Clues on Syntenic Relationship among Some Species of Oryzomyini and Akodontini Tribes (Rodentia: Sigmodontinae)

Sigmodontinae rodents represent one of the most diverse and complex components of the mammalian fauna of South America. Among them most species belongs to Oryzomyini and Akodontini tribes. The highly specific diversification observed in both tribes is characterized by diploid complements, which vary from 2n = 10 to 86. Given this diversity, a consistent hypothesis about the origin and evolution of chromosomes depends on the correct establishment of synteny analyzed in a suitable phylogenetic framework. The chromosome painting technique has been particularly useful for identifying chromosomal synteny. In order to extend our knowledge of the homeological relationships between Akodontini and Oryzomyini species, we analyzed the species Akodon montensis (2n = 24) and Thaptomys nigrita (2n = 52) both from the tribe Akodontini, with chromosome probes of Hylaeamys megacephalus (2n = 54) of the tribe Oryzomyini. The results indicate that at least 12 of the 26 autosomes of H. megacephalus show conserved synteny in A. montensis and 14 in T. nigrita. The karyotype of Akodon montensis, as well as some species of the Akodon cursor species group, results from many chromosomal fusions and therefore the syntenic associations observed probably represent synapomorphies. Our finding of a set of such associations revealed by H. megacephalus chromosome probes (6/21; 3/25; 11/16/17; and, 14/19) provides phylogenetic information for both tribes. An extension of these observations to other members of Akodontini and Oryzomyini tribes should improve our knowledge about chromosome evolution in both these groups.     

Extensive genomic reshuffling involved in the karyotype evolution of genus Cerradomys (Rodentia: Sigmodontinae: Oryzomyini)

Rodents of the genus Cerradomys belong to the tribe Oryzomyini and present high chromosome variability with diploid numbers ranging from 2n=46 to 60. Classical cytogenetics and fluorescence in situ hybridization (FISH) with telomeric and whole chromosome-specific probes of another Oryzomyini, Oligoryzomys moojeni (OMO), were used to assess the karyotype evolution of the genus. Results were integrated into a molecular phylogeny to infer the hypothetical direction of chromosome changes. The telomeric FISH showed signals in telomeres in species that diverged early in the phylogeny, plus interstitial telomeric signals (ITS) in some species from the most derived clades (C. langguthi, C. vivoi, C. goytaca, and C. subflavus). Chromosome painting revealed homology from 23 segments of C. maracajuensis and C. marinhus to 32 of C. vivoi. Extensive chromosome reorganization was responsible for karyotypic differences in closely related species. Major drivers for genomic reshuffling were in tandem and centric fusion, fission, paracentric and pericentric inversions or centromere repositioning. Chromosome evolution was associated with an increase and decrease in diploid number in different lineages and ITS indicate remnants of ancient telomeres. Cytogenetics results corroborates that C. goytaca is not a junior synonym of C. subflavus since the karyotypic differences found may lead to reproductive isolation.     

Phylogeny and biogeography of tetralophodont rodents of the tribe Oryzomyini (Cricetidae: Sigmodontinae)

Oryzomyini is the richest tribe among the Sigmodontine rodents, encompassing 32 living and extinct genera and including an increasing number of recently described species and genera. Some Oryzomyini are tetralophodont showing a reduction in the number of molar folds to four, while most taxa in this tribe retain the plesiomorphic pentalophodont state. We applied phylogenetic methods, molecular dating techniques and ancestral area analyses to members of an oryzomyini clade informally named ‘D’ in former studies and included related fossil tetralophodont forms. Based on 98 morphological characters and sequences of five gene fragments, we found that the tetralophodont condition is paraphyletic. Among living taxa, Pseudoryzomys is sister to Holochilus, and Lundomys is derived from a basal divergence. A clade formed by living Holochilus and the fossils Noronhomys and Carletonomys is sister to Holochilus primigenus, making Holochilus paraphyletic. Therefore, we describe a new genus that accommodates the fossil H. primigenus. Because trans‐Andean taxa currently share a common ancestor with taxa of cis‐Adean distribution, the northern Andes uplift may have worked as a postdispersal barrier. The tetralophodont lineages diverged during the Pliocene from a cis‐Andean ancestor, and the Great Plains in South America may have favoured the diversification of tetralophodont forms adapted to open habitats during the Pliocene.     

Comparative Chromosome Painting in Six Species of Oligoryzomys (Rodentia,Sigmodontinae) and the Karyotype Evolution of the Genus

Oligoryzomys belongs to the tribe Oryzomyini, and contains about 22 species. Diploid numbers range from 2n = 44 in Oligoryzomys sp. 2 to 2n = 72 in O. utiaritensis and phylogenetic relationships are not well defined. The high morphological convergence leads to misidentification of taxonomic entities and the species are often identified by chromosomal characters. Until now, the genus has been studied only by classical cytogenetic approaches. To understand the chromosomal evolution of Oligoryzomys, we developed chromosome probes from a female of Oligoryzomys moojeni (OMO) with 2n = 70 and hybridized to other five Oligoryzomys species. The probes painted 31 segments on O. fornesi (OFO) with 2n = 62; 32 segments on O. microtis (OMI), 2n = 64; 33 segments on O. nigripes (ONI), 2n = 62 and on O. rupestris (ORU), 2n = 46; and 34 on Oligoryzomys sp. 2 (OSP), 2n = 44. OMO probes 4 and 5 showed a syntenic association in O. fornesi, O. microtis and O. nigripes and were also presented in the same pair, although disrupted, in O. rupestris and Oligoryzomys sp. 2. Concerning O. rupestris and Oligoryzomys sp. 2, species with the lowest diploid numbers of the genus, a total of 8 probes hybridized to 11 segments on the largest pair of ORU 1 and 9 probes hybridized to 12 segments on OSP 1. Also, OMO 6 painted three segments in ORU, corresponding to the proximal segment of ORU 2q, and the whole of ORU 19 and 20. In OSP, the segment corresponding to ORU 20 was homologous to OSP 1p. OMO X showed signals of hybridization in both X and Y chromosomes. Extensive chromosomal rearrangements, that could not be detected by classical cytogenetic techniques, such as pericentric inversions or repositioning of centromeres, Robertsonian rearrangements and tandem fusions/fissions, as well as gain/activation or loss/inactivation of centromeres and telomeric sequences have driven the huge genome reshuffling in these closely related species.     

Genetic studies in representatives of genusRhipidomys (Rodentia, Sigmodontinae) from Brazil

Karyotypic polymorphism of five taxa of the rodent genusRhipidomys from the Brazilian Amazon and Cerrado biomes was analysed.Rhipidomys nitela Thomas, 1901 from Amazon has 2n=48, FN=68. The other species, all have 2n=44, but can be separated into two groups, one with high FNs (76, 80) and the other with low FNs (48, 52). Two cytotypes ofR. mastacalis (Lund, 1840) with high FNs were trapped in four localities of the Cerrado, showing 19 and 17 biarmed autosomes, respectively. A low FN (48) was observed inR. leucodactylus (Tschudi, 1844) in two localities of the Cerrado and FN=52 in one locality in the Cerrado and the Amazon. All taxa with 2n=44 have a medium-sized acrocentric X chromosome and a small Y.Rhipidomys nitela is different from the species with 2n=44 by presenting a heterochromatic short arm of the X chromosome. In all karyotypes analysed, the nucleolus organizer regions were located in the short arms of two to six pairs and the (T₂AG₃)_n telomeric probes hybridizedin situ in both the short and long arms of all pairs of the karyotypes.     

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.     

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.     

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.     

A new genus and species of rodent from the Brazilian Atlantic Forest (Rodentia: Cricetidae: Sigmodontinae: Oryzomyini), with comments on oryzomyine biogeography

We describe in this paper a new genus and species of cricetid rodent from the Atlantic Forest of Brazil, one of the most endangered eco‐regions of the world. The new form displays some but not all synapomorphies of the tribe Oryzomyini, but a suite of unique characteristics is also observed. This new forest rat possesses anatomical characteristics of arboreal taxa, such as very developed plantar pads, but was collected almost exclusively in pitfall traps. Phylogenetic analyses of morphological (integument, soft tissue, cranial, and dental characters) and molecular [nuclear – Interphotoreceptor retinoid binding protein (Irbp) – and mitochondrial – cytochrome b– genes] datasets using maximum likelihood and cladistic parsimony approaches corroborate the inclusion of the new taxon within oryzomyines. The analyses also place the new form as sister species to Eremoryzomys polius, an Andean rat endemic to the Maranon valley. This biogeographical pattern is unusual amongst small terrestrial vertebrates, as a review of the literature points to few other similar examples of Andean–Atlantic Forest pairings, in hylid frogs, Pionus parrots, and other sigmodontine rodents. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161 , 357–390.     

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.     

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.     

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.     

Chromosomes in a hybrid zone of Israeli mole rars (Spalax, Rodentia)

Chromosomal novelties and the level of meiotic and mitotic abnormalities were studied in a hybrid zone between two chromosomally differentiated Spalax cytotypes of 2n = 58 and 2n = 52. These cytotypes differ by five Rb fusions, four centromeric shifts accompanied by heterochromatin deletion, one paracentric inversion, and the Y-chromosome reorganization. Among 149 specimens studied, 82 were hybrids with 64 different karyotypes ranging in diploid numbers from 2n = 50 to 2n = 60. Nine hybrid specimens were mosaics for the chromosome numbers due to occurrence of cell lines with different Robertsonian chromosome arrangements, and six specimens possessed variable number of B-chromosomes. Mosaicism of B-chromosomes was found also in meiotic cells however chromatid breaks and abnormal chromosome pairing during meiosis occurred very rarely. All these results imply some local genomic instability resulting in the spontaneous process of reversible Rb fusions.