A new karyotype of Oligoryzomys (Sigmodontinae, Rodentia) from central Brazil

A new karyotype of the genus Oligoryzomys was described for specimens collected in Brazilian Cerrado in Tocantins and Goiás States. Conventional staining, G-, C-banding, and Ag-NOR staining techniques were used for describe this karyotype with 2n=70, AN=74 or 76. The chromosome complement, with the highest diploid number known among Oligoryzomys species, differs from all others previously reported. This small sized Oligoryzomys species can be differentiated from other Brazilian Oligoryzomys not only by chromosomal complement, but also in some morphological attributes. The new species is apparently restricted in distribution and endemic of Brazilian Cerrado, occurring only in Rio Tocantins basin.     

Chromosome relationships in three representatives of the genusHolochilus (Rodentia,Cricetidae) from Brazil

The G- and C-banded karyotype ofHolochilus brasiliensis collected in central Brazil (2n=55, AN=56, acrocentric X and Y) can be regarded as the most representative of the ancestral form of the genus.H. magnus (2n=52, AN=58, acrocentric X and Y) andH. brasiliensis vulpinus (2n=40, AN=56), both living in southern Brazil, would be in different phases of chromosome number reduction due to centric fusions.H. magnus, in addition, shows an X of variable morphology and a metacentric Y. This fact, coupled with its distinct morphology and restricted distribution, suggests that it may represent a distinct trend in the genus' main evolutionary line.     

Description of the karyotype of Rhagomys rufescens Thomas, 1886 (Rodentia, Sigmodontinae) from Southern Brazil Atlantic forest

Rhagomys rufescens (Rodentia: Sigmodontinae) is an endemic species of the Atlantic forest from Southern and Southeastern Brazil. Some authors consider Rhagomys as part of the tribe Thomasomyini; but its phylogenetic relationships remain unclear. Chromosomal studies on eight specimens of Rhagomys rufescens revealed a diploid number of 2n = 36 and a number of autosome arms FN = 50. GTG, CBG and Ag-NOR banding and CMA(3) /DAPI staining were performed on metaphase chromosomes. Eight biarmed and nine acrocentric pairs were found in the karyotype of this species. The X and Y chromosomes were both acrocentric. Most of the autosomes and the sex chromosomes showed positive C-bands in the pericentromeric region. The X chromosome showed an additional heterochromatic block in the proximal region of the long arm. Nucleolus organizer regions (NORs) were located in the pericentromeric region of three biarmed autosomes (pairs 4, 6 and 8) and in the telomeric region of the short arm of three acrocentrics (pairs 10, 12 and 17). CMA (3) /DAPI staining produced fluorescent signals in many autosomes, especially in pairs 4, 6, and 8. This study presents cytogenetic data of Rhagomys rufescens for the first time.     

Genetic studies in the recently divergent Eligmodontia puerulus and E. moreni (Rodentia,Cricetidae, Sigmodontinae) from Puna and Monte deserts of South America

Eligmodontia is a genus of phyllotine rodents adapted to arid environments with seven recognized species. The sister species E. puerulus and E. moreni are distributed in the adjacent highland Puna and lowland Monte deserts respectively, and show remarkable morphological and chromosomal differences. However, analyses of the cytochrome b gene showed important variability, without reciprocal monophyly between them. In order to study the evolutionary processes involved in the diversification of both taxa, we analyzed 1161 bp of the mitochondrial control region and flanking sequences (N = 60), as well as 759 bp of the first exon of the nuclear gene IRBP (N = 14). Individuals of both species from Jujuy, Catamarca and Mendoza Provinces of Argentina were previously karyotyped. Results showed that the mitochondrial sequences present high haplotype and nucleotide diversity within all population, and no haplotype was shared between both species. F_ST indicated that populations of both species were moderately structured. The network was constituted by two major haplogroups, one composed by E. puerulus samples from Jujuy, and the other composed of sequences of all studied populations. The Bayesian analysis showed three clusters, matching the network. Phylogenetic analysis recovered two clades with high support, in coincidence with the network groups. There was only one close join between sequences of both species, corresponding to samples from Catamarca. Thus, mitochondrial data suggested hybridization between both species in Catamarca, with asymmetric introgression. The IRBP showed low variability and, in the phylogenetic analysis, the sequences of E. puerulus form a monophyletic group with intermediate support, whereas those of E. moreni collapse into a basal polytomy. Our data indicated a recent divergence and absence of introgression in the nuclear genomes. The results at the population level with mitochondrial sequences, together with integrative taxonomy at the species level in a biogeographic context, suggest that climatic and geologic changes could have had an important role in the determination of genetic variability patterns observed in these rodents.     

Biochemical Polymorphisms and Genetic Relationships in Rodents of the Genera Oryzomys and Oligoryzomys (Sigmodontinae) from Brazil

An electrophoretic analysis of 12 allozyme systems (14 loci, 40 alleles) was performed in the rodent genera Oryzomys and Oligoryzomys, in order to determine the levels of genetic variability within and among populations and species. One hundred and fifty-five individuals from 16 populations of Oryzomys russatus, Or. angouya, Oligoryzomys flavescens, and O1. nigripes species were trapped in nine Brazilian localities. Genetic divergence among populations, as well as the interpopulational gene diversity, was higher in Oryzomys than in Oligoryzomys. The dendrogram of the phenetic relationships among the 16 populations of these four species displays three clusters: the first one joins the O1. flavescens populations with those of O1. nigripes; the second groups the populations of Or. russatus, and the last assembles the populations of Or. angouya. The genetic parameters analyzed reveal that the species belonging to the genus Oryzomys are genetically more structured than those of Oligoryzomys, the latter presenting lower levels of intrademe genetic differentiation (G(ST)')     

Chromosomal localization of telomeric sequences in three species of Akodon (Rodentia, Sigmodontinae)

The distribution of the vertebrate telomeric sequence T2AG3 in three species of the rodent genus Akodon was examined by FISH with a peptide nucleic acid probe. In addition to the expected telomeric hybridization, non-telomeric signals were observed in the three species. In A. dolores, centromeric signals were visible in two of the four biarmed autosome pairs featuring Robertsonian polymorphism, indicating the retention of at least part of the telomeric sequences during the fusion process, and an interstitial signal of lower intensity was observed in the short arm of another. In A. boliviensis, a strong signal was observed near the centromeric end of the first chromosome pair. The first pair of A. azarae (homologous to the first pair of A. boliviensis) showed a similar but markedly amplified signal, and a subcentromeric signal in the X chromosome corresponding to a heterochromatic region; additionally, interstitial signals of lower intensity were present in one to four chromosomes in the majority of cells examined.     

Genetic and morphological variability in South American rodent Oecomys (Sigmodontinae, Rodentia): evidence for a complex of species

The rodent genus Oecomys (Sigmodontinae) comprises ~16 species that inhabit tropical and subtropical forests in Central America and South America. In this study specimens of Oecomys paricola Thomas, 1904 from Belém and Marajó island, northern Brazil, were investigated using cytogenetic, molecular and morphological analyses. Three karyotypes were found, two from Belém (2n = 68, fundamental number (FN) = 72 and 2n = 70, FN = 76) and a third from Marajó island (2n = 70, FN = 72). No molecular or morphological differences were found between the individuals with differing cytotypes from Belém, but differences were evident between the individuals from Belém and Marajó island. Specimens from Belém city region may represent two cryptic species because two different karyotypes are present in the absence of significant differences in morphology and molecular characteristics. The Marajó island and Belém populations may represent distinct species that have been separated for some time, and are in the process of morphological and molecular differentiation as a consequence of reproductive isolation at the geographic and chromosomal levels. Thus, the results suggest that O. paricola may be a complex of species.     

Trichostrongylina (Nematoda) parasitic in Phyllotis sp. (Rodentia: Sigmodontinae) from Argentina, with description of three new species

The Trichostrongylina parasitic in the leaf-eared mouse Phyllotis sp. (Sigmodontinae) from the Province of Catamarca, Argentina are studied. Three new species of Stilestrongylus Freitas, Lent and Almeida, 1937 (Heligmosomoidea: Heligmonellidae: Nippostrongylinae) are described. Stilestrongylus andalgala n. sp. is distinguished from the most closely related species S. moreli Diaw, 1976 parasitic in Auliscomys boliviensis from Bolivia, by the absence of a common trunk of right rays 2 and 3, by left ray 4 shorter than ray 5, and by the shape of the female tail, without a mucron. Stilestrongylus catamarca n. sp. is distinguished from all the other species in the genus by hypertrophied rays 2, larger than rays 3, and from the closely related species Stilestrongylus barusi Durette-Desset, 1971, parasitic in Sigmodontomys alfari from Colombia, by right ray 4 longer than ray 5 and left ray 4 shorter than ray 5, by a larger spicule length/body length ratio (22.6% vs. 7.2%), and by the presence of a mucron on the female tail. Stilestrongylus gracielae n. sp. most closely resembles Stilestrongylus azarai Durette-Desset and Sutton, 1985, parasitic in Akodon azarae and in Graomys griseoflavus, and Stilestrongylus franciscanus Digiani and Durette-Desset, 2003, also parasitic in G. griseoflavus, both from Argentina. These latter differ from the new species by the right lobe of the caudal bursa: in S. azarai rays 4, 5 and 6 arise at same level from their common trunk and in S. franciscanus right rays 4 and 5 diverge at their distal extremity. Lamanema chavezi Becklund, 1963 (Molineoidea: Molineinae), a parasite of South American camelids, is reported in rodents for the second time.     

Chromosomal aspects and inheritance of the XY female condition in Akodon azarae (Rodentia,Sigmodontinae)

The populations of several species of Akodon present, besides XX females, a variable proportion of fertile XY females. In Akodon azarae, a correspondence exists between the X-chromosome C-banding pattern and the sexual phenotype of XY individuals: males carry a determinate X-chromosome type, defined by its C-banding pattern, and XY females, any of two others. To confirm the relation between X-chromosome type and the XY female condition and to investigate the hereditary transmission of these different X-chromosomes, we analyzed 50 animals captured in the field and 95 individuals corresponding to the F₁ and F₂ offspring of 16 crosses.It was seen that the correlation between X type and the sexual phenotype of XY animals is retained, and that the three X types are transmitted to the progeny. It was also observed that the male offspring of XY females receive the X-chromosome from their male parents and the Y from their mothers. These results strongly support the causal role of an X-borne mutation in A. azarae XY sex reversal, and discard a mutation of the Y-chromosome as the sole basis of this phenomenon.     

A new Heligmonellid (Nematoda: Heligmonellidae) from Oecomys mamorae (Rodentia: Sigmodontinae) in the pantanal and new data on the synlophe of Guerrerostrongylus zetta from the Atlantic Forest, Brazil

A new species of Heligmonellidae nematode, Guerrerostrongylus gomesae n. sp., is described from specimens collected from the small intestine of the rodent Oecomys mamorae Thomas, 1906 in the Brazilian Pantanal. It differs from the 2 other species of the genus by the number of ridges in the synlophe 35-46 and 40-48 at the midbody in the male and female, respectively, by rays 8 arising at midlength of the dorsal trunk, ending near the margin of the caudal bursa, and the dorsal ray divided at the second-third part into 2 branches, each branch divided into 2 subequal subbranches. In addition, specimens of Guerrerostrongylus zetta Travassos, 1937 collected in Oligoryzomys nigripes Olfers, 1818 contain synlophe with 36-42 cuticular ridges in males and 38-42 in females at the midbody. By the characteristics of the synlophe and caudal bursa, Gerrerostrongylus gomesae n. sp. is considered a new species. We suggest that the number of cuticular ridges for this genus broadens the range of the cuticular ridges to at least 35 at midbody.     

Reproduction, development and growth of Akodon lindberghi (Hershkovitz, 1990) (Rodentia, Muridae, Sigmodontinae) raised in captivity

The reproduction, development and growth of Akodon lindberghi were studied in captivity. The colony was derived from animals captured in Sim?o Pereira, Minas Gerais state, which represents a new area of geographical distribution known for this species. Twelve males and twelve females were crossed, producing 144 young in 53 litters. Post-partum oestrus was observed and gestation length was estimated in 23 days. Litter size ranged from 1 to 4 with a mean of 2.72 (SD = 0.97, n = 53) and modal size of 3. Sexual dimorphism was neither present in body mass at birth nor at weaning. There was a significant negative correlation between litter size and mass at birth or weaning. Permanent emergence of adult external appearance occurred at 15 days. Puberty for males and females was 43 and 42 days, respectively, and the first fecundation event for two females was recorded at 47 and 54 days of age. The weight growth was described by fitting a Gompertz model. No significant difference was found in any parameter of growth curves for males and females. Measurements (head-body, tail, hind foot and internal and external ear lengths) obtained for adult individuals also did not reveal the presence of sexual dimorphism.     

Use of space by Necromys lasiurus (Rodentia,Sigmodontinae) in a grassland among Atlantic Forest fragments

The spatial patterns presented by the rodent Necromys lasiurus were studied through capture-mark-recapture in a grassland among Atlantic Coastal Forest fragments in Southeastern Brazil. Males moved farther than females. Males moved similar distances in the dry and wet seasons, whereas females moved significantly longer distances in the dry season. Home ranges sizes were estimated for 32 individuals, and varied from 0.02 to 0.52 ha. Males had larger home ranges than females when all data are pooled and in the wet season; in the dry season home ranges of both sexes were of similar sizes. Home range sizes did not vary significantly between seasons for either sex. Home ranges of males overlapped extensively among themselves and with females, whereas females showed mutually exclusive home ranges. The spatial patterns described here are consistent with the results found for this species in different habitats and they suggest that N. lasiurus has a promiscuous mating system.     

Inheritance of litter size at birth in the Brazilian grass mouse (Akodon cursor,Sigmodontinae, Rodentia)

By means of complex segregation analysis we studied the inheritance of litter size in two large pedigrees of captive-bred colonies of the Brazilian grass mouse Akodon cursor. Genetic analysis has revealed a highly significant influence of genetic factors on the variation of litter size (heritability, h2, was estimated as 0.44). The inheritance followed the classical polygene model: neither the major-gene model nor the polygene with unequal contribution model described the data significantly better.     

Diversification of the climatic niche drove the recent processes of speciation in Sigmodontinae (Rodentia,Cricetidae)

Evolutionary radiations are among the most intriguing natural phenomena. Sigmodontine rodents form a megadiverse group for which doubts exist about the adaptive or non‐adaptive nature of its radiation. We analysed whether or not the rates of diversification of species of Sigmodontinae are related to the rates of diversification of the climatic niches occupied by the species. Our results show a clear association between niche diversification and speciation processes. However, this association is linked to recent and independent processes of diversification in sigmodontines, as opposed to an early link that would indicate a niche‐filling consistent with an adaptive radiation of the subfamily.