Apodemus Speciosus Research Articles

A pilot study to directly estimate radiation-induced mutation in large Japanese field mouse duo sample, mother and offspring, excluding unknown father, using ddRAD sequencing.

DNA mutations are one of the effects of radiation exposure. A large amount of radioactive materials was released into the environment from the Fukushima Daiichi Nuclear Power Plant after a major earthquake and tsunami. Wild animals and plants living in highly radiation-contaminated areas are constantly exposed to high doses of radiation, and concerns occur about its effects on their health and the next generations. As a pilot study, double-digest restriction site-associated DNA (ddRAD) sequencing was conducted to assess the incidence of mutations in wild large Japanese field mice collected from the evacuation area. The optimal combination of restriction enzymes, encompassing the functionally important coding regions, was selected using in silico analysis. These enzymes were used for ddRAD sequence analysis of females and their fetuses to evaluate mutation rates. The results indicated that no significant differences were observed in mutation rates between mothers and fetuses in the study areas.

Mite composition in nests of the Japanese wood mouse, Apodemus speciosus (Rodentia: Muridae).

Acari is a diverse group of arthropods that include well-known parasites of animals. Rodents, particularly, serve as common hosts of mites and ticks, transmitting pathogens to domestic animals and humans. Understanding the ecological dynamics between parasites and rodent hosts is crucial for ecosystem management. Due to limited knowledge about the life history of ectoparasites in wild mouse nests, we collected four nests of Apodemus speciosus, the most common rodent species in the wild areas of Japan, along with soil samples near the nests to study arthropod communities. Mites overwhelmingly populated the mouse nests, comprising approximately 90% of all arthropods, while both mites and collembolans were prevalent in soil. Various species identified in our study, such as those from the families Laelapidae, Pygmephoridae, Cheyletidae, Trombiculidae, Glycyphagidae, and Thyrisomidae align with known ectoparasites or species found in the nests of other rodent species, but most parasitic species were never collected in the surrounding soil except for trombiculids. The dominance of mites in mouse nests suggests selective preference for inhabiting these host environments, although the exact reasons driving this dominance remain unclear. Further investigations into the food web within mouse nests will aid in characterising faunal composition and understanding the ecological interaction among rodents, mites, and other nest symbionts.

Molecular evolution of Hokkaido virus, a genotype of Orthohantavirus puumalaense, among Myodes rodents

Viruses in the genus Orthohantavirus within the family Hantaviridae cause human hantavirus infections and represent a threat to public health. Hokkaido virus (HOKV), a genotype of Orthohantavirus puumalaense (Puumala virus; PUUV), was first identified in Tobetsu, Hokkaido, Japan. Although it is genetically related to the prototype of PUUV, the evolutionary pathway of HOKV is unclear. We conducted a field survey in a forest in Tobetsu in 2022 and captured 44 rodents. Complete coding genome sequences of HOKVs were obtained from five viral-RNA-positive rodents (four Myodes rufocanus bedfordiae and one Apodemus speciosus). Phylogenetic analysis revealed a close relationship between the phylogenies and geographical origins of M. rufocanus-related orthohantaviruses. Comparison of the phylogenetic trees of the S segments of orthohantaviruses and the cytochrome b genes of Myodes species suggested that Myodes-related orthohantaviruses evolved in Myodes rodent species as a result of genetic isolation and host switching.

Ultrastructural Analysis of Large Japanese Field Mouse (Apodemus speciosus) Testes Exposed to Low-Dose-Rate (LDR) Radiation after the Fukushima Nuclear Power Plant Accident.

Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, great attention has been paid to the impact of chronic low-dose-rate (LDR) radiation exposure on biological systems. The reproductive system is sensitive to radiation, with implications connected to infertility. We investigated the testis ultrastructure of the wild large Japanese field mouse (Apodemus speciosus) from three areas contaminated after the FDNPP accident, with different levels of LDR radiation (0.29 µSv/h, 5.11 µSv/h, and 11.80 µSv/h). Results showed good preservation of the seminiferous tubules, comparable to the unexposed animals (controls), except for some ultrastructural modifications. Increases in the numerical density of lipid droplet clusters in spermatogenic cells were found at high levels of LDR radiation, indicating an antioxidant activity rising due to radiation recovery. In all groups, wide intercellular spaces were found between spermatogenic cells, and cytoplasmic vacuolization increased at intermediate and high levels and vacuolated mitochondria at the high-level. However, these findings were also related to the physiological dynamics of spermatogenesis. In conclusion, the testes of A. speciosus exposed to LDR radiation associated with the FDNPP accident showed a normal spermatogenesis, with some ultrastructural changes. These outcomes may add information on the reproductive potential of mammals chronically exposed to LDR radiation.

Heritability of walnut seed size and the benefits of large seeds

AbstractJapanese walnuts (Juglans mandshurica) are known to be seed‐dispersed by the hoarding of two species of rodents, Japanese squirrels (Sciurus lis) and large Japanese field mice (Apodemus speciosus). The previous study showed that the squirrel transported and stored larger walnut seeds farther, whereas the mice transported smaller seeds farther. Divergent selection by different seed dispersers may play an important role in differentiation between local walnut populations, in which understanding seed size heritability is crucial. This study aims to determine how the seed size selected by rodents affects the initial growth of walnuts and to what extent walnut seed size is genetically determined. Sixty walnut seeds were collected from a small seed‐producing tree, and 50 walnut seeds from a large seed‐producing tree were sown and grown for 18 years. The seed size produced from each tree showed that the heritability of seed size was high (h2 = 0.52). When 50 smaller and 50 larger walnut seeds were planted under the same conditions, the dry weight of shoots and roots of the larger seeds was significantly greater than that of the smaller seeds, and root length was also significantly longer in the larger seeds. Seed size is heritable, so if squirrels are inhabiting, larger seeds, which are advantageous for regeneration, will be selected, and walnut trees producing larger seeds will be passed on to the next generation. Because selective hoarding has implications for seed establishment success, inhabiting a suitable seed disperser, the squirrel, is more critical for walnut regeneration than previously assumed.

Morphological Peculiarities of Molars in the Large Japanese Field Mouse (Apodemus speciosus, Rodentia, Muridae) from Kunashir Island

Morphological Peculiarities of Molars in the Large Japanese Field Mouse (Apodemus speciosus, Rodentia, Muridae) from Kunashir Island

Phoretic behavior of the pseudoscorpion Megachernes ryugadensis on the Japanese wood mouse Apodemus speciosus.

Phoresy is a passive transportation behavior where one organism (phoront) disperses to a new location by attaching to another organism. Pseudoscorpions are arthropod predators that mainly live in soil, subterranean habitats, and under tree bark. Some species also live in animal nests and engage in phoresy on small mammals, suggesting close associations with these animals. However, the relationship between phoretic pseudoscorpions and hosts as well as the ecological significance of phoresy remain largely unexplored. Here, to understand the function of phoresy of Megachernes ryugadensis, phoretic on small mammals, their phoretic behavior was investigated in a deciduous forest in northern Japan; individual-level dynamics of phoresy were examined by over 3-year mark-recapture surveys that concurrently marked the host and phoront; and host characteristics, such as sex and age class, were analyzed based on a 2-year small mammal trapping survey. The primary host species was the abundant Japanese wood mouse Apodemus speciosus. Out of 132 pseudoscorpions marked, 5 were recaptured approximately 1month later. No pseudoscorpions were recaptured within the same census period (3-4days) when they were marked, indicating that phoresy events last less than one night, and pseudoscorpions are unlikely to engage in phoresy again within a few weeks of their initial engagement. Furthermore, analysis of host characteristics revealed a tendency for female mice and adult individuals to have a higher probability of being hosts compared with males and subadults, respectively. Based on the findings in this and previous studies, the function of phoresy in this species is discussed.

Interactive effects of two rodent species on the seed dispersal of Japanese walnut

The effects of seed dispersers on plant fitness (seed dispersal effectiveness, SDE) have been evaluated based on the number (quantity) and recruitment probability (quality) of dispersed seeds. Although seeds of most zoochorous species are dispersed by two or more animal species, which may interact with each other, SDE has often been studied assuming a one-plant and one-animal species system. We compared the SDE of Japanese walnut (Juglans ailanthifolia) between squirrel-only and squirrel-mouse sites in natural forests of Hokkaido, Japan, and found that the SDE from the red squirrel (Sciurus vulgaris), considered a primary seed disperser, was altered by an alternative seed disperser species, the Japanese wood mouse (Apodemus speciosus). Seed removal rates at the squirrel-mouse site were significantly higher than those at the squirrel-only site, and both dispersed seeds and seedlings were less aggregated, with a strongly repulsive relationship with adult conspecific trees at the squirrel-mouse site. Seedlings established themselves at a location with fewer medium-sized trees (< 10 cm DBH) at the squirrel-mouse site. These results suggest that the interactive effect of the rodent species affects the SDE of Japanese walnut.

How much do field mice prefer dwarf bamboo seeds? Two-choice experiments between seeds of Sasa borealis and several tree species on the forest floor.

Bambusoideae is a taxon of mass-flowering monocarpic perennials with a long life cycle. Forest ecosystems are affected by Bambusoideae seeding and death events in various ways, including an increased abundance of Apodemus spp. The utilization and preference of dwarf bamboo seeds over tree seeds by field mice remain elusive. Therefore, we aimed to determine whether field mice prefer dwarf bamboo to tree seeds. We examined one dwarf bamboo species (Sasa borealis) against four tree species with varying acorn/fruit traits (Castanea crenata, Quercus crispula, Fagus crenata, and Lindera triloba). The seeds were placed in a container in a forest among dead S. borealis culms, with an automatic camera monitoring the setup. The examined seeds were mainly foraged by two field mouse species, Apodemus speciosus and Apodemus argenteus, with preference in the following order: C. crenata, L. triloba, S. borealis, F. crenata, and Q. crispula. Our findings indicated that during S. borealis mast seeding years, predation pressure on F. crenata and Q. crispula seeds could be considerably reduced. This suggests that mast seeding might disrupt the normal pattern of survival, and seed dispersal patterns, potentially altering the forest vegetation composition.

Utilization of Sasa borealis seeds by Japanese field mouse: discovery of small-seed caching

Bambusoideae are well-known for their large-scale flowering, seeding, and death events that occur after a vegetative phase that can often last for decades. These events are a valuable resource for many animals. Their mast seeding, in particular, affects rodent populations although the causal relationship between them is speculative. Moreover, the species of animals that utilize this resource also remains unclear. Therefore, this study aimed to examine the interaction between mast seeding plants and animals by conducting a feeding test using the seeds of a dwarf bamboo species (Sasa borealis) at different seasons and in different forest types. The results revealed that two mouse species, Apodemus speciosus and A. argenteus, utilized the seeds. They both predated as well as removed and sometimes buried the seeds, exhibiting small-seed caching behavior. Furthermore, A. speciosus was found to exhibit higher seed predation at covered sites and switching to removal behavior at the more exposed, open sites, while A. argenteus showed no distinguishable trends. Additionally, A. speciosus utilized the seeds less frequently in fall than in summer, suggesting a shift in diet towards acorns when available. Collectively, the results of this study provide novel insights into the interaction between mast seeding plants and the animals that utilize them.

The Large Japanese Field Mouse (Apodemus speciosus) as a Consumer and Potential Disperser of Seeds of the Neurotoxic Japanese Star Anise (Illicium anisatum)

The small Japanese field mouse Apodemus argenteus is known to disperse the highly toxic seeds of the Japanese star anise Illicium anisatum (Schisandraceae); however, whether the large Japanese field mouse A. speciosus does so remains unexplored. Seeds of I. anisatum were placed in mesh cages with openings sized only for small rodents and monitored at two forest sites in Tsukuba, Ibaraki Prefecture, Japan, in autumn 2018. Any animals approaching or entering the cages were recorded by automated cameras with infrared motion sensors. Only A. speciosus was recorded entering the cages. At both sites, some seeds were consumed inside the cages or transported outside them. These results show that A. speciosus interacts with seeds of I. anisatum and is a potential agent of seed dispersal. Further study is needed to determine whether it can tolerate anisatin, the main neurotoxin in I. anisatum, and the possible tolerance mechanism.

MORPHOLOGICAL FEATURES OF THE MOLARS OF THE JAPANESE MOUSE, &lt;i&gt;APODEMUS SPECIOSUS&lt;/i&gt; (RODENTIA, MURIDAE), FROM THE KUNASHIR ISLAND

Seven new characters were revealed in 722 molars of the Japanese mouse, Apodemus speciosus, from the Kunashir Island, Kuriles. The absence from the Kunashir population of characters previously identified for the Japanese mouse from the island of Honshu, Japan, both Recent and fossil (Holocene, Middle and Late Pleistocene), as well as the stabilization of new traits, indicates either the Kunashir population could have passed through sharp declines in numbers or a small number of founders, or a new type of nutrition. New features of the molars of the mice supplement the list of the morphological and morphometric characteristics of the Japanese mouse and can serve as the basis for clarifying the morphological features of A. s. ainu, as well as to continue the study of the populations from Hokkaido and the small islands adjacent to it.

Quaternary Environmental Changes Shaped Mitochondrial DNA Diversity in the Large Japanese Wood Mouse Apodemus speciosus in Hokkaido, Japan

Quaternary environmental change provided opportunities for rapid population expansion; however, the process of building the population spatial structures remains poorly understood. In this study, we determined the mitochondrial cytochrome b and control region sequences of 43 individuals of the large Japanese wood mouse (Apodemus speciosus) from Hokkaido, northern Japan and analyzed these data along with those from 40 other individuals. Consistent with the findings of our previous study, we found that two rapid expansion events, after the last glacial maximum (LGM) and Marine Isotope Stage (MIS) 4, shaped population genetic pattern of A. speciosus in Hokkaido. In northeastern Hokkaido, several ancient lineages that originated during MIS 3 were detected, whereas central Hokkaido was dominated by haplotypes descended from a single lineage that survived the LGM, suggesting that the populations of western part of Hokkaido were newly formed by westward migration from eastern Hokkaido during the post-LGM warm period. Alternatively, as post-LGM vegetation recovery is thought to have occurred gradually from west to east in Hokkaido, population expansion started in the west and moved gradually to the east, resulting in eastward haplotype movement; thus, western and eastern Hokkaido may have served as the haplotype source and sink, respectively.

Dietary niche breadth influences the effects of urbanization on the gut microbiota of sympatric rodents.

Cities are among the most extreme forms of anthropogenic ecosystem modification, and urbanization processes exert profound effects on animal populations through multiple ecological pathways. Increased access to human‐associated food items may alter species' foraging behavior and diet, in turn modifying the normal microbial community of the gastrointestinal tract (GIT), ultimately impacting their health. It is crucial we understand the role of dietary niche breadth and the resulting shift in the gut microbiota as urban animals navigate novel dietary resources. We combined stable isotope analysis of hair and microbiome analysis of four gut regions across the GIT to investigate the effects of urbanization on the diet and gut microbiota of two sympatric species of rodents with different dietary niches: the omnivorous large Japanese field mouse (Apodemus speciosus) and the relatively more herbivorous gray red‐backed vole (Myodes rufocanus). Both species exhibited an expanded dietary niche width within the urban areas potentially attributable to novel anthropogenic foods and altered resource availability. We detected a dietary shift in which urban A. speciosus consumed more terrestrial animal protein and M. rufocanus more plant leaves and stems. Such changes in resource use may be associated with an altered gut microbial community structure. There was an increased abundance of the presumably probiotic Lactobacillus in the small intestine of urban A. speciosus and potentially pathogenic Helicobacter in the colon of M. rufocanus. Together, these results suggest that even taxonomically similar species may exhibit divergent responses to urbanization with consequences for the gut microbiota and broader ecological interactions.

Chromosomal Aberrations in Large Japanese Field Mice (Apodemus speciosus) Captured in Various Periods after Fukushima Daiichi Nuclear Power Plant Accident.

After the Fukushima Daiichi Nuclear Power Plant accident, we studied the chromosomal aberrations (dicentrics and translocations) in the splenic lymphocytes of wild mice inhabiting Fukushima prefecture. Here, we report the frequencies of chromosomal aberrations in large Japanese field mice (Apodemus speciosus) captured from 2012 to 2016 in a heavily contaminated area. The chromosomal aberrations were detected using newly developed 4-color FISH (fluorescence in situ hybridization) with A. speciosus chromosome 1-, 3-, 4- and 5-specific painting probes. The frequencies of chromosomal aberrations in mice captured in July 2012 and October 2014 were significantly higher than that in the mice inhabiting the non-contaminated control area; however, the frequency of chromosomal aberrations in mice captured in January 2016 was not. The frequency of chromosomal aberrations in individual mice tended to increase with certain dose rates and accumulated doses. Regression tree analyses suggested increasing chromosomal aberration rate in mice exposed to chronic radiation at dose rates of more than 1.1 mGy day-1 and at accumulated doses of more than 200 mGy. It is concluded that ambient dose rates in the most severely contaminated area of Fukushima prefecture and radiation doses to wild mice inhabiting this area decrease with time; consequently, chromosomal aberrations induced by radiation have not been detected 5 years after the accident.

Evaluation of sperm fertilization capacity of large Japanese field mice (Apodemus speciosus) exposed to chronic low dose-rate radiation after the Fukushima accident

Due to the risk of bias in early developmental processes of radio-sensitive germ cells, the effects of radiation on reproductive ability remain unclear. In this study, wild large Japanese field mice (Apodemus speciosus) within the ex-evacuation zone of the Fukushima Daiichi Nuclear Power Plant (FDNPP) exposed to chronic low dose-rate (LDR) radiation since 2011 were examined. We analyzed the in vitro fertilization (IVF) capacity of cryopreserved A. speciosus sperm from 2012 to 2021 using interspecific zona pellucida (ZP)-free and homogenous wild mice oocytes. The radioactivity concentrations of 134Cs and 137Cs were 3,460–37,630 Bq/kg in the Fukushima wild mice. Frozen-thawed sperm showed generally normal acrosomal and nuclear morphology. No significant differences in sperm penetration rate to that of ZP-free mouse oocytes were observed between the control and Fukushima sperm. When frozen-thawed A. speciosus sperm was used for IVF, 44%–90% homogenous wild mice oocytes fertilized and developed a 2-cell embryo. This study was the first to reveal that long-term chronic LDR radiation exposure (0.18–0.59 mGy/d) associated with the FDNPP accident showed no detrimental effect on the IVF capacity of cryopreserved sperm in wild A. speciosus. This study also provides an alternative framework to estimate radiation effects on wild mice reproduction.

Ancient rivers shaped the current genetic diversity of the wood mouse (Apodemus speciosus) on the islands of the Seto Inland Sea, Japan

The current distributions of organisms have been shaped by both current and past geographical barriers. However, it remains unclear how past geographical factors—currently cryptic on the sea floor—affected the current distributions of terrestrial animals. Here, we examined the effects of currently cryptic ancient rivers on current genetic differentiation of the large Japanese wood mouse, Apodemus speciosus, which inhabits islands in the Seto Inland Sea, Japan. Genome-wide polymorphisms were identified by GRAS-Di (Genotyping by Random Amplicon Sequencing, Direct) analysis of 92 A. speciosus individuals. Maximum-likelihood analysis was performed with 94,142 single nucleotide polymorphisms (SNPs) identified by GRAS-Di analyses. Ancient rivers were visualized by Geographic Information System (GIS) channel analysis. Maximum-likelihood analysis showed strong support for the monophyly of each population in the islands in the Seto Inland Sea; it also showed close relationships between Innoshima-Ikuchijima, Ohmishima-Hakatajima-Oshima, Ohmishima-Hakatajima, Ohsakikamijima-Ohsakishimojima, Kamikamagarijima-Shimokamagarijima, and Kurahashijima-Etajima islands. The principal component analyses of the SNPs also supported these relationships. Furthermore, individuals from islands located on the east and west sides of the main stream of the ancient river were clustered on each side with strong support. These phylogenetic relationships were completely congruent with the paleogeographic relationships inferred from ancient rivers. In conclusion, the findings demonstrated that the current distribution of genetically distinct island lineages was shaped by ancient rivers that are currently submerged beneath the Seto Inland Sea, Japan.

How does the 120-year cycle mast seeding of dwarf bamboo affect the rodent population?

BackgroundIn 2017, large-scale flowering, seeding, and dying events of dwarf bamboo (Sasa borealis) occurred in a wide range in central Japan for the first time in 120 years. This phenomenon of S. borealis, like the mast seeding of trees, could be expected to provide a large amount of food for seed-eating rodents and gradually affect their ecology and population dynamics. We captured rodents in survey plots in a secondary broad-leaved forest with the phenomenon from 2018 to 2019 to investigate the species, the number of individuals, growing stage, sex, and body mass. In addition, we also compared the capture data for 2 years (2018–2019) with that for the mast seeding year (2017) and 6 years before it (2011–2016).ResultsThe mast seeding of S. borealis greatly increased the population size of rodents, especially Apodemus speciosus and A. argenteus. Conversely, Eothenomys smithii did not show such an increase. Most of the captured rodent individuals were already adults at the time of new capture, and the proportion of male juveniles was extremely low. These results suggest that the mast seeding of S. borealis created unusually rich food availability for the population concentration of rodents due to their immigration. However, body mass was not significantly different before and after the mast seeding. In addition, the increased populations of the two Apodemus species did not decline 2 years after the S. borealis masting, contrary to the previously reported decrease of rodent populations after tree masting.ConclusionsOur results indicate that the mast seeding of S. borealis affected the population dynamics of Apodemus species over a long time and their individual composition of different growth stages. However, it did not affect their body mass.

Body Temperature Measurement Reveals the Reproductive Profile of Female Apodemus speciosus under Laboratory and Field Conditions

This study illustrated body temperature (Tb) fluctuation during reproduction and discussed the effectiveness of measuring Tb for predicting the reproductive profile of female Apodemus speciosus both under laboratory and field conditions. Tb fluctuation was monitored for four breeding events in the laboratory and for three in the field. Individual variation in Tb fluctuation during reproduction was larger in the field than in the laboratory, while its temporal pattern was clearer in the field than in the laboratory. Tb fluctuating patterns according to the progress of the reproductive stage were similar between the laboratory and the field. Daily mean Tb increased after the start of pregnancy, decreased during late pregnancy, rapidly increased after parturition, and remained higher through the lactation period. In particular, the following three characteristics should be apparent indices of parturition: increase of daily mean Tb and daily minimum Tb in early pregnancy, decrease of daily max Tb and daily mean Tb in late pregnancy, and increase of daily max Tb at the day of parturition. These results indicate that implanting small-sized loggers enables us to obtain a representative pattern of Tb fluctuation and to predict the reproductive profile of female A. speciosus, both under laboratory and field conditions.

Shell colour diversification induced by ecological release: A shift in natural selection after a migration event.

Ecological release is often attributed to the rapid adaptive diversification of phenotypic traits. However, it is not well understood how natural selection changes its strength and direction through the process of ecological release. Herein, we demonstrated how shell colour of the Japanese land snail Euhadra peliomphala simodae has diversified via a shift in natural selection due to ecological release after migration from the mainland to an island. This snail's shell colour diversified on the island due to disruptive selection after migration from the mainland. We used trail camera traps to identify the cause of natural selection on both the mainland and the island. We then conducted a mark–recapture experiment while collecting microhabitat use data. In total, we captured and marked around 1,700 snails on the mainland, some of which were preyed upon by an unknown predator. The trail camera traps showed that the predator is the large Japanese field mouse Apodemus speciosus, and the predatory frequency was higher on the mainland than on the island. However, this predation did not correlate with shell colour. Microhabitat use on the island was more extensive than on the mainland, with snails on the island using both ground and arboreal microhabitats. A Bayesian estimation showed that the stabilizing selection on shell colour came from factors other than predation. Our results suggest that the course of natural selection was modified due to ecological release after migration from the mainland, explaining one cause of the phenotypic diversification.