Marcelo Weksler

Ten New Genera of Oryzomyine Rodents (Cricetidae: Sigmodontinae)

Abstract In order to achieve a monophyletic classification of oryzomyine rodents, 10 new genera are described for species or species groups previously referred to the polyphyletic genus Oryzomys. The following names are proposed: Aegialomys, n.gen. (for the “xanthaeolus group” of authors); Cerradomys, n.gen. (for the “subflavus group”); Eremoryzomys, n.gen. (for polius); Euryoryzomys, n.gen. (for the “nitidus group”); Hylaeamys, n.gen. (for the “megacephalus group”); Mindomys, n.gen. (for hammondi); Nephelomys, n.gen. (for the “albigularis group”); Oreoryzomys, n.gen. (for balneator); Sooretamys, n.gen. (for angouya); and Transandinomys, n.gen. (for bolivaris and talamancae). All of the new genera thus constituted are morphologically diagnosable and have distinct ecogeographic distributions. Pending revisionary work that is currently in progress by other researchers, six species belonging to the “alfaroi group” (herein construed as including alfaroi, chapmani, melanotis, rhabdops, rostratus, and saturatior) are provisionally referred to Handleyomys. As a result of these changes, the genus Oryzomys is restricted to the “palustris group” of authors, and the tribe Oryzomyini now comprises 28 genera.

Phylogeny of Neotropical oryzomyine rodents (Muridae: Sigmodontinae) based on the nuclear IRBP exon

Sigmodontine rodents are the most diverse family-level mammalian clade in the Neotropical region, with about 70 genera and 320 recognized species. Partial sequences (1266 bp) from the first exon of the nuclear gene encoding the Interphotoreceptor Retinoid Binding Protein (IRBP) were used to infer the phylogenetic relationships among 44 species representing all 16 currently recognized genera of the largest sigmodontine tribe, the Oryzomyini. Monophyly of the tribe was assessed relative to 15 non-oryzomyine sigmodontine taxa representing all major sigmodontine lineages. Twelve taxa from seven muroid subfamilies were used as outgroups. The resulting matrix included 71 taxa and 386 parsimony-informative characters. Phylogenetic analysis of this matrix resulted in 16 equally parsimonious cladograms, which contained the following well-supported groups: (i). a monophyletic Oryzomyini, (ii). a clade containing all oryzomyines except Scolomys and Zygodontomys, (iii). a clade containing Oecomys, Handleyomys, and several species of forest-dwelling Oryzomys, and (iv). a clade containing the remaining oryzomyine taxa. The last clade is composed of two large subclades, each with lower nodal support, containing the following taxa: (i). Microryzomys, Oligoryzomys, Neacomys, and Oryzomys balneator; (ii). Holochilus, Lundomys, Pseudoryzomys, Nectomys, Amphinectomys, Sigmodontomys, and several species of open-vegetation or semiaquatic Oryzomys. Regarding relationships among non-oryzomyine taxa, sigmodontines, neotomines, and tylomyines do not form a monophyletic group; a clade containing Rheomys and Sigmodon is basal relative to all other sigmodontines; and the remaining sigmodontines are grouped in three clades: the first containing Thomasomyini, Akodontini, and Reithrodon; the second containing Abrothrichini, and Phyllotini, plus Wiedomys, Juliomys, Irenomys, and Delomys; and the third containing the oryzomyines. No conflict is observed between IRBP results and previous robust hypotheses from mitochondrial data, while a single case of incongruence is present between the IRBP topology and robust hypothesis from morphological studies.

In the Wake of Invasion: Tracing the Historical Biogeography of the South American Cricetid Radiation (Rodentia, Sigmodontinae)

The Great American Biotic Interchange (GABI) was greatly influenced by the completion of the Isthmus of Panama and impacted the composition of modern faunal assemblages in the Americas. However, the contribution of preceding events has been comparatively less explored, even though early immigrants in the fossil records are evidence for waif dispersals. The cricetid rodents of the subfamily Sigmodontinae are a classic example of a species-rich South American radiation resulting from an early episode of North American invasion. Here, we provide a temporal and spatial framework to address key aspects of the historical biogeography and diversification of this diverse mammal group by using mitochondrial and nuclear DNA datasets coupled with methods of divergence time estimation, ancestral area reconstruction and comparative phylogenetics. Relaxed-clock time estimates indicate that divergence of the Sigmodontinae began in the middle–late Miocene (ca. 12–9 Ma). Dispersal-vicariance analyses point to the arrival of a single lineage of northern invaders with a widespread ancestral distribution and imply that the initial differentiation between Central and South America gave rise to the most basal groups within the subfamily. These two major clades diversified in the late Miocene followed by the radiation of main tribes until the early Pliocene. Within the Oryzomyalia, tribes diverged initially in eastern South America whereas multiple dispersals into the Andes promoted further diversification of the majority of modern genera. A comparatively uniform background tempo of diversification explains the species richness of sigmodontines across most nodes, except for two akodontine genera with recent increases in diversification rates. The bridging of the Central American seaway and episodes of low sea levels likely facilitated the invasion of South America long before the onset of the post-Isthmian phase of the GABI.

Unexpected evolutionary diversity in a recently extinct Caribbean mammal radiation

Identifying general patterns of colonization and radiation in island faunas is often hindered by past human-caused extinctions. The insular Caribbean is one of the only complex oceanic-type island systems colonized by land mammals, but has witnessed the globally highest level of mammalian extinction during the Holocene. Using ancient DNA analysis, we reconstruct the evolutionary history of one of the Caribbeans now-extinct major mammal groups, the insular radiation of oryzomyine rice rats. Despite the significant problems of recovering DNA from prehistoric tropical archaeological material, it was possible to identify two discrete Late Miocene colonizations of the main Lesser Antillean island chain from mainland South America by oryzomyine lineages that were only distantly related. A high level of phylogenetic diversification was observed within oryzomyines across the Lesser Antilles, even between allopatric populations on the same island bank. The timing of oryzomyine colonization is closely similar to the age of several other Caribbean vertebrate taxa, suggesting that geomorphological conditions during the Late Miocene facilitated broadly simultaneous overwater waif dispersal of many South American lineages to the Lesser Antilles. These data provide an important baseline by which to further develop the Caribbean as a unique workshop for studying island evolution.

STATUS OF PROECHIMYS ROBERTI AND P. ORIS (RODENTIA: ECHIMYIDAE) FROM EASTERN AMAZONIA AND CENTRAL BRAZIL

Abstract Proechimys roberti has been considered an isolate representative of the guyannensis group of species of the genus Proechimys in the Cerrado of central Brazil, or regarded as a member of the longicaudatus group of species. We present new karyologic, morphometric, morphologic, and molecular data from populations spanning the gap between the previously known isolated distribution of P. roberti and the core distribution of the guyannensis group. All specimens had the same basic karyotype, 2n = 30, with fundamental number (FN) = 54–55. FN variation was due to a pericentric inversion affecting a small autosomal pair. Specimens from the eastern Amazon Basin and the Cerrado did not have any differentiation in external, cranial, and dental characters. In contrast, samples showed a significant multivariate morphometric variation, with a high heterogeneity among all populations, independent of Cerrado or Amazon location. Our results, coupled with phylogenetic analysis of cytochrome-b sequences, indicate that P. roberti is a valid species of the guyannensis group and that the species of this group occurring in eastern Amazon, P. oris, is probably a junior synonym of P. roberti.

A newly recognized clade of trans-Andean Oryzomyini (Rodentia: Cricetidae), with description of a new genus.

Abstract We expand upon recent studies on relationships within the Oryzomyini, in particular, those involving taxa currently assigned to the genus Sigmodontomys. In recent years, Sigmodontomys has been treated as including 2 species, alfari (J. A. Allen, 1897) and aphrastus (Harris, 1932), but throughout their complicated taxonomic history both species also have been placed in the genus Oryzomys, and alfari independently in Nectomys. Using morphological (98 external, cranial, dental, and postcranial) and molecular (nuclear interphotoreceptor retinoid–binding protein gene and mitochondrial cytochrome-b and ribosomal 12S RNA genes) characters, we infer the phylogenetic position of these 2 species within Oryzomyini. We document that alfari and aphrastus do not form a monophyletic group. Sigmodontomys alfari is most closely related to Melanomys, and aphrastus is either the sister to that clade, or to the extinct Caribbean genus Megalomys. Thus, aphrastus is best regarded as representing a new genus, which is described and named herein. This new genus falls within the Sigmodontomys–Melanomys–Aegialomys–Nesoryzomys clade, which forms a monophyletic group of mainly southern Central American and northern South American taxa primarily restricted to lowland to midelevation montane trans-Andean habitats and possessing a marked ability to cross expanses of salt water. The new genus occurs at middle elevations from north-central Costa Rica to northwestern Ecuador and along with some populations of Aegialomys and Melanomys occupies the highest elevations for members of this group.

Redescription of the enigmatic long-tailed rat Sigmodontomys aphrastus (Cricetidae: Sigmodontinae) with comments on taxonomy and natural history

ABSTRACT Sigmodontomys aphrastus, the long-tailed rat, is an exceedingly rare rodent species from montane regions of Central and South America of which very little is known ecologically or systematically. It has been variously placed in the genera Oryzomys, Nectomys, and Sigmodontomys based on the five previously known specimens. Two new individuals were collected in northwestern Costa Ricas Cordillera de Tilarán. These new specimens and the other five known specimens are used to redescribe the species, detail measurements of external and cranial morphology, and compare S. aphrastus to similarly appearing sympatric species (Nephelomys albigularis and N. devius) and proposed closely related species (Sigmodontomys alfari, Mindomys hammondi, and Melanomys caliginosus). New ecological data is presented and the general knowledge of its natural history is summarized. The phylogenetic relatedness of S. aphrastus with purported sister taxa remains unresolved until combined molecular and morphological analyses are conducted.

On the Taxonomic Status of Oryzomys curasoae McFarlane and Debrot, 2001, (Rodentia: Cricetidae: Sigmodontinae) with Remarks on the Phylogenetic Relationships of O. gorgasi Hershkovitz, 1971

Abstract. The type material of Oryzomys curasoae McFarlane and Debrot, 2001, an allegedly extinct species from the island of Curaçao, Netherlands Antilles, is morphologically indistinguishable from O. gorgasi Hershkovitz, 1971, a Recent mangrove-inhabiting species from mainland Venezuela. A phylogenetic analysis suggests that the latter belongs to the genus Oryzomys in its strictest sense, and that “curasoae” is not close to Oecomys (formerly ranked as a subgenus of Oryzomys) as previously suggested. Evidence that Rattus rattus may now be displacing O. gorgasi from coastal habitats in Venezuela suggests that black rats might have been directly responsible for the extinction of the insular population of O. gorgasi on Curaçao.

Response to comment on "The placental mammal ancestor and the post-K-Pg radiation of placentals".

Tree-building with diverse data maximizes explanatory power. Application of molecular clock models to ancient speciation events risks a bias against detection of fast radiations subsequent to the Cretaceous-Paleogene (K-Pg) event. Contrary to Springer et al., post–K-Pg placental diversification does not require “virus-like” substitution rates. Even constraining clade ages to their model, the explosive model best explains placental evolution.

PHYLOGENETIC ANALYSES OF SPINY POCKET MICE (HETEROMYIDAE: HETEROMYINAE) BASED ON ALLOZYMIC AND MORPHOLOGICAL DATA

Abstract The subfamily Heteromyinae (spiny pocket mice) represents a well-defined monophyletic group within the rodent family Heteromyidae. Although 2 genera of spiny pocket mice, Heteromys and Liomys, are recognized in the subfamily, no phylogenetic analysis has demonstrated their reciprocal monophyly. A recent study using DNA-sequence data from the mitochondrial gene cytochrome b suggested that Liomys is paraphyletic but included few species of Heteromys. Here, we conduct phylogenetic analyses of the subfamily with dense taxonomic sampling using allozymic data from a previous study and external and cranial morphological data; our aim is to assess generic monophyly and elucidate phylogenetic structure within the genera, to the degree possible with these data. We also reidentify selected voucher specimens from the allozymic study. Parsimony-based analyses indicate 3 clades in the subfamily: (A) Liomys irroratus, L. pictus, and L. spectabilis; (B) L. adspersus and L. salvini; and (C) all examined species of Heteromys. However, the relationships among these clades are unresolved. The genus Heteromys is characterized by strong support and several unreversed morphological synapomorphies. In contrast, our analyses fail to indicate any synapomorphies for Liomys, but can neither demonstrate nor reject its monophyly. The 3 clades identified here match those recovered from a recent mitochondrial DNA–sequencing study, which found a resolved (B (A + C)) topology. Within Heteromys, we recover 5 lineages, but the relationships among them remain unresolved. The examined South American species of Heteromys formed a clade, but 2 species recently described from Ecuador and Venezuela were not included here. Samples referred to as H. desmarestianus crassirostris and H. d. planifrons were quite distinct from other samples of H. desmarestianus, emphasizing the need for alpha-level taxonomic revision of this species complex. Given the current results, future studies can now examine relationships among species of Heteromys using samples of Liomys as outgroups, but studies of Liomys must take into account its likely paraphyletic nature.