Rodent Akodon Research Articles

A New Metastrongilid Species (Nematoda: Metastrongylidae): A Lungworm from Akodon montensis (Rodentia: Sigmodontinae) in Brazil

Angiostrongylus lenzii n. sp. (Nematoda: Metastrongyloidea) is described and illustrated from the pulmonary artery of the wild rodent Akodon montensis Thomas, 1913, collected in the municipality of Teresopólis, Rio de Janeiro State, Brazil. It represents the 17th species of the genus and the 2nd in South America as parasites in rodents. This new species is the most similar morphologically to Angiostrongylus vasorum, Angiostrongylus gubernaculatus, Angiostrongylus schmidti, and Angiostrongylus morerai. It can be distinguished from them by the morphology of the caudal bursa, ventral rays 2 and 3 with conspicuous knobs, differences in width and length of lateral and dorsal rays, presence of rays 8, and length of spicules.

Pequenos mamíferos não voadores em fragmentos de Mata Atlântica e áreas agrícolas em Viana, Espírito Santo, Brasil

Nós inventariamos os pequenos mamíferos não-voadores em Viana, Espírito Santo, sudeste do Brasil, em 1981-1982 e 2006-2007. Foram capturados 439 pequenos mamíferos não-voadores pertencentes a três ordens (Didelphimorphia, Rodentia e Lagomorpha) e seis famílias (Didelphidae, Sciuridade, Cricetidae, Muridae, Echimyidae e Leporidae). As espécies mais abundantes foram os roedores Akodon cursor e Nectomys squamipes e o marsupial Metachirus nudicaudatus. Registramos pela primeira vez a ocorrência do roedor equimídeo Euryzygomatomys spinosus no estado do Espírito Santo. A riqueza específica (S = 21) e o índice de diversidade de Shannon (H = 2,23) estão entre os maiores registrados para pequenos mamíferos da Mata Atlântica no estado, mesmo quando comparados aos valores obtidos para unidades de conservação. Esses maiores valores de riqueza e diversidade encontrados estão provavelmente associados à heterogeneidade de hábitat e evidenciam a necessidade de conservação dos fragmentos florestais de Viana, os quais são importantes depositários da biodiversidade da Mata Atlântica.

Mamíferos do Parque Estadual do Desengano, Rio de Janeiro, Brasil

Mesmo após a devastação de grande parte das suas matas, o Estado do Rio de Janeiro ainda abriga uma grande riqueza de mamíferos terrestres. Contudo, informações sobre abundância e distribuição da mastofauna do Estado ainda são escassas. Este estudo tem como objetivo inventariar os mamíferos do Parque Estadual do Desengano, localizado na região norte do Estado do Rio de Janeiro, uma das regiões menos amostradas e mais alteradas do Estado. O inventário foi realizado utilizando armadilhas do tipo Sherman e Tomahawk, e armadilhas de queda para os pequenos mamíferos não-voadores, redes de neblina para os morcegos, armadilhas fotográficas, avistamentos e relatos de moradores e funcionários do Parque para os mamíferos de maior porte. Nós registramos 56 espécies de mamíferos, das quais 15 constam na lista da fauna ameaçada de extinção do Estado do Rio de Janeiro e 10 na lista da fauna brasileira ameaçada de extinção, incluindo Brachyteles arachnoides. As espécies mais abundantes foram o morcego Sturnira lilium e o roedor Akodon serrensis. Além disso, Thaptomys nigrita, roedor normalmente pouco capturado no Estado, foi relativamente abundante no Parque Estadual do Desengano. Nossos resultados indicam que o Parque abriga 33,7% das espécies de mamíferos com ocorrência conhecida para o Estado, destacando a importância da área para a conservação da fauna do Rio de Janeiro.

Food habits of Geoffroy's cat (Leopardus geoffroyi) in the central Monte desert of Argentina

Geoffroy's cat (Leopardus geoffroyi) is a little-known South American felid. Here we report the first detailed data on its seasonal food habits in a protected area of the central Monte desert, Argentina. We collected and analyzed 182 scats, identifying a total of 441 prey items, with 2.4±1.5 prey items per scat (range=1–10). Our results confirm that, as with most other felids, Geoffroy's cat is a small-sized predator specialized in capturing vertebrates. Small mammals were the most frequent prey items, representing at least the 63.3% of the food items in each season. The Sigmodontinae rodents Akodon molinae and Calomys musculinus were the most important prey throughout the study period, whereas birds and reptiles were consumed mainly during warmer seasons. Seasonal differences in diet composition were higher between warmer and colder seasons, suggesting that diet composition is constrained by seasonal fluctuation and disponibility of prey.

The use of PCR-RFLP as an identification tool for three closely related species of rodents of the genus Akodon (Sigmodontinae, Akodontini)

Three cryptic species of the rodent Akodon (A. cursor, A. montensis and Akodon sp.) were analyzed. The two former species are sympatric in the Brazilian states of Sao Paulo, Rio de Janeiro and Minas Gerais, where hybrids have already been found. The third species, Akodon sp., occurs in an isolated area in Central Brazil. The identification of these species is difficult by the need of living animals. At present, karyotyping is the only method used in the identification of specimens. We used PCR-RFLP of the mitochondrial cytochrome gene to test the distinctiveness of the three species, which was confirmed by the absence of shared species-specific haplotypes. We also detected a geographical pattern of haplotypes distribution with highly polymorphic populations of A. cursor from Espirito Santo and of A. montensis from Rio Grande do Sul.

Phenotypic plasticity in response to low quality diet in the South American omnivorous rodent Akodon azarae (Rodentia: Sigmodontinae)

We studied the responses in the omnivorous rodent A. azarae submitted to a low quality diet at morphological, physiological and biochemical levels. At short term, a decrease in body mass occurred. A later increase in food consumption constituted a strategy that allowed a temporal recovery of physical condition. However, hyperphagia appeared not to be enough to maintain physical condition after 30 days of low quality diet consumption. At the morphological level, an increase in length (9%) of the anterior portion of the gut occurred, the part of the gut where digestion and absorption take place. A decrease in small intestine weight could be related with the long-term impairment of body condition. Inhibition of sucrase specific activity in small intestine would indicate a down-regulation of sucrase-isomaltase complex. Total maltase specific activity in small intestine was not affected suggesting an up-regulation of sucrase-independent maltase specific activity. A down-regulation of protease specific activity in small intestine occurred in response to low quality diet. The specific activity of disaccharidases in caecum and large intestine was down-regulated. The strategies and constraints at different levels of A. azarae upon low quality diet are discussed.

Fine structure and cytochemical analysis of the intestinal wall along the body of adult female of Litomosoides chagasfilhoi (Nematoda: Filarioidea)

Litomosoides chagasfilhoi is a filariid nematode parasite of the abdominal cavity of the wild rodent Akodon cursor (Winge, 1887), that has been described and used in Brazil as a new model for human filariasis. The fine structure of the intestine of this nematode was analyzed based on observations made by light and transmission electron microscopies of serial sections along the body. Cytochemical analysis was carried out to investigate the composition of the intestinal wall. This structure consisted of a basal lamina and an epithelium of variable thickness, composed of cells that have an irregular shape. The cytoplasm of intestinal cells contains few organelles: vacuoles, lysosomal bodies, spheroid bodies, endoplasmic reticulum, and many large lipid droplets. In the anterior portion of the intestine, the lysosomal bodies, spheroid bodies, and vacuoles presented positive reaction for acid phosphatase, and carbohydrates were detected in lysosomal bodies. The midbody and posterior regions presented less organelles and lipid droplets, and nuclei were more abundant. Residues of L-fucose were detected by Ulex europaeus lectin binding in the midbody sections. Basic proteins were associated to lipid droplets, in the posterior region. In the whole extension of the intestine, carbohydrates were detected on tight junctions. These results indicate that the metabolized material in the epithelium can contribute to the microfilariae development and also probably can be involved with the excretory/secretory mechanism of these nematodes.

Phylogenetic relationships and karyotype evolution in the sigmodontine rodent Akodon (2n = 10 and 2n = 16) from Brazil

Comparison of G-bands from 2n = 10 and 2n = 16 karyotypes of Akodon revealed tandem fusions, pericentric inversions and Robertsonian rearrangement in autosomes and addition/deletion of constitutive heterochromatin in sex chromosomes. Cytochrome-b sequences indicate that the 2n = 10 karyotype is a new species and show it to be a sister taxon of the 2n = 14, 2n = 15 and 2n = 16 karyotypes. Indeed, this group shows a particular evolutionary situation in which a unique taxonomic unit based on morphological data can be detected, but, karyologically, it can be separated into two groups (2n = 14-15-16 and 2n = 10). Cytochrome-b sequences show a finer resolution, indicating that these four karyotypes represent three molecular entities (2n = 14-15, 2n = 16 and 2n = 10) that may be derived from a common ancestor with a 2n = 16 karyotype.

Influence of edge and fire-induced changes on spatial distribution of small mammals in Brazilian Atlantic forest fragments

The spatial distribution of small mammals in relation to distance from forest edges, before and after fire, was studied in Atlantic Forest fragments in Brazil. None of nine species was captured exclusively on either edges or forest interior. Before the fire only the rodents Akodon cursor and Oecomys con-color were captured more often at the edge than randomly expected. After the fire only A. cursor remained associated with edges; it had increased in number and penetrated farther than before into the forest fragments. The marsupial Micoureus demerarae became more restricted to the forest interior than randomly expected. These results suggest that small mammal species tolerant to habitat changes induced by edge effects and fire have better chances to survive in forest fragments.

Effects of cereal harvest on abundance and spatial distribution of the rodent Akodon azarae in central Argentina

Abstract The effects of corn and wheat harvest on abundance, movement, disappearance, sex ratio and age structure of Akodon azarae were studied in cropfield–border systems. Sampling occurred both before and after harvest, in crops, in their weedy margins (borders) and in surrounding areas. The abundance of A. azarae decreased in fields between before and after harvest and increased along borders. Rodents moved among habitat patches, movement being higher as a consequence of harvest. There was no difference among habitats in terms of disappearance and sex ratio but changes occurred in age structure because of harvest. In response to harvest A. azarae was able to move from crops to borders, decreasing the mortality effects. The ability to respond to habitat changes allows A. azarae to maximize fitness in periodically disturbed habitats.

Phenotypic flexibility of digestive morphology and physiology of the South American omnivorous rodent Akodon azarae (Rodentia: Sigmodontinae)

We have determined the occurrence of responses at different levels (morphological, physiological and biochemical) in the omnivorous rodent Akodon azarae upon cold acclimation (15 °C). A short-term enhancement in food consumption appeared to account for the maintenance of both mass and body composition. At the morphological level, the main response was an increase in the dimensions of small intestine, which constitutes the section of the gut where absorption and secretion take place. An increase in sucrase specific activity was only found in small intestine. Sucrose independent maltase activity was very low since 99.8% of total maltase activity was due to sucrase–isomaltase (SI) complex. Protease specific activities were not affected. The fact that resting metabolic rates determined at 15 and 23 °C were similar in cold acclimated animals suggests a change in lower critical temperature. In conclusion, our results show that A. azarae exhibits different strategies to support cold environment that could lead to an enhancement in digestion and absorption efficiency. Furthermore, this work suggests that low temperature is an independent cue of other environmental factors to trigger the strategies allowing the maintenance of body condition in A. azarae.

Offspring sex-ratio and reproductive performance in heterogametic females of the South American field mouse Akodon azarae. Reproduction in heterogametic Akodon azarae females.

We have compared the breeding performance of homogametic (XX) and heterogametic (XY*) females of the South American sigmodontine rodent Akodon azarae under laboratory conditions. XY* females showed an enhanced reproductive performance when compared with normal, XX, females. The XY* females had a longer reproductive lifespan. They started to reproduce early, had more frequent litters, and stopped reproduction later than XX females. Their progeny showed a biased 1:2 male:female sex ratio which may be explained by the early loss of YY* zygotes after fertilization. However, litter size at birth was similar both in XY* and XX females, and no difference in ovulation rate was detected between them. This indicates that an "automatic" rather than an "evolved" reproductive compensation mechanism may be acting in heterogametic females. A separate study has shown that self-synapsis of both the X and Y* chromosomes takes place during meiosis, allowing the oocytes to escape from functional deterioration. It is suggested that self-synapsis and "automatic" reproductive compensation account for the preservation of fertility in heterogametic females in spite of the heteromorphic sex chromosomes and the early embryo loss they experience. However, these mechanisms do not account for the enhancement of reproductive lifespan. The possibility that an intrauterine position phenomenon is acting in A. azarae is discussed.

X;Y translocation revealed by chromosome microdissection and FISH in fertile XY females in the Brazilian rodent Akodon montensis

In a Brazilian population of the neotropical rodent Akodon montensis we found five sex-reversed XY females. These animals were cytogenetically analyzed by chromosome painting using species-specific DNA probes from the Y chromosome, generated by chromosomal microdissection and subsequent use of the degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR). The results showed a chromosome complement with an apparently normal Y chromosome and an X chromosome carrying a translocation that encompasses a large portion of the Y chromosome (seemingly the entire Y). Ovarian histology suggested that these females are fertile. Amplification of the SRY HMG box sequence by PCR shows that at least one copy of the Sry gene is present in the A. montensis XY females. Based on our findings, we suggest that the breakpoint of the X;Y translocation probably altered an X-linked sex-determining locus (or loci), blocking testicular organogenesis in the XY females. Further studies are necessary to determine the precise location and role of this putative sex-determining chromosomal region. Genetic mechanisms of XY sex reversal in A. montensis populations are discussed.

Seasonal Changes in Microhabitat Use and Niche Overlap between Akodon azarae and Calomys laucha (Rodentia, Muridae) in Agroecosystems of Central Argentina

We examined seasonal changes in microhabitat use by the murid rodents Akodon azarae and Calomys laucha in agroecosystems of the former pampa region of Argentina. In summer, trap data showed that both species had low densities and were almost equally distributed between the mature cropfields and their surrounding weedy borders. Analysis of vegetation at the trap sites revealed no selection for microhabitats at the borders, whereas in the cropfields both species shared preferences for covered microhabitats. In contrast, winter data revealed a sharp habitat segregation, being Calomys numerically dominant in post-harvest cropfields and Akodon more abundant in borders. Moreover, there were clear differences in microhabitat selection at the borders, A. azarae occupying the more covered microhabitats and C. laucha the less covered. Breadth and overlap of niches were calculated using principal component analysis, in order to recognize interspecific competition and its influence on community niche space. Available evidence indicates that the structure of this rodent community is strongly influenced by seasonal changes in habitat stucture and rodent abundance. The socially dominant A. azarae retains the best part of the available niche space, a fact becoming more evident under the harsh winter conditions.

Assessing the impact of owl predation on the growth rate of a rodent prey population

Data from a comparative survey of rodent population dynamics and predation by owls were used to estimate the impact of owl predation on the demography of the sigmodontine rodent Akodon azarae. A stage-structured matrix model based on mass classes was used to estimate rodent demographic parameters through the analysis of field estimated transition matrices. Owl pellet analysis allowed estimation of predation rate upon different mass classes of A. azarae and computation of estimated predation-free transition matrices. Owl predation affected exclusively small individuals of initial body mass stages. Although population growth rate in A. azarae was most sensitive to transitions involving initial stages, exclusive consumption of small individuals by owls did not have a significant impact on population growth rate. Owls preyed upon individuals of A. azarae only slightly, possibly on account of specific features of its habitat use and escape response, which may result in predators not affecting prey abundance or demography. The proposed methods can be of special convenience for endangered or rare vertebrate species in which large-scale experimental manipulations might be questionable.

The rodent. Akodon arviculoides, Wagner, 1842 (Cricetidae)--importance in plague foci in Brazil

The occurrence of the rodent Akodon arviculoides Wagner, 1842 in the plague focus of the "Agreste" region of the State of Pernambuco and a report on its ability for survival, reproduction and development in captivity, its susceptibility to Yersinia pestis infection and the role of this rodent species in Brazilian plague foci are reported.

Temporal variation of allele frequencies in populations of Akodon dolores (Rodentia, Cricetidae)

Six population samples of the South American cricetid rodent Akodon dolores were collected at the same site at six-month intervals over a three year period. Changes in density were detected. Seven out of 18 loci analyzed by means of starch gel electrophoresis were polymorphic. Only two of these loci (Est-4 and G6pdh) showed statistically significant variation in allele frequencies following a seasonal pattern. There was no correlation between allele frequencies and population density. When animals were grouped into two classes according to body weight, a clear difference in allele distribution at the Est-4 and G6pdh loci was observed between individuals 39 g or less and those heavier than 39 g. As the first group comprises predominantly younger animals, the data indicate that changes in the age-structure of population, rather than density variations, are responsible for the cyclic pattern of allele frequencies fluctuations.

Interspecies mouse × Akodon Urichi somatic cell hybrids: Comparative immunogenicity of parental and hybrid cells

Immunizing capacity of an interspecies somatic hybrid cell line, HyRA 4 (resulting from in vitro mating of R4/B malignant cells from the BALB/c mice, with normal embryonic cells of the South American rodent Akodon urichi venezuelensis), was compared with that of the parental R4/B cell line. Both R4/B and HyRA 4 cells displayed cross-reacting tumour specific transplantation antigen(s) and were able to induce a relative transplantation resistance against challenge inoculation of R4/B cells in syngeneic BALB/c mice. However, immunization with non-pretreated HyRA 4 hybrid cells carrying twice the number of R4/B chromosomes was not found to be more efficient than the immunization with X-irradiated R4/B parental cells in terms of the transplantation resistance induction. Furthermore, when mice in spite of pretreatment with either R4/B or HyRA 4 cells failed to reject the challenge graft, the resulting tumours showed enhanced growth rates as compared to those growing in non pretreated control mice. Though the HyRA 4 hybrid cells carried only a reduced number of chromosomes from the parental Akodon cell, they strongly displayed Akodon species specific antigen(s), thus rendering them unable to grow in normal BALB/c mice. In exceptional cases, in some mice in which tumours developed from the challenge inoculum of R4/B cells in spite of previous “immunization” with HyRA 4 cells, a delayed tumour outgrowth at the site of HyRA 4 inoculum was observed. These mouse tumour derivatives of HyRA 4 cells were, however, devoid of any detectable Akodon antigenicity as well as of any identifiable complete Akodon chromosome. This phenomenon resulting in the outgrowth of an antigenically “curtailed” hybrid variant is described and discussed.