Systematics of Neotropical Spiny Mice, Genus Neacomys Thomas, 1900 (Rodentia: Cricetidae), from Southeastern Amazonia, with Descriptions of Three New Species

Chromosomal rearrangements played an important role in the speciation of rice rats of genus Cerradomys (Rodentia, Sigmodontinae, Oryzomyini)

Rodents of the genus Cerradomys belong to tribe Oryzomyini, one of the most diverse and speciose groups in Sigmodontinae (Rodentia, Cricetidae). The speciation process in Cerradomys is associated with chromosomal rearrangements and biogeographic dynamics in South America during the Pleistocene era. As the morphological, molecular and karyotypic aspects of Myomorpha rodents do not evolve at the same rate, we strategically employed karyotypic characters for the construction of chromosomal phylogeny to investigate whether phylogenetic relationships using chromosomal data corroborate the radiation of Cerradomys taxa recovered by molecular phylogeny. Comparative chromosome painting using Hylaeamys megacephalus (HME) whole chromosome probes in C. langguthi (CLA), Cerradomys scotii (CSC), C. subflavus (CSU) and C. vivoi (CVI) shows that karyotypic variability is due to 16 fusion events, 2 fission events, 10 pericentric inversions and 1 centromeric repositioning, plus amplification of constitutive heterochromatin in the short arms of the X chromosomes of CSC and CLA. The chromosomal phylogeny obtained by Maximum Parsimony analysis retrieved Cerradomys as a monophyletic group with 97% support (bootstrap), with CSC as the sister to the other species, followed by a ramification into two clades (69% of branch support), the first comprising CLA and the other branch including CVI and CSU. We integrated the chromosome painting analysis of Eumuroida rodents investigated by HME and Mus musculus (MMU) probes and identified several syntenic blocks shared among representatives of Cricetidae and Muridae. The Cerradomys genus underwent an extensive karyotypic evolutionary process, with multiple rearrangements that shaped extant karyotypes. The chromosomal phylogeny corroborates the phylogenetic relationships proposed by molecular analysis and indicates that karyotypic diversity is associated with species radiation. Three syntenic blocks were identified as part of the ancestral Eumuroida karyotype (AEK): MMU 7/19 (AEK 1), MMU 14 (AEK 10) and MMU 12 (AEK 11). Besides, MMU 5/10 (HME 18/2/24) and MMU 8/13 (HME 22/5/11) should be considered as signatures for Cricetidae, while MMU 5/9/14, 5/7/19, 5 and 8/17 for Sigmodontinae.

Genetic diversity analysis in the Brazilian Amazon reveals a new evolutionary lineage and new karyotype for the genus Mesomys (Rodentia, Echimyidae, Eumysopinae)

Morphological, molecular and chromosomal studies in the genera Lonchothrix and Mesomys have contributed to a better understanding of taxonomic design, phylogenetic relationships and karyotypic patterns. Recent molecular investigations have shown a yet undescribed diversity, suggesting that these taxa are even more diverse than previously assumed. Furthermore, some authors have questioned the limits of geographic distribution in the Amazon region for the species M. hispidus and M. stimulax. In this sense, the current study sought to understand the karyotypic evolution and geographic limits of the genus Mesomys, based on classical (G- and C-banding) and molecular cytogenetic analysis (FISH using rDNA 18S and telomeric probes) and through the sequencing of mitochondrial genes Cytochrome b (Cytb) and Cytochrome Oxidase-Subunit I (CO using phylogeny, species delimitation and time of divergence, from samples of different locations in the Brazilian Amazon. The species M. stimulax and Mesomys sp. presented 2n = 60/FN = 110, while M. hispidus presented 2n = 60/FN = 112, hitherto unpublished. Molecular dating showed that Mesomys diversification occurred during the Plio-Pleistocene period, with M. occultus diverging at around 5.1 Ma, followed by Mesomys sp. (4.1 Ma) and, more recently, the separation between M. hispidus and M. stimulax (3.5 Ma). The ABGD and ASAP species delimiters support the formation of 7 and 8 potential species of the genus Mesomys, respectively. Furthermore, in both analyzes Mesomys sp. was recovered as a valid species. Our multidisciplinary approach involving karyotypic, molecular and biogeographic analysis is the first performed in Mesomys, with the description of a new karyotype for M. hispidus, a new independent lineage for the genus and new distribution data for M. hispidus and M. stimulax.

New species of the Spiny Mouse genus Neacomys (Cricetidae, Sigmodontinae) from northwestern Ecuador

Neacomys is a genus of small spiny or bristly sigmodontine rodents that are common components of mammalian faunas in multiple biomes on Central and South America. Recent studies on this group have demonstrated that there is cryptic diversity yet to be discovered within currently recognized species that have not received comprehensive revisions, as well as in areas that have been overlooked. Here we ratify this assertion by describing a new species previously misidentified as the Narrow-footed Spiny Mouse (Neacomystenuipes) from the Chocó biogeographic region in northwestern Ecuador, Neacomysmarci Brito & Tinoco, sp. nov. Distinctiveness of this entity is supported by the combination of the following morphological characters: small size (head-body length 65-85 mm); long tail (69-126% longer than head-body length); pale buff-colored but gray-based belly fur; white throat; hypothenar pad usually absent; long nasals; and a condylar process higher than the coronoid process. Likewise genetic distance analyses and phylogenetic reconstructions based on cytochrome-b (Cytb) sequence data indicate a clear divergence from typical populations of N.tenuipes, and a sister relationship between them. The results presented here increase the diversity of Neacomys to 24 species, placing it among the most diverse genera within the sigmodontine rodents.

Systematics of the rodent genus Neacomys Thomas (Cricetidae: Sigmodontinae): two new species and a discussion on carotid patterns

The taxonomy of the oryzomyine genus Neacomys currently is in a state of flux: systematic studies in the last 20 years increased its diversity from four to 16 species, with an additional several undescribed phylogenetic lineages. Despite this progress, morphological variation and species limits remain poorly known for several species groups and complexes within the genus, such as N. tenuipes, N. musseri, and N. dubosti. Here we analyze the variation of morphological characters and morphometric patterns of two new candidate species of Neacomys for northern Venezuela and Pará state, Brazil, that are characterized by the presence of derived carotid circulation, a rare character state for the genus. Analyzed material included holotypes, type series, and/or topotypes of almost all described species of Neacomys, including type series of N. musseri, the holotype of N. tenuipes, and sequenced specimens of N. xingu. Qualitative comparison showed that 25 characters are informative for the distinction of the two new forms, and morphometric analyses corroborated the morphological separation of the new species. Results also point that N. tenuipes might represent a species complex. The new forms are sympatric with other species, including N. tenuipes and N. xingu, and can be identified by a suite of morphological characters, including the presence of a derived carotid pattern and, for the Pará form, by a unique morphology of the first upper molar. We review the occurrence of carotid circulation patterns within Sigmodontinae and discuss its use for the systematics of the subfamily. Further studies involving the new Neacomys species will contribute to understanding the biogeographic patterns and evolutionary trends within this distinct and diverse genus.