Ulyses F. J. Pardiñas

Genetic footprints of late Quaternary climate change in the diversity of Patagonian‐Fueguian rodents

Species are impacted by climate change at both ecological and evolutionary time scales. Studies in northern continents have provided abundant evidence of dramatic shifts in distributions of species subsequent to the last glacial maximum (LGM), particularly at high latitudes. However, little is known about the history of southern continents, especially at high latitudes. South America is the only continent, other than Antarctica, that extends beyond 40 degrees S. Genetic studies of a few Patagonian species have provided seemingly conflicting results, indicating either postglacial colonization from restricted glacial refugia or persistence through glacial cycles and in situ differentiation. Using mitochondrial DNA sequences of 14 species of sigmodontine rodents, a major faunal ensemble of Patagonia and Tierra del Fuego, we show that at least nine of these species bear genetic footprints of demographic expansion from single restricted sources. However, timing of demographic expansion precedes the LGM in most of these species. Four species are fragmented phylogeographically within the region. Our results indicate that (i) demographic instability in response to historical climate change has been widespread in the Patagonian-Fueguian region, and is generally more pronounced at high latitudes in both southern and northern continents; (ii) colonization from lower latitudes is an important component of current Patagonian-Fueguian diversity; but (iii) in situ differentiation has also contributed to species diversity.

DEFINITION AND DIAGNOSIS OF A NEW TRIBE OF SIGMODONTINE RODENTS (CRICETIDAE: SIGMODONTINAE), AND A REVISED CLASSIFICATION OF THE SUBFAMILY

A new tribe of sigmodontine rodents is formally defined and diagnosed. The tribe contains the extant genera Abrothrix (including Chroeomys), Chelemys, Geoxus, Pearsonomys, and Notiomys, a group of taxa distributed in the central and southern Andes and lowlands both east and west of the cordillera. The new tribe presents a unique combination of characters including nasals and premaxilla usually projected anterior to the incisors, moderately trumpeted; zygomatic plate with upper free border reduced or obsolete; and third upper molar reduced and sub-cylindrical in out-line with an internal ring-like enamel fossette. Molecular phylogenetic analyses indicate that Abrothrix is sister to a clade containing the remaining genera of the new tribe.

PHYLOGEOGRAPHY OF OLIGORYZOMYS LONGICAUDATUS (RODENTIA: SIGMODONTINAE) IN TEMPERATE SOUTH AMERICA

Abstract Phylogeographic relationships were evaluated at the intraspecific level using nucleotide sequence data from the mitochondrial cytochrome b gene of representative specimens of “colilargo” (Oligoryzomys longicaudatus) from 31 localities, along its distributional range over a large part of the western Andes and southern Argentina. Based on approximately 1,000 base pairs (bp), we recognized a single species on both the Chilean and the Argentinean side as far as at least latitude 51°S, rejecting the subspecific distinctiveness of longicaudatus and philippi. We thus placed the latter in full synonymy with O. longicaudatus as earlier studies proposed, and enlarged its range as far as Torres del Paine, about 51°S. The occurrence of subspecies in this range is doubtful given the low sequence divergence values and the absence of significant associations between haplotypes and their geography. Additionally, we hypothesized that the entrance of this species into the Chilean side of the Andes mountains occurred through the Patagonian forests of southern Argentina, with further dispersal to the north from the south.

Dating an impressive Neotropical radiation: Molecular time estimates for the Sigmodontinae (Rodentia) provide insights into its historical biogeography.

With about 400 living species and 82 genera, rodents of the subfamily Sigmodontinae comprise one of the most diverse and more broadly distributed Neotropical mammalian clades. There has been much debate on the origin of the lineage or the lineages of sigmodontines that entered South America, the timing of entrance and different aspects of further diversification within South America. The ages of divergence of the main lineages and the crown age of the subfamily were estimated by using sequences of the interphotoreceptor retinoid binding protein and cytochrome b genes for a dense sigmodontine and muroid sampling. Bayesian inference using three fossil calibration points and a relaxed molecular clock estimated a middle Miocene origin for Sigmodontinae (∼12Ma), with most tribes diversifying throughout the Late Miocene (6.9-9.4Ma). These estimates together results of analyses of ancestral area reconstructions suggest a distribution for the most recent common ancestor of Sigmodontinae in Central-South America and a South American distribution for the most recent common ancestor of Oryzomyalia.

A phylogenetic appraisal of Sigmodontinae (Rodentia, Cricetidae) with emphasis on phyllotine genera: systematics and biogeography

Here, we present a comprehensive phylogenetic analysis based on nuclear and mitochondrial DNA sequences of rodents of the subfamily Sigmodontinae. The emphasis is placed on the large tribe Phyllotini; sampling includes for the first time in any molecular‐based phylogenetic analysis representatives of several genera traditionally considered to be phyllotines. Given the broad taxonomic sampling, results provide substantial improvements in our knowledge on both the structure of the sigmodontine radiation and of phyllotine phylogenetic relationships. For instance, the tribe Ichthyomyini was not recovered monophyletic. Similarly, in a novel hypothesis on the contents of the tribe Phyllotini, it is shown that unlike Galenomys, the genera Chinchillula, Neotomys and Punomys are not phyllotines. The later genera together with Andinomys, Euneomys, Irenomys and Juliomys form part of novel generic clades of mostly Andean sigmodontine rodents. More in general, results strongly suggest the occurrence of several instances of putative morphological convergence among distinct sigmodontine lineages (e.g. among now considered to be ichthyomyines; between Phyllotini and some Andean taxa; among Euneomys‐Neotomys and Reithrodon). Finally, we suggest that the historical biogeography of the sigmodontine rodents is far more complex than earlier envisioned.

SYSTEMATICS OF NECROMYS (RODENTIA, CRICETIDAE, SIGMODONTINAE): SPECIES LIMITS AND GROUPS, WITH COMMENTS ON HISTORICAL BIOGEOGRAPHY

Abstract We present the most comprehensive systematic study to date of Necromys, a rodent genus distributed in open areas north and south of Amazonia and in Andean grasslands. The study is based on sequences of the cytochrome-b gene that were analyzed by parsimony and Bayesian approaches. The analyses include sequences of 62 specimens from 51 localities from Argentina, Bolivia, Brazil, Colombia, Paraguay, Peru, Uruguay, and Venezuela, representing all but 1 of the species currently recognized in the genus. Necromys was recovered as a monophyletic group and we found a large polytomy at its base that involves 3 lineages. One, represented by the Andean N. lactens, shows a marked phylogeographic pattern. The 2nd clade is formed by N. urichi from the northern grasslands of South America and N. amoenus from the central Andes. Results suggest that each of these taxa may represent more than 1 biological species. The 3rd clade is formed by lowland species found south of Amazonia. Within this clade N. obscurus is sister to the remaining species. Haplotypes recovered from specimens assigned to N. benefactus, N. temchuki, and N. lasiurus form a clade, but these taxa do not form reciprocally monophyletic groups, nor does this large clade possess geographic structure. These genealogical results, discussed in the context of genetic variation, are the basis of taxonomic (e.g., N. benefactus and N. temchuki are regarded as junior synonyms of N. lasiurus) and biogeographic considerations.

SYSTEMATICS OF ARGENTINEAN, PARAGUAYAN, AND URUGUAYAN SWAMP RATS OF THE GENUS SCAPTEROMYS (RODENTIA, CRICETIDAE, SIGMODONTINAE)

Abstract We present a systematic study of Scapteromys populations from Argentina, Paraguay, and Uruguay, based on molecular and morphological evidence. First, based on DNA sequences (801 base pairs) from the mitochondrial cytochrome-b gene, we found that Scapteromys populations fall into 2 main clades; 1 formed by Argentinean and Paraguayan populations together with 1 population from western Uruguay, and the other constituted by the remaining Uruguayan populations. Analysis of molecular variance showed that 86.93% of the genetic variation uncovered in Scapteromys is explained by differences between clades. Morphological analyses corroborated the existence of 2 main morphotypes among Scapteromys specimens. The phylogeographic break identified is mostly congruent with patterns of morphological and chromosomic variation. In light of these results, we propose that S. aquaticus be elevated to the rank of species, we redefine the known distributions of S. aquaticus and S. tumidus, and we provide a list of character states that allow an unambiguously diagnosis of both species.

A NEW SPECIES OF AKODON (RODENTIA, CRICETIDAE) FROM THE NORTHERN CAMPOS GRASSLANDS OF ARGENTINA

Abstract Integrated analyses of morphological, chromosomal, and molecular data resulted in the description of a new species of the genus Akodon, endemic to the Northern Campos in southern Misiones, Argentina. The new species presents a unique combination of characters, including a narrow zygomatic plate almost without a free upper border, short rostrum, short tail (40% of the head–body length), small and flat auditory bullae, diploid complement of 2n = 36, and several molecular synapomorphies. Phylogenetic analysis, based on cytochrome-b gene sequences, indicates that the Brazilian species Akodon lindberghi is sister to the new species, although this relationship is weakly supported. We comment on the conservation significance of our study, considering that these perisylvan grasslands are suffering substantial human disturbance by agriculture and burning practices.

A role for suppressed incisor cuspal morphogenesis in the evolution of mammalian heterodont dentition.

Changes in tooth shape have played a major role in vertebrate evolution with modification of dentition allowing an organism to adapt to new feeding strategies. The current view is that molar teeth evolved from simple conical teeth, similar to canines, by progressive addition of extra “cones” to form progressively complex multicuspid crowns. Mammalian incisors, however, are neither conical nor multicuspid, and their evolution is unclear. We show that hypomorphic mutation of a cell surface receptor, Lrp4, which modulates multiple signaling pathways, produces incisors with grooved enamel surfaces that exhibit the same molecular characteristics as the tips of molar cusps. Mice with a null mutation of Lrp4 develop extra cusps on molars and have incisors that exhibit clear molar-like cusp and root morphologies. Molecular analysis identifies misregulation of Shh and Bmp signaling in the mutant incisors and suggests an uncoupling of the processes of tooth shape determination and morphogenesis. Incisors thus possess a developmentally suppressed, cuspid crown-like morphogenesis program similar to that in molars that is revealed by loss of Lrp4 activity. Several mammalian species naturally possess multicuspid incisors, suggesting that mammals have the capacity to form multicuspid teeth regardless of location in the oral jaw. Localized loss of enamel may thus have been an intermediary step in the evolution of cusps, both of which use Lrp4-mediated signaling.

The genus Akodon (Muroidea: Sigmodontinae) in Misiones, Argentina

Our understanding of the rodent diversity from the Argentinean Atlantic rainforest and its surrounding environments (Misiones Province) is still relatively poor. Here we present an update on the species inhabiting Misiones of the genus Akodon. Integrated analyses of morphology, chromosomal and molecular data allow us to document the existence of four species of Akodon in the province. One of these is apparently undescribed, and is known only from one locality in the ‘campos’ phytogeographic unit of southern Misiones. A second species, A. montensis, has a large distribution and is one of the dominant sigmodontine species in primary and secondary forest. Akodon sp. 2, previously reported as either A. serrensis and A. paranaensis, is known from only one locality at the northeast of the province, where it is sympatric with A. montensis. Also in sympatry with A. montensis at one locality in central Misiones there is a similar species with a gall bladder referred here to A. cursor. Finally, the previous reference of one specimen to A. serrensis must be discarded due to the referred specimen is lost, and the data at hand are insufficient to corroborate the identity of the specimen under question. As Misiones has the largest continuous patches of southern interior Atlantic rainforest remaining in Argentina we pose comments on the conservation significance of our study.