Sixty polymorphic microsatellite markers for the oldfield mouse developed in Peromyscus polionotus and Peromyscus maniculatus

We isolated and characterized 60 novel microsatellite markers from the closely related oldfield mouse (Peromyscus polionotus) and deer mouse (Peromyscus maniculatus) for studies of conservation, ecological, quantitative and population genetics. We assessed all 60 markers in a wild population of Peromyscus polionotus rhoadsi (N = 20) from central Florida and found an average of nine alleles per marker and an observed heterozygosity (H_O) of 0.66 (range = 0.00–1.00). These polymorphic markers contribute to the growing number of genomic resources for Peromyscus, an emerging model system for ecological and evolutionary research.     

Vasopressin and aggression in cross-fostered California mice (Peromyscus californicus) and white-footed mice (Peromyscus leucopus)

To examine how developmental experiences alter neural pathways associated with adult social behavior, we cross-fostered pups between the more aggressive and monogamous California mouse (Peromyscus californicus) and the less aggressive and polygamous white-footed mouse (P. leucopus). Cross-fostered males became more like their foster parents when tested as adults. Male white-footed mice became more aggressive only in an aggression test in a neutral arena, whereas the territorial California mice became less aggressive in resident-intruder aggression test, as measured by attack latency. Only the species that displayed a change in resident-intruder aggression showed a change in arginine vasopressin (AVP) levels: cross-fostered California mice had significantly lower levels of AVP-immunoreactive (AVP-ir) staining than controls in the bed nucleus of the stria terminalis (BNST) and the supraoptic nucleus (SON) and a nonsignificant trend toward lower levels in the medial amygdala (MA). Neither species showed changes in AVP-ir staining in a control area, the paraventricular nucleus (PVN). The changes in AVP-ir staining in the BNST and SON may not be caused by stress because cross-fostering was not associated with changes in adult plasma concentrations of two steroid hormones, corticosterone and testosterone, that have been associated with stress-related alterations in AVP pathways. These results suggest that manipulating the early parental environment can directly alter both a neurotransmitter system and species-typical patterns of social behavior, but that these effects may vary between species and under different social contexts.     

Species recognition in wild-caught,laboratory-reared and cross-fostered Peromyscus californicus and Peromyscus eremicus (Rodentia,Cricetidae)

Experiments were performed to compare homospecific and heterospecific species choice in two closely related species of white-footed mice, Peromyscus californicus and P. eremicus. Both species significantly chose the homospecific stimulus animal. Significant homospecific choice was made by mice from sympatric but not from allopatric populations. Reciprocal cross-fostering between the two species resulted in significant choice for the heterospecific (foster) species by P. eremicus, and random choice by cross-fostered P. californicus. Laboratory-reared controls chose significantly for the homospecific chamber. No significant difference in choice performance was demonstrated between males and females, even when the oestrus stages of the females (both stimulus and test animals) were statistically controlled. A comparison of different test durations and temporal regimes of data collection was performed and 90 min was found to be the most efficient experiment duration with our apparatus.     

Immunological factors in Peromyscus speciation

Reciprocal interspecific F1 hybrids of deermice (Peromyscus maniculatus) and oldfield mice (P. polionotus) differ significantly and substantially in fetal and placental, as well as adult, size and weight. Hybrid fetal mortality is associated with large conceptus size. Skin grafts were exchanged between and within the two species to ascertain whether any relationship exists between mean graft retention time and body size of fetuses and adults. P. maniculatus skin grafted to P. polionotus rejected significantly earlier than the reciprocal xenograft. All interspecific graft combinations rejected significantly earlier than intraspecific grafts. Pre-immunization of female P. maniculatus with con- and trans-specific paternal spleen cell antigens reduced fetal, placental, neonatal, and ten-day size compared with controls. Size, weight, fertility, and graft rejection data were compared with several theoretical models. The data were consistent with the hypothesis that immunological disparity between the species could produce marked size variations in reciprocal hybrids. Multiple minor histocompatibility factors can account for large placental size and fetal mortality in Peromyscus hybrids. Physiological reproductive isolation may result from immunological differences between closely allied species.     

Rapid morphological and genetic change in Chicago-area Peromyscus

We report rapid change of morphology and mitochondrial genes in white-footed mice (Peromyscus leucopus) in the Chicago (Illinois, USA) region. We sequenced mitochondrial DNA COX2 from 55 museum skins of white-footed mice caught in the Chicago area since 1855 and from 44 mice recently trapped in the same locations. We found consistent directional genotype replacement at five separate collection locations. We later focused on a single one of these locations (Volo Bog State Natural Area) and sequenced mitochondrial D-loop control region from 58 museum skins of mice collected in 1903-1976 and 32 mice recently trapped there. We found complete and more recent replacement of D-loop haplotypes, apparently occurring between 1976 and 2001. We tested whether these genetic changes were mirrored by changes in morphology by comparing 15 external and cranial traits. We found no significant morphological differences between mice collected in 1903-1976; however, mice collected in 2001-2003 showed 9 of 15 measurements to be significantly changed relative to the earlier samples. Recent mice were longer in total length, with broader, longer noses, and longer but shallower skulls(1). Discriminant function analysis allowed for 100% correct classification using these traits. Principal components analysis shows variance over time is well distributed across both external and cranial measures. The sequential replacements of haplotypes and the rapid change of morphology can best be explained by replacement of the regional population with immigrants from genetically distinct neighbouring populations, likely facilitated by the large environmental changes occurring over the time period. Replacement with genotypes from external populations may be a common mechanism of evolution of newly adaptive local forms in an increasingly human-impacted world.     

Sexual imprinting and speciation between two Peromyscus species

Sexual isolation, a reproductive barrier, can prevent interbreeding between diverging populations or species. Sexual isolation can have a clear genetic basis; however, it may also result from learned mate preferences that form via sexual imprinting. Here, we demonstrate that two sympatric species of mice—the white‐footed mouse (Peromyscus leucopus) and its sister species, the cotton mouse (P. gossypinus)—hybridize only rarely in the wild despite co‐occurrence in the same habitat and lack of any measurable intrinsic postzygotic barriers in laboratory crosses. We present evidence that strong conspecific mating preferences in each species result in significant sexual isolation. We find that these preferences are learned in at least one species: P. gossypinus sexually imprints on its parents, but in P. leucopus, additional factors influence mating preferences. Our study demonstrates that sexual imprinting contributes to reproductive isolation that reduces hybridization between otherwise interfertile species, supporting the role for learning in mammalian speciation.     

Five hundred microsatellite loci for Peromyscus

Mice of the genus Peromyscus, including several endangered subspecies, occur throughout North America and have been important models for conservation research. We describe 526 primer pairs that amplify microsatellite DNA loci for Peromyscus maniculatus bairdii, 467 of which also amplify in Peromyscus polionotus subgriseus. For 12 of these loci, we report diversity data from a natural population. These markers will be an important resource for future genomic studies of Peromyscus evolution and mammalian conservation.     

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.     

Medroxyprogesterone, immunosuppression, and conceptus size in Peromyscus

Effects of exogenous medroxyprogesterone acetate (MPA) on skin transplant retention and conceptus size were studied in the deer mouse, Peromyscus maniculatus, and the oldfield mouse Peromyscus polionotus. Daily injections of 2 mg MPA significantly prolonged survival of allografts in P. maniculatus and transspecific grafts between P. maniculatus and P. polionotus. Allografts were retained significantly longer than transspecific grafts (17.2 vs 12.4 days) on MPA-treated P. maniculatus. Near-term fetal and placental sizes and weights were not detectably influenced by daily 1-mg MPA injections given to the mother from the 5th through the 19th day of pregnancy. The data are discussed relative to the possible immunosuppressive role of progestins in protecting the allogeneic conceptus from maternal immune rejection.     

Histone acetylation and heterochromatin content of cultured Peromyscus cells

In order to explore the relationship between unacetylated arginine-rich histones and condensed chromatin structure, the extent of histone acetylation was examined in cultured cell lines derived from three species of deer mice. These species differ considerably in their genomic content of heterochromatin but contain essentially the same euchromatin content. Cells of Peromyscus eremicus, containing 34–36% more constitutive heterochromatin than Peromyscus boylii or Peromyscus crinitus cells were found to contain 28–35% more unacetylated histone H4, 22–29% more unacetylated histone H3, and 18–22% more unacetylated histone H2B. This relationship between unacetylated histones and heterochromatin content was further explored by inducing hyperacetylation of P. eremicus and P. boylii histones through treatment of cells with 15 mM sodium butyrate for 24 h. It was found that the percentages of unacetylated histones H3 and H4 remaining after butyrate treatment were proportional to the amount of constitutive heterochromatin in the genome. These data support the concept that a small core of histones in constitutive heterochromatin is inaccessible to acetylation. It was also found that the acetylated state of isolated histones was sensitive to the method of histone extraction. Thus concern must be given to preparative procedures when studying histone acetylation in order to minimize these acetate losses.