Cytonuclear discordance among Southeast Asian black rats (Rattus rattus complex)

Black rats are major invasive vertebrate pests with severe ecological, economic and health impacts. Remarkably, their evolutionary history has received little attention, and there is no firm agreement on how many species should be recognized within the black rat complex. This species complex is native to India and Southeast Asia. According to current taxonomic classification, there are three taxa living in sympatry in several parts of Thailand, Cambodia and Lao People's Democratic Republic, where this study was conducted: two accepted species (Rattus tanezumi, Rattus sakeratensis) and an additional mitochondrial lineage of unclear taxonomic status referred to here as ‘Rattus R3’. We used extensive sampling, morphological data and diverse genetic markers differing in rates of evolution and parental inheritance (two mitochondrial DNA genes, one nuclear gene and eight microsatellite loci) to assess the reproductive isolation of these three taxa. Two close Asian relatives, Rattus argentiventer and Rattus exulans, were also included in the genetic analyses. Genetic analyses revealed discordance between the mitochondrial and nuclear data. Mitochondrial phylogeny studies identified three reciprocally monophyletic clades in the black rat complex. However, studies of the phylogeny of the nuclear exon interphotoreceptor retinoid‐binding protein gene and clustering and assignation analyses with eight microsatellites failed to separate R. tanezumi and R3. Morphometric analyses were consistent with nuclear data. The incongruence between mitochondrial and nuclear (and morphological) data rendered R. tanezumi/R3 paraphyletic for mitochondrial lineages with respect to R. sakeratensis. Various evolutionary processes, such as shared ancestral polymorphism and incomplete lineage sorting or hybridization with massive mitochondrial introgression between species, may account for this unusual genetic pattern in mammals.     

Trophoblast Deportation to the Lungs of Cotton Rats (Sigmodon hispidus)

Cotton rats (Sigmodon hispidus) have been used to study a variety of infectious agents, particularly human respiratory viral pathogens. During the course of comprehensive pathologic evaluations of aging breeders from our breeding colony, 6 of 22 (27%) female cotton rats had histologic evidence, limited to the lungs, of embolized cells that were confirmed to be trophoblastic in origin by HSD3B1 immunoreactivity. When pulmonary trophoblast emboli were numerous, they usually were associated with additional histologic findings in the lungs, including pulmonary edema and hemorrhage, endothelial hypertrophy, fibrinoid vascular necrosis, and abundant alveolar macrophages containing fresh fibrin and hemolyzing erythrocytes. Of the 6 cotton rats with pulmonary trophoblast emboli, 5 (83%) were at 8 to 18 d of the 27-d gestation period, with the greatest number of emboli per lung present between days 10 through 14. The remaining cotton rat had a focal pulmonary trophoblast embolus and was not pregnant but had delivered a litter 3 mo previously. Three other cotton rats in either the early or late stages of gestation showed no histologic evidence of pulmonary trophoblast deportation. This report is the first to document pulmonary trophoblast emboli in cotton rats. This finding suggests that cotton rats may be an alternative animal model for the study of normal and aberrant trophoblast deportation in routine pregnancies and gestational pathologic conditions in women.Abbreviations: HSD3B1, hydroxyl-C-5-steroid dehydrogenaseCotton rats (Sigmodon hispidus) are a relevant animal model for the study of human respiratory²³ viral pathogens, with increasing usage by academic and industrial institutions. The hemochorial placentation in Sigmodontinae²² is similar to that of humans and several laboratory animal species including mice, rats, hamsters, rabbits, guinea pigs, chinchillas, and nonhuman primates.^{10,20,40,42,44,49} In these species, one or more layers of analogous trophoblast types comprise the interhemal barrier between maternal and fetal blood supplies. Placental trophoblasts perform a number of critical functions during gestation, including mediation of uterine implantation and invasion, nutrient exchange, regulation of maternal blood flow, and hormone production.^{1,19,26-28,35,38,46,47}As a consequence of their inherent invasiveness, placental trophoblasts migrate into maternal uterine blood vessels, after which syncytiotrophoblasts (syncytial knots) are normally deported daily to the lungs in humans.^2,3,17 Deportation increases with frequency as gestation progresses,^3,4 with gestational pathologic conditions such as preeclampsia and eclampsia,^2,3,18,36 and after cesarean sections⁵⁰ and abortions.⁴⁸ The current thinking is that these syncytial knots undergo programmed cell death and apoptotic shedding during routine pregnancy, in contrast to conditions like preeclampsia and eclampsia, during which aberrant intervillous hemodynamics resulting in hypoxia favor necrosis and associated inflammation.^18,25,29,30 In addition, spontaneous trophoblast emboli have been documented in the lungs and a few other tissues, including uterus, adrenal gland, spleen, and liver of chinchillas,^6,11,52 hamsters,^7,41 and porcupines.²⁴ Experimentally, trophoblast invasion has been further studied in mice^8,9 and hamsters.⁵ To our knowledge, pulmonary trophoblast emboli in cotton rats have not previously been reported.Pairs of cotton rat breeders were maintained for the production of animals to be used in various studies investigating human respiratory viruses, including measles, respiratory syncytial, and parainfluenza viruses. During the course of comprehensive pathologic evaluations of aging breeders, 6 female cotton rats were incidentally found to have pulmonary trophoblast emboli. The purposes of the present case series were to characterize the embolized trophoblasts and associated pulmonary histopathology in these cotton rats and to correlate the incidence with gestational stage.     

Sugar preferences of "black rats", Rattus rattus L.

The choice of "black rats", Rattus rattus L ., for common sugars - sucrose, jaggery, glucose, fructose and lactose are described. In laboratory colonies, the sugars were preferred in the order glucose > sucrose > jaggery > fructose > lactose; but in another sequence - sucrose > jaggery > glucose > fructose > lactose, in the free living colonies. The discrepancy is accounted for by the characteristics of sugars which influence consummatory behaviour in the two situations.     

Strongyloidiasis in cotton rats (Sigmodon hispidus) from central Oklahoma

Thirty-one of 40 cotton rats (Sigmodon hispidus) collected from central Oklahoma were infected with Strongyloides sp. (78% prevalence). Larvae of Strongyloides sp. (rhabditiform or filariform) were not demonstrable in intestinal contents and scrapings. Female nematodes recovered from intestinal contents and scrapings had morphological similarities with Strongyloides sigmodontis. Cotton rats infected with Strongyloides sp. were indistinguishable clinically from non-infected hosts. Infected animals had no significant gross lesions, but the presence of Strongyloides sp. in the intestinal mucosa was associated with villus atrophy and mild to moderate infiltration of the lamina propria by lymphocytes, plasma cells and occasional eosinophils. Other organs or tissues examined were free from lesions induced by Strongyloides sp.     

Multiple Cryptosporidium genotypes detected in wild black rats (Rattus rattus) from northern Australia

As part of a broader investigation into the potential role of black rats (Rattus rattus) as disease vectors into native small mammal populations of northern Australia, blood and faecal samples from wild black rats were screened by molecular methods, for piroplasms (Babesia and Theileria), trypanosomes and the enteric parasite Cryptosporidium. While piroplasms and trypanosomes were not detected in the blood of these animals, the overall prevalence of Cryptosporidium 18S rDNA in faecal samples was 8.2% (7/85). Co-occurrence of multiple genotypes was observed in 57.1% of the infected individuals (4/7); cloning and re-sequencing resulted in 14 sequences which broadly grouped with Cryptosporidium sp. rat-genotypes II and III. A novel rat-derived Cryptosporidium sp. genotype at the actin locus was also obtained from five animals. The relatively low infection rate detected, and the epidemiological data on cryptosporidiosis, do not conclusively support a current threat to native Australian mammals from black rats carrying Cryptosporidium. However, this observation is based on sampling limited isolates, in limited regions. Further studies, also including sampling of native mammals, are required on larger sample sizes and from wider geographic areas, to determine the significance of these findings, including the public health importance of Cryptosporidium spp. from rodents.     

Frugivory by introduced black rats (Rattus rattus) promotes dispersal of invasive plant seeds

Oceanic islands have been colonized by numerous non-native and invasive plants and animals. An understanding of the degree to which introduced rats (Rattus spp.) may be spreading or destroying seeds of invasive plants can improve our knowledge of plant-animal interactions, and assist efforts to control invasive species. Feeding trials in which fruits and seeds were offered to wild-caught rats were used to assess the effects of the most common rat, the black rat (R. rattus), on 25 of the most problematic invasive plant species in the Hawaiian Islands. Rats ate pericarps (fruit tissues) and seeds of most species, and the impacts on these plants ranged from potential dispersal of small-seeded (≤1.5 mm length) species via gut passage (e.g., Clidemia hirta, Buddleia asiatica, Ficus microcarpa, Miconia calvescens, Rubus rosifolius) to predation where <15% of the seeds survived (e.g., Bischofia javanica, Casuarina equisetifolia, Prosopis pallida, Setaria palmifolia). Rats consumed proportionally more seed mass of the smaller fruits and seeds than the larger ones, but fruit and seed size did not predict seed survival following rat interactions. Although invasive rat control efforts focus on native species protection, non-native plant species, especially those with small seeds that may pass internally through rats, also deserve rat control in order to help limit the spread of such seeds. Black rats may be facilitating the spread of many of the most problematic invasive plants through frugivory and seed dispersal in Hawaii and in other ecosystems where rats and plants have been introduced.     

Developmental Plasticity of Postweanling Cotton Rats (Sigmodon hispidus) as an Adaptation to Nutritionally Stochastic Environments

Elucidating interrelationships between rate of growth and sexual maturation in unpredictable or stochastic environments could increase our understanding of life-history strategies of small mammals. It has been hypothesized that species living in environments where food availability is unpredictable might become sexually mature at smaller sizes and channel excess energy into reproduction rather than into compensatory growth. We explored this hypothesis in female cotton rats (Sigmodon hispidus) by feeding variable levels of dietary protein during early postweanling development (14–45days of age) and monitoring compensatory growth and fitness after nutritional rehabilitation (45–100days of age). Growth was optimum in females fed diets containing 16% protein, with minimal requirements estimated to be 12%. Females fed diets containing <12% protein exhibited suppressed development, including delayed puberty. However, these nulliparous females demonstrated compensatory growth during the early period of nutritional rehabilitation, regardless of the severity of previous restrictions in protein. No long-lasting fitness consequences from postweanling nutritional restrictions were apparent as we observed no difference in date of conception, body mass of dams at parturition, litter size, or rate of growth of neonates. We offer a possible adaptive explanation for this observed plasticity in growth and development. This revised version was published online in July 2006 with corrections to the Cover Date.     

Echinococcus multilocularis: responses to infection in cotton rats (Sigmodon hispidus)

Various aspects of the responses of cotton rats to intraperitoneal infections with Echinococcus multilocularis were examined to determine if they could be related to the progress of the infection. At 14 weeks post-infection, infected animals had enlarged spleens; there was a slight decrease in packed cell volume, but no reticulocytosis. The number of all four types of leukocytes in the peripheral blood (lymphocytes, monocytes, neutrophils and eosinophils) increased during the course of the infection. In the peritoneal fluid, the numbers of neutrophils increased, monocytes and basophils decreased, and lymphocytes and eosinophils remained unchanged. Antibodies to E. multilocularis were detected in the serum of infected cotton rats as early as 2 weeks post-infection. The mean levels of transaminases (SGOT and SGPT) in the serum of infected animals were higher than in controls, and 5'-nucleotidase levels were elevated in heavily infected animals. There were no differences in responses between male and female animals. Comparison with results previously obtained suggest that both the outcome of the infection, and responses to it, may be under host control.     

Fatal Caryospora bigenetica (Apicomplexa: Eimeriidae) infections in cotton rats, Sigmodon hispidus

Four groups of cotton rats, Sigmodon hispidus, were shown to be suitable secondary hosts for the viperid coccidium, Caryospora bigenetica, following oral inoculation of a mixture of oocysts and sporocysts. Swelling of the face, ears, and scrota and hemorrhagic ears were the predominant clinical signs and some cotton rats died in 3 of 4 experiments. Developmental stages of C. bigenetica were found in connective tissue components of the ear, nose, cheeks, anal skin, scrotum, and penile sheath of all cotton rats in which these tissues were examined. Additionally, developmental stages of C. bigenetica were found in connective tissue components of the following tissues examined from some cotton rats: tongue, lung, testicle, epididymis, rectum, base of the tail, footpad, and bone marrow. The present study shows that C. bigenetica can be pathogenic for cotton rats and demonstrates many new anatomic sites for developmental stages of this parasite in the secondary host.     

Trichogramma species (Hymenoptera: Trichogrammatidae) parasitoids of Lepidopteran eggs in Lara State, Venezuela

This work was conducted to determine the occurrence of Trichogramma wasp species in Lara State, Venezuela. Lepidopteran egg samples collected from various crop leaves were observed daily under laboratory conditions for emergence of Trichogramma species adult wasps. Trichogramma were also obtained from traps containing eggs from the hosts Sitotroga cerealella (Olivier) (Lepidoptera: Gelechiidae) and Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae). In the laboratory, specimens of Trichogramma species were mounted on microscope slides to show male genitalia and other morphological characters used for its identification. Trichogramma atopovirilia Oatman y Platner emerged from S. frugiperda eggs collected in a cornfield at La Palomera, Humocaro Alto and at El Parchal, Humocaro Bajo, Municipio Morán. T. atopovirilia also emerged from S. frugiperda eggs collected in a cornfield at Sabana Grande and from S. cerealella eggs used as traps in a pepper culture at Tintinal, Municipio Andrés Eloy Blanco. T. exiguum Pinto y Platner emerged from S. frugiperda eggs used as traps or collected in cornfield at Totumito and at La Palomera, Humocaro Alto. T. pretiosum Riley emerged from Phthorimaea operculella Zeller eggs collected on stored potatoes at Monte Carmelo, Sanare. The identification of Trichogramma species in Lara State would be useful in the development of biological control programs for lepidopteran pests.     

Environmental Influences on Mass Dynamics of the Cotton Rat (Sigmodon hispidus) Thymus Gland

The influence of season on thymus gland mass was examined relative to captivity, gender, and age in 921 cotton rats (Sigmodon hispidus) from free-ranging and laboratory populations. Age-related involution of the thymus gland was evident in free-ranging males and females and captive females. A distinct seasonal cycle in thymus mass dynamics was apparent among adult cotton rats. Mass of the thymus gland was greatest from late fall to early winter before declining 2-4 fold during spring. Thymus gland mass remained low through spring and summer in adult cotton rats when reproductive activity was maximum. No seasonal cycle in thymus mass was apparent among juveniles. Possible involvement of sex hormones in regulating thymus size is discussed.     

Invasion genetics of the introduced black rat (Rattus rattus) in Senegal,West Africa

An understanding of the evolutionary history and dynamics of invasive species is required for the construction of predictive models of future spread and the design of biological management measures. The black rat (Rattus rattus) is a major vertebrate invader with a worldwide distribution. Despite the severe ecological, economic and health impacts of this species, its evolutionary history has been little studied. We carried out extensive specimen sampling in Senegal, West Africa, and used microsatellite markers to describe the pattern and processes of invasion in this large continental area. The genetic data obtained were combined with historical knowledge concerning the presence of this species in Senegal. Data were analysed by a combination of Bayesian clustering and approximate Bayesian computation methods. The invasion pathways closely paralleled the history of human trade routes in Senegal. In several places, we detected the occurrence of multiple introductions from genetically different sources. Long‐distance migration between towns and villages was also observed. Our findings suggest that genetic bottlenecks and admixture have played a major role in shaping the genetics of invasive black rats. These two processes may generate genetic novelty and favour rapid evolution along the invasion pathways.     

Black rats (Rattus rattus) from medieval Mertola (Baixo Alentejo, Portugal)

This paper is about the analysis and interpretation of micromammal samples from archaeological sites while reporting on a black rat assemblage from XIII^th century Muslim levels in the city of Mertola. Questions concerning skeletal representation, size plus age estimations and bone fragmentation are discussed from a taphonomic standpoint. It is concluded that the tendency to interpret rodent accumulations in anthropic sites as evidence of preybird activities might be an oversimplification in need of substantial revision.     

Karyotypes and serum transferrin patterns of hybrids between Asian and Oceanian black rats,Rattus rattus

Karyotypes and serum transferrin patterns were examined in Asian and Oceanian black rats (R. rattus). Japanese R. r. tanezumi and Malayan R. r. diardii had 2n=42, but Australian and New Guinea R. r. rattus showed 2n=38 chromosomes. F₁ hybrids between Japanese and Australian rats and Malayan and New Guinea rats had 2n=40 chromosomes which consists of the two genomes of both parents. Although various matings between the F₁ hybrids were made, only one F₂ male rat with 2n=39 chromosomes was obtained. The F₁ hybrids seem to be semisterile. Parental transferrin phenotypes were TfR in Japanese rats and TfCD in Oceanian rats. F₁ hybrids examined showed TfRD in both male and female and one F₂ hybrid had TfR type transferrin. Based on the above investigations, it is suggested that Asian and Oceanian black rats are geographically isolated and evolved different chromosomal and serum transferrin characteristics, but the sexual isolation of the two groups is incomplete at the present time.Contribution No. 826 from the National Institute of Genetics, Japan. Supported by a grant-in-aid from the Ministry of Education of Japan (Scientific Expedition in 1968, No. 8801 in 1969 and No. 9001 in 1970).     

Effects of social environment on sexual maturation in female cotton rats (Sigmodon hispidus)

Age at sexual maturation among female cotton rats was measured in a variety of intraspecific social environments. Two experiments were conducted. In Experiment I, female cotton rats attained vaginal perforation and first estrus at younger ages and lighter body masses when paired from weaning with a conspecific juvenile male than when caged alone. In Experiment II, these findings were replicated and extended. Females housed with juvenile males matured at the youngest ages, while those housed alone matured at the oldest ages. Females housed with adult males matured at intermediate ages. Presence of a second juvenile female during maturation was significantly associated with early vaginal opening but not with early first estrus. The results of this study are discussed in context of similar social environmental effects on female sexual maturation that have been identified in other rodent species.