Open Access

The phylogenetic position of the enigmatic Atlantic forest-endemic spiny mouse Abrawayaomys (Rodentia: Sigmodontinae)

  • Karen Ventura1,
  • Maria José J Silva2,
  • Lena Geise3,
  • Yuri LR Leite4,
  • Ulyses FJ Pardiñas5,
  • Yatiyo Yonenaga-Yassuda1 and
  • Guillermo D'Elía6Email author
Zoological Studies201352:55

https://doi.org/10.1186/1810-522X-52-55

Received: 18 February 2013

Accepted: 30 October 2013

Published: 9 December 2013

Abstract

Background

The phylogenetic position of the sigmodontine genus Abrawayaomys, historically assigned to the tribe Thomasomyini or considered a sigmodontine incertae sedis, was assessed on the basis of nuclear and mitochondrial DNA sequences obtained from four individuals from different localities in the Atlantic forest of Brazil. Sequences of Abrawayaomys were analyzed in the context of broad taxonomic matrices by means of maximum-likelihood (ML) and Bayesian analyses (BA).

Results

The phylogenetic position of Abrawayaomys differed depending on the gene analyzed and the analysis performed (interphotoreceptor retinoid-binding protein (IRBP) ML: sister to Thomasomyini; IRBP BA: sister to Akodontini; cytochrome (Cyt) b ML: sister to Neotomys; and Cyt b BA: sister to Reithrodontini). With the sole exception of the BA based on Cyt b sequences, where the Abrawayaomys-Reithrodon clade had strong support, all sister-group relationships involving Abrawayaomys lacked any significant support.

Conclusions

As such, Abrawayaomys constitutes the only representative so far known of one of the main lineages of the sigmodontine radiation, differing from all other Atlantic forest sigmodontine rodents by having a unique combination of morphological character states. Therefore, in formal classifications, it should be regarded as a Sigmodontinae incertae sedis.

Keywords

AkodontiniAtlantic forestCricetidaePhylogenyThomasomyini

Background

With about 86 living genera, cricetids of the subfamily Sigmodontinae are one of the most diversified and taxonomically complex groups of mammals. Predominantly distributed in South America, sigmodontines also reach Central and North America, and one extant genus is endemic to the Galapagos Islands (D'Elía 2003a). Remarkably, new sigmodontine genera are still being erected on the basis of both revisionary museum work and from newly discovered species (e.g., Pardiñas et al. 2009a; Percequillo et al. 2011; Pine et al. 2012; Alvarado-Serrano and D'Elía 2013; see comments in D'Elía and Pardiñas 2007). Similarly, statuses of several sigmodontine taxonomic forms at the species level are unclear (e.g., Alarcón et al. 2011; Bonvicino et al. 2012).

Sigmodontine genera have been arranged into different groups, most of which have been given the formal rank of tribes (e.g., Reig 1980). In the last two decades, phylogenetic analyses using either morphological or molecular data or both were used to set the limits and contents of these groups and determine the timing of their diversification (e.g., Braun 1993; Smith and Patton 1999; Steppan 1995;D'Elía 2003b; Pacheco 2003; Weksler 2003; D'Elía et al. 2003, D'Elia et al. 2006a, b; Martínez et al. 2012; Parada et al. 2013; Salazar-Bravo et al. 2013). Those studies caused a number of major reconsiderations on the limits and contents of these groups. Currently, 12 extant genera are considered as Sigmodontinae incertae sedis (see the most recently published classification in D'Elía et al. (2007) and the modification prompted by the description of a new genus by Alvarado-Serrano and D'Elía (2013)); one of these is Abrawayaomys Cunha and Cruz 1979.

Abrawayaomys is a poorly known sylvan sigmodontine genus that stands out within the sigmodontine radiation due to its spiny pelage and unusual craniodental morphology. It is found in the Atlantic forest of Argentina and Brazil and is known from a handful of trapped specimens and a few osteological remains gathered from owl pellets (Pardiñas et al. 2009b). Two species are recognized: the type species Abrawayaomys ruschii Cunha and Cruz 1979 and the recently described Abrawayaomys chebezi (Pardiñas et al. 2009a,2009b).

Abrawayaomys displays a striking combination of morphological features that was referred by Musser and Carleton (2005, p. 1,088) in the following terms ‘Diagnostic traits seem to combine aspects of Neacomys, Oryzomys, and Akodon, and both Reig (1987) and Smith and Patton (1999) acknowledged the enigmatic affinities of Abrawayaomys as uncertain.’ Recently, Pardiñas et al. (2009b) evaluated the morphology of Abrawayaomys in detail and noted a certain resemblance to the bauplan of the tribe Akodontini but also to the thomasomyine genera Chilomys and Rhagomys. Similarities with Akodontini were regarded as convergences since Abrawayaomys was assigned to Thomasomyini given that independent sets of data placed Chilomys and Rhagomys in Thomasomyini (D'Elía et al. 2006a; Salazar-Bravo and Yates 2007) and because morphology-based phylogenetic analyses placed Abrawayaomys in this tribe (Pacheco 2003; Salazar-Bravo and Yates 2007). The lack of molecular data for Abrawayaomys has prevented an assessment of its position within the available comprehensive sigmodontine trees generated on the basis of mitochondrial and nuclear DNA sequences (e.g., Engel et al. 1998; Smith and Patton 1999; D'Elía 2003b; Weksler 2003; D'Elia et al. 2006a, b).

In recent fieldwork in the states of Rio de Janeiro (Pereira et al. 2008), Minas Gerais (Passamani et al. 2011), and São Paulo (see below) in southeastern Brazil, four specimens referred to as A. ruschii were collected. Based on mitochondrial and nuclear DNA sequences gathered from these specimens, we present the first phylogenetic analyses to test previous hypotheses concerning the placement of Abrawayaomys within the sigmodontine radiation. In addition, we provide some taxonomic judgments based on the resulting phylogeny and comments on the evolution of some morphological traits.

Methods

DNA sequences corresponding to the cytochrome (Cyt) b gene and the first exon of the nuclear interphotoreceptor retinoid-binding protein (IRBP) gene were used as evidence. We sequenced four specimens of A. ruschii deposited in the following Brazilian collections: Museu Nacional, Univ. Federal do Rio de Janeiro (MN 67557; Brazil, Rio de Janeiro, Aldeia Sapucai) (Pereira et al. 2008); Museu de Zoologia, Univ. de São Paulo (MZUSP 32319; Brazil, São Paulo, Biritiba-Mirim; BO 27; Brazil, São Paulo, Estação Ecológica de Boracéia); and Coleção de Mamíferos da Univ. Federal de Lavras (CMUFLA 906; Brazil, Minas Gerais, Caeté) (Passamani et al. 2011). Sequences were gathered following the protocol of Pardiñas et al. (2003) and D'Elía et al. (2006b). We found minor differences among Cyt b sequences (see ‘Results’ below), but in the phylogenetic analyses, we used that of specimen MN 67557 because it was the only complete one (i.e., 1,140 bp; the other being 801 bp long). We found no variation in IRBP sequences of the four specimens analyzed, and therefore, Abrawayaomys was represented by a single terminal (MN 67557) in the phylogenetic analyses. New sequences were submitted to GenBank [GenBank: JX949182 to JX949189].

To appraise the phylogenetic position of Abrawayaomys within the radiation of the Sigmodontinae, we sought to ensure that sigmodontine diversity was represented as comprehensively as possible. According to the current classification, our sampling only lacked for both matrices the incertae sedis genera Phaenomys and Wilfredomys; the akodontines Gyldenstolpia and Podoxymys; the ichthyomyines Anotomys, Chibchanomys, Neusticomys, and Ichthyomys; and the oryzomyine Mindomys. In addition, the IRBP matrix lacked the thomasomyine Chilomys. Meanwhile, the ichthyomyine Rheomys, the oryzomyine Microakodontomys, and the thomasomyine Aepeomys were also missing from the Cyt b matrix. Therefore, both the IRBP (1,181 characters) and Cyt b (1,134 characters) matrices respectively included representatives of 76 and 74 sigmodontine genera (including sequences of the recently described genus Neomicroxus). The IRBP matrix lacked a sequence for Neusticomys available in GenBank [GenBank: EU649036] because a recent inspection of it indicated that it may be in fact a composite of an ichthyomyine and an oryzomyine sequence. Although the monophyly of the Sigmodontinae is well corroborated (e.g., Engel et al. 1998; Steppan et al. 2004; Parada et al. 2013), the identity of its sister group is unclear. Therefore, we integrated the outgroup with two representatives of each of the other four main lineages that, together with the Sigmodontinae, compose the family Cricetidae: arvicolines (Arvicola and Microtus), cricetines (Cricetulus and Phodopus), neotomines (Neotoma and Scotinomys), and tylomyines (Nyctomys and Tylomys). All taxa represented in the analyses, along with the GenBank accession numbers of their DNA sequences, are listed in Table 1.
Table 1

List of taxa and the DNA sequences of which were included in the phylogenetic analyses

Tribe

Species

IRBP

Cyt b

Abrotrichini

Abrothrix longipilis

AY163577

U03530

Abrotrichini

Chelemys macronyx

AY277441

U03533

Abrotrichini

Geoxus valdivianus

AY277448

AY275116

Abrotrichini

Notiomys edwardsii

AY163602

U03537

Abrotrichini

Pearsonomys annectens

AY851749

AF108672

Akodontini

‘Akodon’ serrensis

EF626799

AY273908

Akodontini

Akodon azarae

AY163578

DQ444328

Akodontini

Bibimys chacoensis/labiosus

AY277435

DQ444329

Akodontini

Blarinomys breviceps

AY277437

AY275112

Akodontini

Brucepattersonius soricinus

AY277439

AY277486

Akodontini

Deltamys kempi

AY277444

AY195862

Akodontini

Juscelinomys huanchacae

AY277453

AF133667

Akodontini

Kunsia tomentosus

AY277455

AY275120

Akodontini

Lenoxus apicalis

AY277456

U03541

Akodontini

Necromys lasiurus

AY277459

AY273912

Akodontini

Oxymycterus nasutus

AY277468

EF661854

Akodontini

Scapteromys tumidus

AY163637

AY275133

Akodontini

Thalpomys cerradensis

AY277481

AY273916

Akodontini

Thaptomys nigrita

AY277482

AF108666

Ichthyomyini

Rheomys raptor

AY163635

-

Incertae sedis

Abrawayaomys ruschii

JX949185

JX949189

Incertae sedis

Andinomys edax

JQ434400

AF159284

Incertae sedis

Chinchillula sahamae

JQ434409

JQ434422

Incertae sedis

Delomys sublineatus

AY163582

AF108687

Incertae sedis

Euneomys chinchilloides

AY277446

AY275115

Incertae sedis

Irenomys tarsalis

AY163587

U03534

Incertae sedis

Juliomys pictipes

AY163588

AF108688

Incertae sedis

Neomicroxus latebricola

QCAZ4160

QCAZ4160

Incertae sedis

Neotomys ebriosus

HM061605

HM061604

Incertae sedis

Punomys kofordi

JQ434414

JQ434426

Oryzomyini

Aegialomys xanthaeolus

GQ178247

EU579479

Oryzomyini

Amphinectomys savamis

AY163579

EU579480

Oryzomyini

Cerradomys scotti

EU649040

EU579482

Oryzomyini

Drymoreomys albimaculatus

EU649042

EU579487

Oryzomyini

Eremoryzomys polius

AY163624

EU579483

Oryzomyini

Euryoryzomys macconnelli

AY163620

EU579484

Oryzomyini

Handleyomys intectus

AY163584

EU579490

Oryzomyini

Holochilus brasiliensis

AY163585

EU579496

Oryzomyini

Hylaeamys megacephalus

AY163621

EU579499

Oryzomyini

Lundomys molitor

AY163589

EU579501

Oryzomyini

Melanomys caliginosus

EU649052

EU340020

Oryzomyini

Microakodontomys transitorius

EU649054

-

Oryzomyini

Microryzomys minutus

AY163592

AF108698

Oryzomyini

Neacomys spinosus

AY163597

EU579504

Oryzomyini

Nectomys squamipes

AY163598

EU340012

Oryzomyini

Nephelomys albigularis

EU649057

EU579505

Oryzomyini

Nesoryzomys fernandinae

EU649058

EU579506

Oryzomyini

Oecomys bicolor

AY163604

EU579509

Oryzomyini

Oligoryzomys fulvescens

AY163611

DQ227457

Oryzomyini

Oreoryzomys balneator

AY163617

EU579510

Oryzomyini

Oryzomys palustris

AY163623

EU074639

Oryzomyini

Pseudoryzomys simplex

AY163633

EU579517

Oryzomyini

Scolomys ucayalensis

AY163638

EU579518

Oryzomyini

Sigmodontomys alfari

AY163641

EU340016

Oryzomyini

Sooretamys angouya

AY163616

EU579511

Oryzomyini

Tanyuromys aphrastus

JF693878

JF693877

Oryzomyini

Transandinomys talamancae

AY163627

EU579514

Oryzomyini

Zygodontomys brevicauda

AY163645

EU579521

Phyllotini

Andalgalomys pearsoni

EU649038

AF159285

Phyllotini

Auliscomys pictus

AY277434

JQ434420

Phyllotini

Calomys lepidus

AY163580

EU579473

Phyllotini

Eligmodontia typus

AY277445

AF108692

Phyllotini

Galenomys garleppi

JQ434410

JQ434423

Phyllotini

Graomys griseoflavus

EU649037

EU579472

Phyllotini

Loxodontomys micropus

AY277457

AY275122

Phyllotini

Phyllotis xanthopygus

AY163632

AY275128

Phyllotini

Salinomys delicatus

JQ434415

EU377608

Phyllotini

Tapecomys primus

JQ434416

AF159287

Phyllotini

Phyllotini n. gen.

JQ434417

JQ434425

Reithrodontini

Reithrodon auritus

AY163634

EU579474

Sigmodontini

Sigmodon alstoni

EU635698

AF293397

Thomasomyini

Aepeomys lugens

DQ003722

-

Thomasomyini

Chilomys instans

-

AF108679

Thomasomyini

Rhagomys longilingua/rufescens

DQ003723

AY206770

Thomasomyini

Rhipidomys macconnelli

AY277474

AY275130

Thomasomyini

Thomasomys aureus

AY277483

U03540

Wiedomyini

Wiedomys pyrrhorhinos

AY163644

EU579477

Outgroup

Arvicola terrestris

AY277407

AY275106

Cricetus cricetus

AY277410

AY275109

Microtus socialis

FM162055

AY513830

Neotoma albigula

AY277411

AF108704

Nyctomys sumichrasti

AY163603

AY195801

Phodopus sungorus

AY163631

JN015007

Scotinomys xerampelinus

AY277416

AF108706

Tylomys nudicaudus

AY163643

DQ179812

Sequences of the genera Bibimys and Rhagomys of each gene were gathered from different species. GenBank accession numbers for each gene (IRBP and Cyt b) are indicated in the last two columns. Tribal assignations follow D'Elía et al. 2007 (see also D'Elía and Pardiñas 2007) and results of the present study.

Alignment was done with Clustal X (Thompson et al. 1997) using default parameters for all alignment parameters. Uncorrected genetic distances (p distances) with pairwise deletions were computed using MEGA 5 (Tamura et al. 2011). Each matrix was subjected to maximum-likelihood (ML) (Felsenstein 1981) and Bayesian analyses (BA) (Rannala and Yang 1996). The ML analysis was conducted in Treefinder (Jobb et al. 2004; Jobb 2008). The best fitting models of nucleotide substitution (IRBP: TVM[Optimum, Empirical]:G[Optimum]:5; Cyt b: GTR[Optimum, Empirical]:G[Optimum]:5) (see Jobb 2008) were selected with the Akaike information criterion in Treefinder using the ‘propose model’ routine. The best tree was searched under the model of nucleotide substitutions previously selected using search algorithm 2 implemented in Treefinder version March 2011; nodal support was estimated with 1,000 bootstrap pseudoreplicates (BS). The BA was conducted using MrBayes 3.1.2 (Ronquist and Huelsenbeck 2003) by means of two independent runs with three (IRBP) or seven (Cyt b) heated and one cold Markov chains each. Considering the model selected by Treefinder and models specified in MrBayes, a model with six categories of a base substitution, a gamma-distributed rate parameter, and a proportion of invariant sites was specified for both matrices; all model parameters were estimated using MrBayes. Runs were allowed to proceed for 20 million (IRBP) and 28 million (Cyt b) generations, and trees were sampled every 1,000 (IRBP) and 2,000 (Cyt b) generations. Log-likelihood values were plotted against the generation time to check that runs converged on a stable log-likelihood value. The first 25% of sampled trees were discarded as burn-in; the remaining trees were used to compute a 50% majority rule consensus tree and obtain posterior probability (PP) estimates for each clade.

Results

Cyt b gene sequences of Abrawayaomys gathered from the three specimens collected in the states of São Paulo and Rio de Janeiro varied by 0.5% ~ 0.7%; while comparisons involving the Cyt b sequence of the specimen from Minas Gerais ranged 2.6% ~ 3.0%. As mentioned above, IRBP sequences of the four specimens analyzed were identical.

Abrawayaomys, as represented by sequences of specimen MN 67557, was highly divergent from all compared sigmodontines for both analyzed genes. For the Cyt b gene, observed pairwise values involving Abrawayaomys ranged 19.0% ~ 25.9% for comparisons with Neotomys and Lundomys. For the IRBP gene, observed pairwise values involving Abrawayaomys ranged 3.42% ~ 6.65% for comparisons with Brucepattersonius and Rheomys.

Phylogenetic analyses recovered general results congruent with those of previous IRBP- and/or Cyt b-based studies (e.g., D'Elia et al. 2006a, b; Percequillo et al. 2011; Martínez et al. 2012; Parada et al. 2013; Salazar-Bravo et al. 2013). For descriptions and discussions of these findings (e.g., a strongly supported Sigmodontinae, a strongly supported Oryzomyalia, the polyphyly of the Reithrodon group, and differences between gene trees), we refer the reader to those studies because herein we focus on the phylogenetic position of Abrawayaomys.

Depending on the gene analyzed, the phylogenetic position of Abrawayaomys varied from being sister to Akodontini (IRBP BA; PP = 0.62; Figure 1), sister to Thomasomyini (IRBP ML; BS < 50), sister to Reithrodontini (Cyt b BA; PP = 0.98; Figure 2), or sister to Neotomys (Cyt b ML; BS < 50). As noted, with the sole exception of the BA based on Cyt b sequences, where the Abrawayaomys-Reithrodon clade had strong support, all sister-group relationships involving Abrawayaomys lacked any significant support. The most inclusive and well-supported clade containing Abrawayaomys was that corresponding to Oryzomyalia (sensu Steppan et al. 2004).
Figure 1

Results of the Bayesian analysis of IRBP gene sequences. Numbers indicate posterior probability (left of the diagonal) and ML bootstrap values (right of the diagonal) of adjacent nodes. Only bootstrap values of >50% are shown. Dashes signal nodes that were not recovered in the ML topology (ln = -9,764.8433). Dots at terminals indicate genera with species possessing dorsal spines.

Figure 2

Results of the Bayesian analysis of Cyt b gene sequences. Numbers indicate posterior probability (left of the diagonal) and ML bootstrap values (right of the diagonal) of adjacent nodes. Only bootstrap values of >50% are shown. Dashes signal nodes that were not recovered in the ML topology (ln = -33,444.453).

Discussion

The diversity of sigmodontine forms has long captivated students of New World mammals, but at the same time, it has seriously defied those attempting to classify them according to their evolutionary history. Problems range from species boundaries to relationships among species and genera, and limits and contents of higher taxa (e.g., tribes). These issues have direct implications for the study of the history of the diversification of the group, which in turn arguably constitutes one of the most controversial debates in muroid systematics (Voss 1993; D'Elía 2003b).

Herein, we showed that the phylogenetic position of Abrawayaomys varies from being sister to Akodontini, Thomasomyini, Reithrodontini, or Neotomys, depending on the gene analyzed (IRBP or Cyt b) and the analysis performed (ML or BA). Given that previous studies showed discrepancies between the topologies of different sigmodontine gene trees (e.g., Feijoo et al. 2010; Teta et al. 2011), the fact that different datasets (i.e., IRBP or Cyt b matrices with slightly different taxonomic sampling) provide different relationships for Abrawayaomys is not unexpected. Importantly, with the exception of the BA based on Cyt b sequences (sister to Reithrodontini), no sister relationship involving Abrawayaomys was recovered that had good support. In light of the analytical results, Abrawayaomys cannot be placed with certainty in any more inclusive clade than that of Oryzomyalia (sensu Steppan et al. 2004).

Pacheco (2003, p. 130), in a phylogenetic analysis based on morphological characters, found the genus Abrawayaomys to be sister to Rhagomys within the Thomasomyini clade. This relationship was supported by the following character states: a broad zygomatic plate, an interorbital region convergent with the supraorbital margins squared or weakly beaded, a long jugal, the absence of mesolophids (but see below), a masseteric crest anterior to the procingulum of the first lower molar, and a deeply excavated retromolar region of the mandible. Pacheco (2003) highlighted the retromolar region condition, i.e., broad and fenestrated, as a synapomorphy of Abrawayaomys + Rhagomys. We agree with Pacheco (2003) (see Musser and Carleton 2005) in the general resemblance between Abrawayaomys and Rhagomys and to a lesser extent to the remainder of the Thomasomyini, but we suggest that this similarity is not remarkable and more important and that these shared character states are not synapomorphies of a putative Abrawayaomys and Rhagomys clade. Almost all of those character states listed by Pacheco (2003) are also present in many sigmodontines, indicating the large amount of homoplasy existing within this group. In addition, we assert that the molar morphologies of both genera are quite distinct, having only the widespread brachyodont condition in common. Rhagomys has very well-developed mesolophs/phids (cf. Luna and Patterson (2003) vs. Pacheco (2003)), procingula of the first upper molars clearly crossed by a deep anteromedian flexus, well-developed posterolophs, a slightly reduced third lower molar with respect to the second lower molar, and several other traits found among taxa displaying the dental bauplan of the pentalophodont type (sensu Hershkovitz 1962), which is clearly distinguishable from the unequivocally tetralophodont molar of Abrawayaomys (cf.; Pardiñas et al. 2009b). Additional differences between these two genera are more than trenchant, including incisive foramina and palate extensions, parapterygoid plate morphology, and carotid circulatory pattern (Table 2) (see also Pardiñas et al. 2009b, Table three). Similarly, several trenchant character states are present in Abrawayaomys and representatives of other sigmodontine tribes. The external morphology of Abrawayaomys resembles that of many akodonts (cf. Pereira et al. 2008, Figure one), although it has a moderately longer tail, at least in some individuals. A morphological description of the stomach (Finotti et al. 2003) suggests a hemiglandular-unilocular type, a widespread condition among sigmodontines (Carleton 1973). Finally, Pacheco (2003) also indicated that Abrawayaomys has a peculiar genal vibrissa (called genal vibrissa 2), which is also present in the oryzomyine Oecomys and the akodontine Kunsia, and a reduced fifth pedal digit. Similarly, karyotypic evidence sheds no conclusive light on the phylogenetic position of Abrawayaomys, as the diploid number of 2n = 58 (Pereira et al. 2008) found in A. ruschii is also present in several distantly related oryzomyine species, such as Euryoryzomys lamia (Andrades-Miranda et al. 2000), Holochilus brasiliensis (Yonenaga-Yassuda et al. 1987), Nectomys squamipes (Yonenaga-Yassuda et al. 1987), Oecomys trinitatis (Patton et al. 2000), Oligoryzomys chacoensis (Myers and Carleton 1981), and Sooretamys angouya (Andrades-Miranda et al. 2000). Taking all this evidence as a whole, Abrawayaomys cannot be placed with certainty in any clade less inclusive than the large clade Oryzomyalia (see also Voss 1993). Therefore, in light of all of the evidence at hand, in formal classifications, Abrawayaomys should be kept as an incertae sedis sigmodontine (D'Elía et al. 2007). Results of future phylogenetic analyses, especially those including sequences of two other Atlantic forest inhabitants, Phaenomys and Wilfredomys, which were also considered part of the tribe Thomasomyini (e.g., Pacheco 2003) and are now regarded as Sigmodontinae incertae sedis (e.g., D'Elía et al. 2007), may prompt changes in this classification.
Table 2

Morphological comparisons among Abrawayaomys and other members of the Sigmodontinae

Character

Abrawayaomys

Rhagomys b

Phaenomys d

Thomasomys f

Aepeomys g

Akodon

Reithrodon

Plantar pads

6

6

6

6

6

6

4

Hindfoot surface

Smooth?

Smooth

Squamated

Smooth

Smooth

Squamated

Squamated

Mammae

6a

6

8

6

6

8

8

Spines

Present

Presentc

Absent

Absent

Absent

Absent

Absent

Relation of tail length (LT) to head-body length (HB)

LT < = > HB

LT ≤ HB

LT > > HB

LT < = > HB

LT > HB

LT ≤ HB

LT < < HB

Rostrum

Short

Short

Long

Long

Long, rostral tube developed

Moderate

Moderate

Interorbit

Hourglass-shaped or slightly convergent, with rounded margins

Convergent, with beaded margins

Hourglass-shapede, with beaded margins

Hourglass-shaped, with rounded margins

Hourglass-shaped, with rounded margins

Hourglass-shaped, with rounded margins

Symmetrically constricted, with parallel margins

Palate

Typically short

Long

Short

Short

Short

Short

Long

Mesopterygoid fossa

Not fenestrated

Not fenestrated

Not fenestrated

Not fenestrated

Not fenestrated

Fenestrated

Fenestrated

Alisphenoid strut

Typically present

Present

Absent

Present

?

Typically present

Present

Tegmen tympani

Overlaps squamosal

Overlaps squamosal

Overlaps squamosal

Overlaps squamosal

Overlaps squamosal

Overlaps squamosal

Does not overlap squamosal

Carotid circulation

Pattern 1

Pattern 3

Pattern 1

Pattern 1

Pattern 1

Pattern 1

Pattern 3

Capsular process

Present

Present

Absent

Absent

Absent

Typically present

Present

Retromolar fossa

Enlarged

Enlarged

Not enlarged

Not enlarged

Not enlarged

Not enlarged

Not enlarged

Molar design

Intermediate to alternate, crested

Opposite, crested

Opposite, crested

Opposite, crested

Opposite, crested

Intermediate, crested to terraced

Alternate, plane

Anteromedian flexus

Patent

Patent

Patent

Patent

Patent

Patent

Not patent

Mesoloph on M1

Present, small

Present, large

Present, large

Present, large

Present, large

Present, small

Absent

M3 reduction to M2

Much reduced

Moderately

Weakly

Moderately

Moderately to reduced

Much reduced

Weakly

Incisors

Orthodont to proodont

Orthodont

Opisthodont

Opisthodont

Opisthodont

Typically opisthodont

Opisthodont

Incisive foramina

Reaching anterior face M1

Very short

Reaching anterior face M1

Reaching anterior face M1

Reaching anterior face M1

Reaching protocone M1

Reaching protocone M1

Subsquamosal foramen

Present

Present

Absent

Present

Present

Present

Present

Number of ribs

12

13

12

13

13

13

12

Gall bladder

Absent

Absent

Present

Present

Present

Typically presenth

Present

aInguinal, abdominal, and postaxial pairs (Pardiñas, unpublished data). bBased on Rhagomys longilingua, after Luna and Patterson (2003). c Rhagomys rufescens lacks spines (cf. Luna and Patterson 2003). dData from Voss et al. (2002) and Pardiñas (unpublished data). eScored as ‘convergent’ by Pacheco (2003, p. 43). fVariation in this genus is remarkable (cf. Pacheco 2003); herein, we follow Voss (1993). gBased on Aepeomys lugens, after Voss et al. (2002). h Akodon montensis lacks gall bladder (cf. Geise et al. 2004).

The prevailing biogeographic view is that the Andes played a major role in sigmodontine diversification, in which the main sigmodontine lineages originated there and later colonized lowlands of South America (Reig 1984,1986; see also Salazar-Bravo et al. 2013). However, Abrawayaomys, an Atlantic forest endemic, is the sole living representative of one of the main sigmodontine lineages (i.e., those classified at the tribal rank in formal classifications or those genera that do not belong to any recognized tribe). A similar scenario was found for two other Atlantic forest endemics, the genera Delomys and Juliomys (Figures 1 and 2) (Voss 1993; D'Elía et al. 2006a; see also the classification in D'Elía et al. 2007). The other main sigmodontine lineages are distributed outside the Atlantic forest (e.g., Abrotrichini and Sigmodontini) or, even when present in this biome, are not endemic to it (e.g., Akodontini and Oryzomyini). As mentioned above, the Atlantic forest-endemic Phaenomys and mostly Atlantic forest-resident Wilfredomys have not been included in any molecular-based phylogenetic analysis. Until their phylogenetic position is assessed, it is unclear whether they in fact represent additional main sigmodontine lineages almost endemic to the Atlantic forest or simply constitute additional genera belonging to other already identified main lineages of the Sigmodontinae either already known from the Atlantic forest or not. Whatever this result is, the finding that at least three unrelated main sigmodontine lineages, those currently represented by Abrawayaomys, Delomys, and Juliomys, are endemic to the southern Atlantic forest supports early claims (Smith and Patton 1999; D'Elía 2003b; see also Salazar-Bravo et al. 2013) highlighting the role of the Atlantic forest in harboring sigmodontine phylogenetic diversity. Future studies should be designed to test if these lineages originated in the Atlantic forest or simply invaded it after originating elsewhere.

Spines of varying hardness and architectures are present in several rodents (Chernova and Kuznetsov 2001) and are conspicuous in some Neotropical groups, such as porcupines (Erethizontidae) and spiny rats (Echimyidae). The vast majority of sigmodontine rodents have soft fur, but a few genera and species have dorsal spines. These spines are present in both species of Abrawayaomys, both species of Scolomys, all eight species of Neacomys, but only in one of the two species of Rhagomys. Rhagomys longilingua from the Andes has spiny fur, but Rhagomys rufescens from the Atlantic forest has soft fur (Luna and Patterson 2003). Considering the phylogeny portrayed here, we concluded that these spines are the result of evolutionary convergence, and this trait evolved at least four times in sigmodontines (Figure 1). None of these four genera are sister groups, and each one of them shares a more recent common ancestor with soft-furred genera or species (in the case of Rhagomys). The functional significance of spines remains unknown. Unlike porcupine quills, spines of muroid or echimyid rodents are insufficiently rigid to provide much protection against predators, including snakes, birds, and mammals, which are known to prey heavily on many spiny species (Hoey et al. 2004). Patterson and Velazco (2008) suggested a thermoregulatory interpretation based on the geographic distribution of echimyid rodents: the spiniest members occur in tropical lowland forests, while many of the softest-haired members of the family range into high elevations or latitudes, but they also noted several exceptions. The same pattern does not occur in sigmodontine rodents, since soft-furred taxa occur at all latitudes and elevations throughout the Neotropics. Regarding spiny taxa, both Neacomys and Scolomys are lowland forest genera found at lower latitudes (Patton et al. 2000); Abrawayaomys also occurs in lowland forests, but at higher latitudes in the Atlantic forest (Pardiñas et al. 2009b); and the spiny R. longilingua is found in Andean cloud forests at 1,900 ~ 2,100 m in elevation (Luna and Patterson 2003), while the soft-furred R. rufescens occurs mostly in montane Atlantic forest at 500 ~ 1,000 m in elevation (Steiner-Souza et al. 2008). Therefore, we still lack robust hypotheses for the ecological role, if any, of spiny fur in sigmodontine rodents, but the character distribution mapped on a phylogenetic tree presented here is the first step toward understanding its evolutionary importance.

Conclusions

Phylogenetic analyses show that Abrawayaomys constitutes the single representative so far known of one of the main lineages of the radiation of Sigmodontinae. In addition, it differs from all other Atlantic forest sigmodontine rodents by having a unique combination of morphological character states. Therefore, in formal classifications, it should be regarded as a Sigmodontinae incertae sedis.

Finally, the observed variation of the four Cyt b sequences analyzed and their geographic pattern, where the one gathered from the northernmost-collected specimen (in the state of Minas Gerais) was the most divergent, are enticing to further explore variation of a larger sample of A. ruschii sequences. Such a study would clarify into which of the already known phylogeographic patterns of Atlantic forest mammals (e.g., Colombi et al. 2010; Ventura et al. 2012; Valdez and D'Elía 2013; see reviews in Martins 2011; Costa and Leite 2012) would A. ruschii fit, or if this species presents a so far undescribed pattern. Similarly, such a study would help assess the alpha diversity of Abrawayaomys (Pardiñas et al. 2009b). Now, that A. ruschii is becoming more frequent in specimen collections, such a study seems feasible.

Declarations

Acknowledgements

We thank André Almeida Cunha, Arlei Marcili, Caroline Cotrim Aires, Laerte Bento Viola, Marcelo Passamani, Patricia B. Bertola, Renata Pardini, and Sandra Favorito for support with fieldwork and collecting specimens, and Juliana F. Justino for producing some of the DNA sequences. Two anonymous reviewers provided valuable comments on an earlier version of this contribution. This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-BEX 3795/08-0) (to LG), Fundação de Amparo a Pesquisa do Estado de São Paulo BP.PD 2009/54300-0 (to KV), JP 05/07553-8 (to MJJS), Fundação de Amparo à Pesquisa do Espírito Santo (to YLRL), Fundação de Amparo do Estado do Rio de Janeiro (FAPERJ-E-26/111.525/2010) (to LG), PICT (Agencia) 2008–547 (to UFJP), Fondo Nacional de Desarrollo Científico y Tecnólogico 1110737 (to GD), and MECESUP AUS1203 (to GD).

Authors’ Affiliations

(1)
Departamento de Genética e Biologia Evolutiva, Instituto de Biociências, Universidade de São Paulo
(2)
Laboratório de Ecologia e Evolução, Instituto Butantan
(3)
Laboratório de Mastozoologia, Departamento de Zoologia, Instituto de Biologia, Universidade do Estado do Rio de Janeiro
(4)
Laboratório de Mastozoologia e Biogeografia, Departamento de Ciências Biológicas, Centro de Ciências Humanas e Naturais, Universidade Federal do Espírito Santo
(5)
Unidad de Investigación Diversidad, Sistemática y Evolución, Centro Nacional Patagónico
(6)
Instituto de Cs. Ambientales y Evolutivas, Facultad de Ciencias, Universidad Austral de Chile

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© Ventura et al.; licensee Springer. 2013

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.