Testicular histomorphometric patterns and spermatogenesis dynamics of Oecomys bicolor tomes, 1860 (Rodentia: Cricetidae)

Although the order Rodentia does not present a high risk of extinction compared to mammals as a whole, several families demonstrate high levels of threat and/or data deficiency, therefore highlighting the need for targeted research and the application of ecological and reproductive data to the development of conservation actions. The order Rodentia, the largest among mammals, includes 9 families, and the family Cricetidae is the most diverse of the Brazilian rodents. In Brazil, 12 of the 16 genera of Oecomys are found. Oecomys bicolor is known in Brazil as the 'arboreal rat' and is, found in dry, deciduous and tropical forests. The mean body weight of Oecomys bicolor was 35.8 g and the gonadal, tubular and epithelial somatic indexes were, 0.53%, 0.47% and 0.37%, respectively. Seminiferous tubules volume density was 89.72% and the mitotic and meiotic indexes corresponded to 8.59 and 2.45 cells, respectively, and the yield of spermatogenesis was 23.83 cells. The intertubular compartment represented 10.28% of the testis parenchyma and around 5% of the interstitial space was occupied by Leydig cells, whose number per gram of testis was 11.10 × 107 cells. By evaluating the biometric and histomorphometric characteristics of the testis, there is evidence that this species has a high investment in reproduction. Due to the high contribution of the seminiferous epithelium and the intertubular compartment in this species, compared to the others of the same family, it is possible to infer that the species Oecomys bicolor has a promiscuous reproductive behaviour.

The emergence of a new sex-system (XX/XY1Y2) suggests a species complex in the "monotypic" rodent Oecomys auyantepui (Rodentia, Sigmodontinae)

X-autosome translocation (XY₁Y₂) has been reported in distinct groups of vertebrates suggesting that the rise of a multiple sex system within a species may act as a reproductive barrier and lead to speciation. The viability of this system has been linked with repetitive sequences located between sex and autosomal portions of the translocation. Herein, we investigate Oecomys auyantepui, using chromosome banding and Fluorescence In Situ Hybridization with telomeric and Hylaeamys megacephalus whole-chromosome probes, and phylogenetic reconstruction using mtDNA and nuDNA sequences. We describe an amended karyotype for O. auyantepui (2n = 64♀65♂/FNa = 84) and report for the first time a multiple sex system (XX/XY₁Y₂) in Oryzomyini rodents. Molecular data recovered O. auyantepui as a monophyletic taxon with high support and cytogenetic data indicate that O. auyantepui may exist in two lineages recognized by distinct sex systems. The Neo-X exhibits repetitive sequences located between sex and autosomal portions, which would act as a boundary between these two segments. The G-banding comparisons of the Neo-X chromosomes of other Sigmodontinae taxa revealed a similar banding pattern, suggesting that the autosomal segment in the Neo-X can be shared among the Sigmodontinae lineages with a XY₁Y₂ sex system.

A new species of Mindomys (Rodentia, Cricetidae) with remarks on external traits as indicators of arboreality in sigmodontine rodents

The diversity of the oryzomyine rat Mindomys (Cricetidae, Sigmodontinae, Oryzomyini), is doubled here with the description of a new species from the remote Cordillera de Kutukú (Ecuador). The novel form can be easily differentiated from Mindomys hammondi –type species of the genus– by a large set of anatomical traits including, among others, larger jugals, parietal “wings” extending to zygomatic roots, larger otic capsules, well-exposed petrosals, narrow zygomatic plates almost without upper free borders, foramen magnum caudally oriented, larger molars, and accessory root of first upper molar present. Until now, the records of Mindomys were restricted to western Andean foothills. The material from Kutukú highlights an Amazonian species and reinforces the valuable biological significance of isolated mountain ranges in eastern Ecuador. Since Mindomys shows some external traits classically related to arboreal life, here we present a brief reappraisal of this poorly explored topic. A partially neglected anatomical system in sigmodontine studies, the fore feet, encloses crucial information reflecting arboreality.

Integrative analysis supports a new species of the Oecomys catherinae complex (Rodentia, Cricetidae) from Amazonia

The sigmodontine rodent Oecomys catherinae has been recognized as a species complex that includes five different lineages distributed throughout Brazil south of the Amazon river, named in literature according to their geographic position (central, eastern, northern, western, and westernmost). The western lineage also has been referred as Oecomys aff. catherinae in the literature, because it may represent a distinct species. Herein, we used molecular and morphological data to investigate the possible species distinctiveness within the O. catherinae complex. From molecular analyses, we recovered the western lineage as a monophyletic group with high genetic divergence from the other lineages of the complex. This lineage can be differentiated from other lineages of the O. catherinae complex and other congeners through a combination of mensural and morphological characters that includes: smaller size; posterior margin of nasals usually surpassing the maxillary–frontal suture; supraorbital crest moderately developed, similar to the temporal crest; parietal slightly expanded laterally; subsquamosal fenestra present; alisphenoid strut absent; and anterior cingulum of M1 present. The western lineage occurs in the southern Amazonia, in sympatry with the westernmost lineage and other species of Oecomys. Herein we describe this lineage as a new species, increasing to 19 the number of species within Oecomys.

A new genus of oryzomyine rodents (Cricetidae, Sigmodontinae) with three new species from montane cloud forests, western Andean cordillera of Colombia and Ecuador

The Andean cloud forests of western Colombia and Ecuador are home to several endemic mammals; members of the Oryzomyini, the largest Sigmodontinae tribe, are extensively represented in the region. However, our knowledge about this diversity is still incomplete, as evidenced by several new taxa that have been described in recent years. Extensive field work in two protected areas enclosing remnants of Chocó montane forest recovered a high diversity of small mammals. Among them, a medium-sized oryzomyine is here described as a new genus having at least three new species, two of them are named and diagnosed. Although externally similar to members of the genera Nephelomys and Tanyuromys, the new genus has a unique molar pattern within the tribe, being characterized by a noticeable degree of hypsodonty, simplification, lamination, and third molar compression. A phylogeny based on a combination of molecular markers, including nuclear and mitochondrial genes, and morphological data recovered the new genus as sister to Mindomys, and sequentially to Nephelomys. The new genus seems to be another example of a sigmodontine rodent unique to the Chocó biogeographic region. Its type species inhabits cloud forest between 1,600 and 2,300 m in northernmost Ecuador (Carchi Province); a second species is restricted to lower montane forest, 1,200 m, in northern Ecuador (Imbabura Province); a third putative species, here highlighted exclusively by molecular evidence from one immature specimen, is recorded in the montane forest of Reserva Otonga, northern Ecuador (Cotopaxi Province). Finally, the new genus is also recorded in southernmost Colombia (Nariño Department), probably represented there also by a new species. These species are spatially separated by deep river canyons through Andean forests, resulting in marked environmental discontinuities. Unfortunately, Colombian and Ecuadorian Pacific cloud forests are under rapid anthropic transformation. Although the populations of the type species are moderately abundant and occur in protected areas, the other two persist in threatened forest fragments.

Karyotypic divergence reveals that diversity in the Oecomys paricola complex (Rodentia, Sigmodontinae) from eastern Amazonia is higher than previously thought

The genus Oecomys (Rodentia, Sigmodontinae) is distributed from southern Central America to southeastern Brazil in South America. It currently comprises 18 species, but multidisciplinary approaches such as karyotypic, morphological and molecular studies have shown that there is a greater diversity within some lineages than others. In particular, it has been proposed that O. paricola constitutes a species complex with three evolutionary units, which have been called the northern, eastern and western clades. Aiming to clarify the taxonomic status of O. paricola and determine the relevant chromosomal rearrangements, we investigated the karyotypes of samples from eastern Amazonia by chromosomal banding and FISH with Hylaeamys megacephalus (HME) whole-chromosome probes. We detected three cytotypes for O. paricola: A (OPA-A; 2n = 72, FN = 75), B (OPA-B; 2n = 70, FN = 75) and C (OPA-C; 2n = 70, FN = 72). Comparative chromosome painting showed that fusions/fissions, translocations and pericentric inversions or centromeric repositioning were responsible for the karyotypic divergence. We also detected exclusive chromosomal signatures that can be used as phylogenetic markers. Our analysis of karyotypic and distribution information indicates that OPA-A, OPA-B and OPA-C are three distinct species that belong to the eastern clade, with sympatry occurring between two of them, and that the “paricola group” is more diverse than was previously thought.

A revision of Neusticomys peruviensis (Rodentia: Cricetidae) with the description of a new subspecies

Neusticomys peruviensis is a poorly known sigmodontine rodent of the tribe Ichthyomyini, represented in collections by only five specimens collected in five localities from lowland forests of central and southern Peru. Recent expeditions in Llanchama, in northern Peru, north of the Río Amazonas, and near Allpahuayo Mishana Natural Reserve (Loreto, Peru), were successful in obtaining three specimens of Neusticomys. Based on morphological and meristic data, we found the population at Llanchama is distinct from the allopatric populations of N. peruviensis, and other species of Neusticomys. A species distribution model also shows the population at Llanchama is not highly predicted by the set of variables of the known localities of N. peruviensis. However, sequence data from the mitochondrial cytochrome-b gene indicate that genetic distinctiveness is low. Because intraspecific variability is important to understand evolutionary and biogeographic processes, and in concordance with the polytypic species concept, we interpret the population at Llanchama to represent a new subspecies of N. peruviensis that we describe in this paper.

Discovery of a new genus record for Paraguay, the Atlantic Forest endemic rodent Abrawayaomys (Cricetidae, Sigmodontinae)

The mammals of Paraguay are still poorly known. We report a new genus for the country, Abrawayaomys, a spiny cricetid rodent endemic for the Interior Atlantic Forest. This record extends the distribution of the genus approximately 100 km, and west of the Paraná River. This addition increases the number of mammals in Paraguay to 184 and verifies that rodents are the most diverse group of mammals in the country. We outline identifying morphological characters of Abrawayaomys and elaborate on the significance of this finding for conservation and biogeography in the region.

Advances in cytogenetics of Brazilian rodents: cytotaxonomy, chromosome evolution and new karyotypic data

Rodents constitute one of the most diversified mammalian orders. Due to the morphological similarity in many of the groups, their taxonomy is controversial. Karyotype information proved to be an important tool for distinguishing some species because some of them are species-specific. Additionally, rodents can be an excellent model for chromosome evolution studies since many rearrangements have been described in this group.This work brings a review of cytogenetic data of Brazilian rodents, with information about diploid and fundamental numbers, polymorphisms, and geographical distribution. We point out that, even with the recent efforts on cytogenetic studies in this group, many species lack karyotypic data. Moreover, we describe for the first time the karyotype of Carterodon sulcidens (Lund, 1838) (Family Echimyidae), a new fundamental number for an undescribed species of Neacomys Thomas, 1900 (Family Cricetidae, Subfamily Sigmodontinae), and illustrate the karyotype of a Brazilian specimen of Mus musculus Linnaeus, 1758 (Family Muridae). This review compiles the cytogenetic data on Brazilian rodents reported in the last three decades, after the last revision published in 1984, including synonyms, chromosomal variations, and geographic distribution. Additionally, it also reinforces that Brazilian biodiversity is still poorly known, considering the new data reported here.

How many species of mammals are there in Brazil? New records of rare rodents (Rodentia: Cricetidae: Sigmodontinae) from Amazonia raise the current known diversity

BACKGROUND

Since 1996, when Vivo questioned how many species of mammals occur in Brazil, there has been a huge effort to assess this biodiversity. In this contribution, we present new records for rare species of the sigmodontine rodent genera Rhagomys and Neusticomys previously unknown to Brazilian Amazon. We provided detailed information on the morphologic variation to allow the proper identification of these species. We also furnished updated information on their collection, aiming to establish hypothesis of their geographic distribution, based on SDM's, aiming to hypothesize potential occurrence areas for these species.

METHODS

Rodent specimens were sampled in separate inventories in two sites of Rondônia State (Hydroelectric Dam Jirau and Parque Nacional de Pacaás Novos) and one site in Pará State (Pacajá), Brazil, and were compared to specimens from museum collections to apply appropriate names. The SDM were conducted using two algorithms for rare species, MaxEnt and randomForest (RF), and were based on seven localities for Rhagomys, and 10 for Neusticomys.

RESULTS

All specimens were collected with pitfall traps. One specimen of genus Rhagomys was trapped in the Hydroelectric Dam Jirau. We identified this specimen as R. longilingua, and the SDM species indicates suitable areas for its occurrence at high elevations near on the Andes and lowlands of Amazon Basin to the South of the Rio Amazonas. Two specimens of Neusticomys were recorded, and we identified the specimen from Pacaás Novos as N. peruviensis, with SDM suggesting main areas of occurrence on Western Amazon. We applied the name N. ferreirai to the specimen from Pacajá, with SDM recovering suitable areas in Eastern Amazon.

DISCUSSION

We reinforced the importance of pitfall traps on the study of Neotropical rodents. We described morphologic variation within and among all species that do not invalidate their specific status, but in the near future a re-evaluation will be mandatory. The new records extended the species distribution considerably. SDM was successful to predict their distributions, as the two algorithms presented important differences in range size recovered by the models that can be explained by differences in the thresholds used for the construction of the models. Most suitable areas coincide with the areas facing most of the deforestation in Amazon. We added two rare species of sigmodontine rodents to the list of Brazilian Mammals, which now comprises 722 species (or 775 valid nominal taxa). Although more information is available than in 1996, it is essential that mammal experts maintain inventory and revisionary programs to update and revise this information. This is even more important, as changes in Brazilian environmental legislation are being discussed, suggesting reduced need for environmental impact reports prior to beginning commercial enterprises, resulting in the loss of information about native biodiversity in the affected areas.