-chromosome requires 40 chromosomal segments of the Peromyscus map. Furthermore, a single Peromyscus linkage group corresponds to about 91% of the rat and only 76% of the mouse X-chromosomes.

Natural genetic variation underlying differences in Peromyscus repetitive and social/aggressive behaviors.

Peromyscus maniculatus (BW) and P. polionotus (PO) are interfertile North American species that differ in many characteristics. For example, PO exhibit monogamy and BW animals are susceptible to repetitive behaviors and thus a model for neurobehavioral disorders such as Autism. We analyzed these two stocks as well as their hybrids, a BW Y(PO) consomic line (previously shown to alter glucose homeostasis) and a natural P. maniculatus agouti variant (A(Nb) = wide band agouti). We show that PO animals engage in far less repetitive behavior than BW animals, that this trait is dominant, and that trait distribution in both species is bi-modal. The A(Nb) allele also reduces such behaviors, particularly in females. PO, F1, and A(Nb) animals all dig significantly more than BW. Increased self-grooming is also a PO dominant trait, and there is a bimodal trait distribution in all groups except BW. The inter-stock differences in self-grooming are greater between males, and the consomic data suggest the Y chromosome plays a role. The monogamous PO animals engage in more social behavior than BW; hybrid animals exhibit intermediate levels. Surprisingly, A(Nb) animals are also more social than BW animals, although A(Nb) interactions led to aggressive interactions at higher levels than any other group. PO animals exhibited the lowest incidence of aggressive behaviors, while the hybrids exhibited BW levels. Thus this group exhibits natural, genetically tractable variation in several biomedically relevant traits.

Aberrant growth and pattern formation in Peromyscus hybrid placental development.

Crosses between the North American deer mouse species Peromyscus maniculatus (BW) and P. polionotus (PO) produce dramatic asymmetric developmental effects. BW females mated to PO males (female bw × male po) produce viable growth-retarded offspring. In contrast, PO females mated to BW males (female PO × male BW) produce overgrown but dysmorphic conceptuses. Most female PO × male BW offspring are dead by midgestation; those surviving to later time points display numerous defects reminiscent of several diseases. The hybrid effects are particularly pronounced in the placenta. Here we examine placental morphological defects via histology and in situ hybridization as well as the relationship between growth and mortality in the female PO × male BW cross. These assays indicate altered hybrid fetal:placental ratios by the equivalent of mouse (Mus) Embryonic Day (E) 13 and disorganization and labyrinth defects in female PO × male BW placentas and confirm earlier suggestions of a severely reduced junctional zone in the female bw × male po hybrids. Further, we show that both cellular proliferation and death are abnormal in the hybrids through BrdU incorporation and TUNEL assays, respectively. Together the data indicate that the origin of the effects is prior to the equivalent of Mus E10. Finally, as the majority of these assays had not previously been performed on Peromyscus, these studies provide comparative data on wild-type placentation.

A high methyl donor diet affects physiology and behavior in Peromyscus polionotus.

Folic acid and other dietary methyl donors are widely supplemented due to their ability to prevent neural tube defects. Dietary methyl donors are also added to other consumables such as energy drinks due to energy-promoting attributes and other perceived benefits. However, there is mounting evidence that indicates developmental exposure to high levels of dietary methyl donors may have deleterious effects. We assessed whether behavior was affected in the social North American rodent species Peromyscus polionotus exposed to a diet enriched with folic acid, Vitamin B12, choline, and betaine/trimethylglycine(TMG). P. polionotus (PO) animals are very social and exhibit little repetitive behavior, particularly compared to their sister species, P. maniculatus. We assayed the effects of dietary methyl-donor supplementation on anxiety-like repetitive and social behaviors by testing young adult animals for novel cage behavior and in social interaction tests. Animals of both sexes exposed to the diet had increased repetitive behaviors and reduced social interactions. Males exposed to the diet became more aggressive compared to their control counterparts. Since methyl-diet animals were larger than control animals, DEXA scans and hormone analyses were performed. Animals exposed to the diet had increased body fat percentages and experienced hormonal changes typically associated with excess fat storage and anxiety-like behavior changes. Therefore, these data suggest the wide use of these dietary supplements makes further investigation imperative.

Comparative genome mapping of the deer mouse (Peromyscus maniculatus) reveals greater similarity to rat (Rattus norvegicus) than to the lab mouse (Mus musculus).

BACKGROUND: Deer mice (Peromyscus maniculatus) and congeneric species are the most common North American mammals. They represent an emerging system for the genetic analyses of the physiological and behavioral bases of habitat adaptation. Phylogenetic evidence suggests a much more ancient divergence of Peromyscus from laboratory mice (Mus) and rats (Rattus) than that separating latter two. Nevertheless, early karyotypic analyses of the three groups suggest Peromyscus to be exhibit greater similarities with Rattus than with Mus. RESULTS: Comparative linkage mapping of an estimated 35% of the deer mouse genome was done with respect to the Rattus and Mus genomes. We particularly focused on regions that span synteny breakpoint regions between the rat and mouse genomes. The linkage analysis revealed the Peromyscus genome to have a higher degree of synteny and gene order conservation with the Rattus genome. CONCLUSION: These data suggest that: 1. the Rattus and Peromyscus genomes more closely represent ancestral Muroid and rodent genomes than that of Mus. 2. the high level of genome rearrangement observed in Muroid rodents is especially pronounced in Mus. 3. evolution of genome organization can operate independently of more commonly assayed measures of genetic change (e.g. SNP frequency).

Changes in cell cycle and extracellular matrix gene expression during placental development in deer mouse (Peromyscus) hybrids.

Crosses between two species of the rodent genus Peromyscus produce defects in both growth and development. The defects are pronounced in the hybrid placentas. Peromyscuys maniculatus (strain BW) females mated to P. polionotus (strain PO) males produce placentas half the size of the parental species, as well as growth-retarded embryos. In contrast, PO females mated to BW males result in defective conceptuses that display embryonic and placental overgrowth. These 'parent-of-origin'-dependent phenotypes are consistent with previous studies that demonstrated altered expression of imprinted genes and genetic linkage of the overgrowth phenotypes to imprinted domains. In the present study, we take a broader approach in assessing perturbations in hybrid placental gene expression through the use of Mus musculus cDNA microarrays. In verifying classes of genes identified in microarray screens differentially regulated during hybrid placental development, we focused on those influencing the cell cycle and extracellular matrix (ECM). Our work suggests that cell cycle regulators at the G(1)/S phase check-point are downregulated in the large hybrid placenta, whereas the small hybrid placenta is more variable. The ECM genes are typically downstream targets of cell cycle regulation and their misregulation is consistent with many of the dysmorphic phenotypes. Thus, these data suggest imbalances in proliferation and differentiation in hybrid placentation.

Assays to determine allelic usage of gene expression in the placenta.

Mammalian placentas express a large number of so-called imprinted genes. Imprinting refers to mono-allelic or biased expression based on which parent contributed the allele. Many of these imprinted loci encode factors involved in growth and cell-cycle regulation, as well as maternal behavior. In general, paternally expressed genes tend to enhance growth, whereas maternally expressed genes inhibit growth. Methods are described for developing assays to test the allelic usage of a gene. The approaches described are best utilized within a system where multiple strains are available, and it is possible to perform reciprocal crosses. Only polymerase chain reaction-based methods are examined in any detail.

Consequences of dietary methyl donor supplements: Is more always better?

Epigenetic mechanisms are now recognized to play roles in disease etiology. Several diseases increasing in frequency are associated with altered DNA methylation. DNA methylation is accomplished through metabolism of methyl donors such as folate, vitamin B12, methionine, betaine (trimethylglycine), and choline. Increased intake of these compounds correlates with decreased neural tube defects, although this mechanism is not well understood. Consumption of these methyl donor pathway components has increased in recent years due to fortification of grains and high supplemental levels of these compounds (e.g. vitamins, energy drinks). Additionally, people with mutations in one of the enzymes that assists in the methyl donor pathway (5-MTHFR) are directed to consume higher amounts of methyl donors to compensate. Recent evidence suggests that high levels of methyl donor intake may also have detrimental effects. Individualized medicine may be necessary to determine the appropriate amounts of methyl donors to be consumed, particularly in women of child bearing age.

Maternal methyl supplemented diets and effects on offspring health.

Women seeking to become pregnant and pregnant women are currently advised to consume high amounts of folic acid and other methyl donors to prevent neural tube defects in their offspring. These diets can alter methylation patterns of several biomolecules, including nucleic acids, and histone proteins. Limited animal model data suggests that developmental exposure to these maternal methyl supplemented (MS) diets leads to beneficial epimutations. However, other rodent and humans studies have yielded opposing findings with such diets leading to promiscuous epimutations that are likely associated with negative health outcomes. Conflict exists to whether these maternal diets are preventative or exacerbate the risk for Autism Spectrum Disorders (ASD) in children. This review will discuss the findings to date on the potential beneficial and aversive effects of maternal MS diets. We will also consider how other factors might influence the effects of MS diets. Current data suggest that there is cause for concern as maternal MS diets may lead to epimutations that underpin various diseases, including neurobehavioral disorders. Further studies are needed to explore the comprehensive effects maternal MS diets have on the offspring epigenome and subsequent overall health.

Caring for Peromyscus spp. in research environments.

Peromyscus spp. are the most abundant native North American mammals. They have gained popularity as research animals in the last 20 years, and this trend is expected to continue as new research tools, such as whole genome sequences, baseline physiological data and others, become available. Concurrently, advances have been made in the recommendations for the care of laboratory animals. The authors provide insight into how the Peromyscus Genetic Stock Center successfully breeds and maintains several stocks of deer mice and related species. This information is beneficial to researchers that plan to include Peromyscus spp. in their research programs.

Pleiotropic effects of a methyl donor diet in a novel animal model.

Folate and other methyl-donor pathway components are widely supplemented due to their ability to prevent prenatal neural tube defects. Several lines of evidence suggest that these supplements act through epigenetic mechanisms (e.g. altering DNA methylation). Primary among these are the experiments on the mouse viable yellow allele of the agouti locus (A(vy)). In the Avy allele, an Intracisternal A-particle retroelement has inserted into the genome adjacent to the agouti gene and is preferentially methylated. To further test these effects, we tested the same diet used in the Avy studies on wild-derived Peromyscus maniculatus, a native North American rodent. We collected tissues from neonatal offspring whose parents were fed the high-methyl donor diet as well as controls. In addition, we assayed coat-color of a natural variant (wide-band agouti = A(Nb)) that overexpresses agouti as a phenotypic biomarker. Our data indicate that these dietary components affected agouti protein production, despite the lack of a retroelement at this locus. Surprisingly, the methyl-donor diet was associated with defects (e.g. ovarian cysts, cataracts) and increased mortality. We also assessed the effects of the diet on behavior: We scored animals in open field and social interaction tests. We observed significant increases in female repetitive behaviors. Thus these data add to a growing number of studies that suggest that these ubiquitously added nutrients may be a human health concern.

Hematologic and serum biochemical values of 4 species of Peromyscus mice and their hybrids.

Deer mice (Peromyscus maniculatus) and congeneric species are used in a wide variety of research applications, particularly studies of developmental, physiologic, and behavioral characteristics associated with habitat adaptation and speciation. Because peromyscine mice readily adapt to colony conditions, animals with traits of interest in the field are moved easily into the laboratory where they can be studied under controlled conditions. The purpose of this study was to determine the serum chemistry and hematologic parameters of 4 frequently used species from the Peromyscus Genetic Stock Center species (P. californicus, P. leucopus, P. maniculatus, and P. polionotus) and to determine quantitative differences in these parameters among species and between sexes. Triglyceride values were substantially higher in female compared with male mice in all 4 species. Similar cross-species differences in MCH were present. Overall there was considerable interspecific variation for most blood parameters, with little evidence for covariation of any 2 or more parameters. Because crosses of P. maniculatus and P. polionotus produce fertile offspring, segregation analyses can be applied to determine the genetic basis of any traits that differ between them, such as their 3.8- and 2.1-fold interspecific differences in cholesterol and triglyceride levels, respectively. The current data provide a set of baseline values useful for subsequent comparative studies of species experiencing different circumstances, whether due to natural variation or anthropogenic environmental degradation. To enable such comparisons, the raw data are downloadable from a site maintained by the Stock Center (http://ww2.biol.sc.edu/∼peromyscus).

Peromyscus (deer mice) as developmental models.

Deer mice (Peromyscus) are the most common native North American mammals, and exhibit great natural genetic variation. Wild-derived stocks from a number of populations are available from the Peromyscus Genetic Stock Center (PGSC). The PGSC also houses a number of natural variants and mutants (many of which appear to differ from Mus). These include metabolic, coat-color/pattern, neurological, and other morphological variants/mutants. Nearly all these mutants are on a common genetic background, the Peromyscus maniculatus BW stock. Peromyscus are also superior behavior models in areas such as repetitive behavior and pair-bonding effects, as multiple species are monogamous. While Peromyscus development generally resembles that of Mus and Rattus, prenatal stages have not been as thoroughly studied, and there appear to be intriguing differences (e.g., longer time spent at the two-cell stage). Development is greatly perturbed in crosses between P. maniculatus (BW) and Peromyscus polionotus (PO). BW females crossed to PO males produce growth-restricted, but otherwise healthy, fertile offspring which allows for genetic analyses of the many traits that differ between these two species. PO females crossed to BW males produce overgrown but severely dysmorphic conceptuses that rarely survive to late gestation. There are likely many more uses for these animals as developmental models than we have described here. Peromyscus models can now be more fully exploited due to the emerging genetic (full linkage map), genomic (genomes of four stocks have been sequenced) and reproductive resources.

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.

Peromyscus as a Mammalian epigenetic model.

Deer mice (Peromyscus) offer an opportunity for studying the effects of natural genetic/epigenetic variation with several advantages over other mammalian models. These advantages include the ability to study natural genetic variation and behaviors not present in other models. Moreover, their life histories in diverse habitats are well studied. Peromyscus resources include genome sequencing in progress, a nascent genetic map, and >90,000 ESTs. Here we review epigenetic studies and relevant areas of research involving Peromyscus models. These include differences in epigenetic control between species and substance effects on behavior. We also present new data on the epigenetic effects of diet on coat-color using a Peromyscus model of agouti overexpression. We suggest that in terms of tying natural genetic variants with environmental effects in producing specific epigenetic effects, Peromyscus models have a great potential.

Patterns of hybrid loss of imprinting reveal tissue- and cluster-specific regulation.

BACKGROUND: Crosses between natural populations of two species of deer mice, Peromyscus maniculatus (BW), and P. polionotus (PO), produce parent-of-origin effects on growth and development. BW females mated to PO males (bwxpo) produce growth-retarded but otherwise healthy offspring. In contrast, PO females mated to BW males (POxBW) produce overgrown and severely defective offspring. The hybrid phenotypes are pronounced in the placenta and include POxBW conceptuses which lack embryonic structures. Evidence to date links variation in control of genomic imprinting with the hybrid defects, particularly in the POxBW offspring. Establishment of genomic imprinting is typically mediated by gametic DNA methylation at sites known as gDMRs. However, imprinted gene clusters vary in their regulation by gDMR sequences. METHODOLOGY/PRINCIPAL FINDINGS: Here we further assess imprinted gene expression and DNA methylation at different cluster types in order to discern patterns. These data reveal POxBW misexpression at the Kcnq1ot1 and Peg3 clusters, both of which lose ICR methylation in placental tissues. In contrast, some embryonic transcripts (Peg10, Kcnq1ot1) reactivated the silenced allele with little or no loss of DNA methylation. Hybrid brains also display different patterns of imprinting perturbations. Several cluster pairs thought to use analogous regulatory mechanisms are differentially affected in the hybrids. CONCLUSIONS/SIGNIFICANCE: These data reinforce the hypothesis that placental and somatic gene regulation differs significantly, as does that between imprinted gene clusters and between species. That such epigenetic regulatory variation exists in recently diverged species suggests a role in reproductive isolation, and that this variation is likely to be adaptive.

Adaptive genetic variation, stress and glucose regulation.

Elevated glucose levels in the presence of insulin are indicative of type 2 diabetes and the more inclusive metabolic syndrome. Alleles conferring susceptibility to these and other common conditions may be adaptations to past environments. It is possible that other mammals exhibiting environmental diversity harbor similar variants; therefore, we assessed glucose regulation in two species of deer mice (Peromyscus), a diverse endemic North American group. The prairie deer mouse, P. maniculatus bairdii (BW), and the Oldfield mouse, P. polionotus subgriseus (PO) differ in sexual dimorphism, behavior and habitat. PO animals exhibit better regulatory ability than BW animals, particularly among males, although both species display equivalent insulin levels/responses and non-fasted glucose levels. Hybrid males exhibit a PO glucose challenge response and subsequent analysis of consomic animals implicates Y chromosome variation as the genetic cause. Two pieces of evidence indicate that the male glucose regulatory differences are mediated by stress response: (1) fasting and handling alone account for most of the variation; (2) an inhibitor of glucocorticoid (GC) stress hormone synthesis eliminates these differences. PO males have GC levels that are twice those of BW males, indicating the presence of alleles that attenuate the GC response. We hypothesize that the interspecific physiological and behavioral differences are interrelated and that similar human variants exist.

Harvesting sperm and artificial insemination of mice.

Rodents of the genus Peromyscus (deer mice) are the most prevalent native North American mammals. Peromyscus species are used in a wide range of research including toxicology, epidemiology, ecology, behavioral, and genetic studies. Here they provide a useful model for demonstrations of artificial insemination. Methods similar to those displayed here have previously been used in several deer mouse studies, yet no detailed protocol has been published. Here we demonstrate the basic method of artificial insemination. This method entails extracting the testes from the rodent, then isolating the sperm from the epididymis and vas deferens. The mature sperm, now in a milk mixture, are placed in the female's reproductive tract at the time of ovulation. Fertilization is counted as day 0 for timing of embryo development. Embryos can then be retrieved at the desired time-point and manipulated.Artificial insemination can be used in a variety of rodent species where exact embryo timing is crucial or hard to obtain. This technique is vital for species or strains (including most Peromyscus) which may not mate immediately and/or where mating is hard to assess. In addition, artificial insemination provides exact timing for embryo development either in mapping developmental progress and/or transgenic work. Reduced numbers of animals can be used since fertilization is guaranteed. This method has been vital to furthering the Peromyscus system, and will hopefully benefit others as well.

Retrieval of mouse oocytes.

To date, only a few studies have reported successful manipulations of Peromyscus embryogenesis or reproductive biology. Together with the Peromyscus Genetic Stock Center (http://stkctr.biol.sc.edu), we are characterizing the salient differences needed to develop this system. A primary goal has been to optimize oocyte/early embryo retrieval.

Assessment and disease comparisons of hybrid developmental defects.

Rodents of the genus Peromyscus are among the most common North American mammals. Crosses between natural populations of two of these species, P. maniculatus (BW) and P. polionotus (PO), produce parent-of-origin effects on growth and development. BW females mated to PO males produce growth-retarded offspring. In contrast, PO females mated to BW males produce overgrown but dysmorphic conceptuses. Variation in imprinted loci and control of genomic imprinting appear to underlie the hybrid effects. Prior morphological and genetic analyses have focused on placental and post-natal growth. Here, we assess the frequency and scope of embryonic defects. The most frequent outcome of the PO x BW cross is death prior to embryonic day 13. Conceptuses lacking an embryo proper are also observed as in gestational trophoblast disease. Among the common embryonic phenotypes described and tabulated are edema, blood vessel enlargement/hemorrhaging, macroglossia, retention of nucleated erythrocytes, placentomegaly. We investigate expression of loci known to be mis-regulated in human growth/placental disorders and/or mouse knockouts with similar phenotypes. These loci are Igf2, Cdkn1c, Grb10, Gpc3, Phlda2 and Rb1. All exhibited significant differences in either placental or embryonic expression levels at one or more of the three timepoints examined. The data underscore the importance of placental gene expression on embryonic defects. We suggest that the hybrid defects offer a novel system to understand how natural allelic combinations interact to produce disease phenotypes. We propose that such interactions and their resulting epimutations may similarly underlie the phenotypic and causal heterogeneity seen in many human diseases.