This review was done after an extensive revision of the literature, including Master’s and Ph.D. theses, when available (Table 1). Abstracts from congresses and conferences were not considered, since karyotype pictures were only available during the events and access to this kind of material is restricted. Chromosome rearrangements in Table 1 were named as described in the literature (for example Robertsonian rearrangement, centric fusion, etc.). However, in the text, we refer to centric fusion/fission as a synonym of Robertsonian rearrangement (Sumner 2003). Except for the species that have not been formally described (e.g. Thaptomys sp., Proechimys gr. goeldii, etc.), the taxonomical classification follows the one proposed by Patton et al. (2015) and Fabre et al. (2016), that recently included Myocastor Kerr, 1792 within the Family Echimyidae.

The single female of Carterodon sulcidens (lab number: CIT787/ field number: APC58) was captured in Serra da Mesa, State of Goiás, Brazil (13°53'S, 48°19'W), a region characterized by the Cerrado biome. Additionally, five males of Mus musculus (field number: PCH4078, 4079, 4094–96) were captured in Guará, São Paulo State, Brazil (20°29'S, 47°51'W), a transitional region between the Cerrado and Atlantic Forest.

Regarding Neacomys, four specimens of N. amoenus amoenus Thomas, 1903 were captured in Mato Grosso State, Brazil, in a transitional area between Cerrado and Amazonian Rainforest. Two specimens of Neacomys sp. were captured, one at Vila Rica (Mato Grosso State), and the other at Igarapé-Açu (Amazonas State), Brazil (field number, locality, and coordinates are presented in Suppl. material 1).

Chromosome preparations of Carterodon sulcidens, the five samples of Mus musculus, four Neacomys a. amoenus, and a specimen of Neacomys from Vila Rica, Mato Grosso State, were obtained in vivo from bone marrow and spleen, following Ford and Hamerton (1956) or in vitro from fibroblast culture (Freshney 1986). Conventional Giemsa staining was performed to determine the diploid and fundamental numbers, and C-banding and Ag-NOR were performed according to Sumner (1972) and Howell and Black (1980), respectively.

DNA was extracted from the liver or muscle with Chelex 5% (Bio-Rad) (Walsh et al. 1991) of five specimens of Neacomys. DNA of the specimen from Vila Rica, Mato Grosso State, was extracted from fibroblast cell culture using DNeasy Blood and Tissue kit (Qiagen, catalog number 69506).

PCR was performed in a thermal cycler (Eppendorf Mastercycler ep Gradient, Model 5341) using primers MVZ05 (5-CGA AGC TTG ATA TGA AAA ACC ATC GTT G-3) and MVZ16 (5-AAA TAG GAA RTA TCA YTC TGG TTT RAT-3) (Irwin et al. 1991, Smith and Patton 1993, respectively). PCR mixture contained 30 ng of DNA, 25 pmol of each primer, 0.2 mM of dNTP, 2.52 µL of reaction buffer (50 mM KCl, 2.5 mM MgCl₂, 10 mM Tris-HCl; pH 8.8) and 0.2 units of Taq DNA polymerase (Invitrogen). Thirty-nine amplification cycles were performed, consisting of denaturation at 94 °C for 30 s, annealing at 48 °C for 45 s, extension at 72 °C for 45 s and the final extension at 72 °C for 5 min. The PCR products were separated using 1% agarose gel in TAE buffer. Sequencing was conducted using BigDye (DNA “Big Dye Terminator Cycle Sequencing Standart,” Applied Biosystems) and an ABI PRISM 3100 Genetic Analyzer (Applied Biosystems). All sequences were submitted to a comparative similarity search on BLAST (Basic Local Alignment Search Tool) before the alignment. Alignments were performed by using Muscle (Edgar, 2004) implemented in Geneious 4.8.5 (Biomatters). GenBank access numbers are provided in Suppl. material 1.

Models of nucleotide substitution were selected using Bayesian Information Criterion (BIC), implemented in PartitionFinder, version 1.1.1 (Lanfear et al. 2012). Approximately 673 bp were used to perform Maximum Likelihood (ML) in GARLI 2.0 (Bazinet et al. 2014) and Bayesian Inference (BI) in MrBayes 3.04b (Ronquist and Huelsenbeck 2003), using 69 additional Neacomys sequences downloaded from GenBank, plus sequences of Euryoryzomys russatus (Wagner, 1848), Holochilus brasiliensis (Desmarest, 1819) and Oligoryzomys nigripes (Olfers, 1818) as the outgroup (see Suppl. material 1).

From a total of ten species, cytogenetic data is lacking for only one: Galea flavidens (Brandt, 1835). The diploid number varied from 2n = 52 in Kerodon acrobata Moojen, 1997 and K. rupestris (Wied-Neuwied, 1820) to 2n = 66 in Hydrochoerus hydrochaeris (Linnaeus, 1766). Currently, polymorphism of autosomal chromosomes has been described for Cavia porcellus (Linnaeus, 1758), pericentric inversions for C. magna Ximénez, 1980 and K. rupestris, and Robertsonian rearrangement for C. magna (Maia 1984, Gava et al. 2011) (Table 1).

This group comprises the genera Abrawayaomys F. Cunha & Cruz, 1979, Delomys Thomas, 1917, Juliomys E. M. González, 2000, Phaenomys Thomas, 1917, and Wilfredomys Avila-Pires, 1960, which could not be inserted into any other tribes, according to phylogenetic and morphological analyses (Musser and Carleton 2005, Patton et al. 2015). Cytogenetic information is available for all species, except one, Wilfredomys oenax (Thomas, 1928), and is helpful for distinguishing species of the genus Delomys and Juliomys.

This family comprises a single genus, Ctenomys, which presents a great variation in diploid numbers, especially C. lami T. R. O. Freitas, 2001, C. minutus Nehring, 1887 and C. torquatus Lichtenstein, 1830 for which Robertsonian rearrangements and in tandem fusions were described (Freitas and Lessa 1984, Fernandes et al. 2009). The diploid number varied from 36 in Ctenomys nattereri Wagner, 1848 to 58 in C. lami. Only one species out of eight lacks karyotype information. Cytogenetic data was useful for recognizing Ctenomys bicolor Miranda- Ribeiro, 1914, C. ibicuiensis T. R. O. Freitas, Fernandes, Fornel & Roratto, 2012 and C. nattereri, because it presents exclusive karyotype (Stoulz 2012). Pericentric inversion has been described for C. lami and in tandem fusions for C. minutus.

This family is represented by a single species, Cuniculus paca (Linnaeus, 1766), with a wide distribution and unique karyotype (2n = 74, FN = 98) (Giannoni et al. 1991, Bonvicino et al. 2008).

This family comprises two genera: Dasyprocta Illiger, 1811, with nine species, and Myoprocta Thomas, 1903, with two species (Patton and Emmons 2015). There is no cytogenetic data known for three species (Table 1). The diploid number in the Family varied from 62 to 65, and in the genus Dasyprocta, from 64 to 65, due to the presence of B chromosomes in four species (Ramos et al. 2003).

This family possesses only one species, Dinomys branickii Peters, 1873, to which the karyotype is 2n = 64, FN = 98 (Table 1).

Even being the second largest Brazilian rodent family, a remarkable gap regarding cytogenetic data of this family still remains, with 14 species out of 68 lacking such information. This represents about 37% of all the unknown karyotypic information of all Brazilian rodents.

Diploid numbers varied from 2n = 15 in Proechimys goeldii Thomas, 1905 to 118 in Dactylomys boliviensis. B chromosomes have been described for one species: Trinomys iheringi (Thomas, 1911) (Yonenaga-Yassuda et al. 1985), pericentric inversion for seven species, and Robertsonian rearrangement for three. A multiple sex chromosome system was described for Proechimys cf. longicaudatus (Amaral et al. 2013), and addition/deletion of constitutive heterochromatin was described for Clyomys laticeps (Thomas, 1909) and P. longicaudatus (Rengger, 1830) (Souza and Yonenaga-Yassuda 1984, Bezerra et al. 2012, Machado et al. 2005). Secondary constriction is a characteristic feature of several species, occurring in Carterodon sulcidens (this work), Clyomys laticeps, Mesomys occultus Patton, da Silva & Malcolm, 2000, Makalata didelphoides (Desmarest, 1817), Proechimys gardneri M. N. F. da Silva, 1998, all five Thrichomys E.- L. Trouessart, 1880 species, and seven species of Trinomys Thomas, 1921.

Within this family, there are also cases in which different diploid numbers are assigned to the same name. In the case of Clyomys laticeps, the 2n = 34, FN = 58, 60, 62 and 2n = 32, FN = 54, the karyotypes are very similar, and differ by a Robertsonian rearrangement and pericentric inversion (2n = 32). Also, species such as Phyllomys pattoni Emmons, Leite, Kock & Costa, 2002 and Proechimys guyannensis E. Geoffroy, 1803 should be investigated by molecular phylogeny and morphology, because they are prone to either represent species-complex or have taxonomic misidentification.

In this work, the karyotype of Carterodon sulcidens is being described for the first time, showing 2n = 66. Since the animal was a female, it was not possible to recognize the X chromosomes and the exact morphology of the small pair, so we could not establish the fundamental number. Karyotype is composed of 32 acrocentric pairs decreasing in size and presumably one biarmed pair (pair 33). Also, the fourth largest pair possesses a remarkable secondary constriction (Fig. 3a). Constitutive heterochromatin is located in the pericentromeric region of all autosomes (Fig. 3b). Ag-NOR showed signals in the secondary constriction of pair 4 (Fig. 3b inset).

Figure 3.

Karyotype of a female of Carterodon sulcidens with 2n=66 from Serra da Mesa, Goiás State, Brazil. a Giemsa-staining. Inset: Pair 4 with evident secondary constriction b C-banding. Inset: Pair 4 after silver nitrate staining.

Within the Echimyidae Family, the only other species with 2n = 66 described so far is Makalata didelphoides, but its karyotype presents 20 pairs of metacentric chromosomes, which clearly differs from the karyotype of Carterodon sulcidens.

Family Erethizontidae

Three out of eight species lack cytogenetic information. The diploid number varied from 42 in Coendou spinosus (F. Cuvier, 1823) to 74 in C. prehensilis (Linnaeus, 1758) (Lima 1994, Mendes-Pontes et al. 2013) (Table 1).

This family (represented by the genera Mus and Rattus) was introduced from Europe, and even though it is not a native, it is currently widespread throughout Brazil (Musser and Carleton 2005).

Little is known about the cytogenetics of the Mus musculus Brazilian populations because this species seems to be negglected. The present paper features the first picture of Mus musculus karyotype from Brazil. This species presented 2n = 40, FN = 38, with all chromosomes acrocentrics. C-banding was restricted to the centromeric region of all chromosomes (Fig. 4). Sex chromosomes could only be recognized after G-banding (not showed) because they have similar morphology compared to the autosomes.

Figure 4.

Karyotype after C-banding of a male of Mus musculus with 2n=40, FN=38, from Guará, São Paulo State, Brazil.

For the black rat Rattus rattus Linnaeus, 1758, diploid number of South America population is the same as those from Oceania (2n = 38), and Kasahara and Yonenaga-Yassuda (1981) described pericentric inversion for individuals from São Paulo, Brazil.

Cytogenetic data is unknown for almost the entire family. For the two species to which chromosome information is known, diploid number is 2n = 40, and pericentric inversion has been described for one of them, Guerlinguetus brasiliensis (Gmelin, 1788) (Lima and Langguth 2002, Fagundes et al. 2003) (Table 1).

The last cytogenetic revision on Brazilian rodents, published in 1984, described the karyotype of 62 species, mainly from South and Southeast Brazil (Kasahara and Yonenaga-Yassuda 1984). This paper compiles the karyotype of 271 species distributed throughout Brazil, representing an increase of more than 300%.

Since then, new cytotypes have been attributed to already known species. For instance, new diploid numbers were described for Ctenomys torquatus and new fundamental numbers for Oligoryzomys nigripes (described as Oryzomys nigripes – see references in Table 1). B chromosomes were described for Sooretamys angouya and also for four species of Dasyprocta. Undescribed rearrangements, including multiple sex chromosome system, were also detected (see Table 1). Moreover, new karyotypes that could not be correlated to any name were published, evidencing the possibility that an undescribed species may exist (e.g.: Akodon sp. n., Deltamys sp., Thaptomys sp., Euryoryzomys sp., Neacomys sp., Oecomys sp. 1 – 4, Oligoryzomys sp., Juliomys sp., Dasyprocta sp. Proechimys sp. – see Table 1). Additionally (as we will mention below) there are many species with a different diploid number associated that do not represent polymorphisms, which need to be revised (e.g. Euryoryzomys lamia, Euryoryzomys macconnelli, Hylaeamys yunganus, Oecomys auyantepui, Oecomys cleberi, Oecomys paricola, Oecomys roberti, Zygodontomys brevicauda, Rhipidomys nitela, Phyllomys pattoni, Proechimys guyannensis, etc.).

Since 1984, many species’ names have been redescribed or validated (e.g. Zygodontomys lasiurus was named as Bolomys lasiurus for a long time, and nowadays is Necromys lasiurus – see synonyms of Table 1). Also, due to the progress of molecular biology during the 1990, associated to morphological information, the number of species described has increased exponentially. It is important to emphasize that molecular phylogeny hitherto has contributed to better understand the cryptic diversity of Brazilian rodents, recognizing monophyletic clades. For instance, new candidate species of Akodon (Silva and Yonenaga-Yassuda 1998a, Silva et al. 2006), Oecomys (Suárez-Villota et al. under revision), Oligoryzomys (Andrades-Miranda et al. 2001a, Miranda et al. 2008), Neacomys (Silva et al. 2015, present paper), Thaptomys (Ventura et al. 2004, 2010), etc. were recognized based on new karyotypes associated to the monophyly of the samples. Even new genera were described based on multidisciplinary approaches: Drymoreomys (Percequillo et al. 2011) and Calassomys (Pardiñas et al. 2014).

Technological advances with fluorescent in situ hybridization (developed at the end of 1980’s but more used during 2000’s to date), made it possible to characterize chromosome rearrangements more precisely.

In this paper, we provide a new fundamental number for an undescribed species of Neacomys. The karyotype presented here (FN = 66) is similar to the one described by Silva et al. (2015) with FN = 64, except that we found five biarmed pairs and the distribution of constitutive heterochromatin in autosomes was restricted to pericentric regions. We suggest that differences in fundamental numbers are due to pericentric inversions in a small pair, since C-banding evidenced constitutive heterochromatin at the pericentromeric regions, and the morphology of chromosomes was accurately defined. Sex chromosomes presented the same morphology, although the Y was heterochromatic in the short arm (present paper), while it was entirely heterochromatic in the samples described by Silva et al. (2015).

Karyotype information was the first to point out that this specimen may represent a new species, since 2n = 58, FN = 66, has never been described for any Neacomys species. Although we used only one molecular marker (incomplete cyt-b), which was the same used by Silva et al. (2015), the phylogeny corroborates this information, since all samples with 2n = 58 clustered in a monophyletic high supported the clade. This included two well-supported structured clades, one with samples with FN = 70 (Chaves, Marajó Island) and the other with samples with FN = 64 and 66 (Marabá, Pará State and Vila Rica, Mato Grosso State, respectively), both sister clade to the sample from Igarapé-Açu, Amazonas State. Whether these samples belong to the same undescribed entity with strong population structure or whether they represent at least three different species must be clarified with further phylogeographic and morphological studies, including samples from other localities. This shows the importance of integrative approaches.

In fact, Neacomys have a greater diversity than previously known. Recently, based on morphology and molecular phylogeny, Hurtado and Pacheco (2017) demonstrated that Neacomys spinosus is a species complex and considered the subespecies Neacomys spinosus amoenus a valid species. After this revision, Neacomys spinosus is restricted to populations from Peruvian Amazon, and Neacomys amoenus encompasses two subspecies: Neacomys a. amoenus (from Brazilian Cerrado and Bolivia) and Neacomys a. carceleni (from Amazon basin of Ecuador, Brazil and Peru). Thus, sequences related to N. spinosus from central Brazil, and transition areas of Cerrado and Amazonia correspond to N. amoenus. Also, a new species, N. vargasllosai, from southern Peru and Bolivia was described. In this same revision, authors recovered three new species pending formal description (the first from Pará, Brazil, the second from Amazonas, Brazil, and the third from Peru and Ecuador). The one from Pará corresponds to the clade composed of samples with 2n = 58 (Fig. 2), reiteraiting the lack of knowledge in this genus.

The description of the karyotype of Carterodon sulcidens (a rare species) also corroborates the lack of knowledge for some species, and the importance of fieldwork in discovering new data.

We also show the picture of the karyotype of the exotic species Mus musculus for the first time. Despite the noteworthy variation in diploid numbers in Western Europe and Mediterranean populations because of Robertsonian rearrangements (Nachman et al. 1994), in Brazil, the only diploid number described was the standard one (2n = 40).

During the beginning of the 1970s (although banding techniques had already been described), karyotypes of Brazilian rodents were studied mainly through conventional staining and the description was limited to diploid and fundamental numbers. Even so, the idea of a wide chromosomal variability already existed. From the 1980s until now, comparative cytogenetics with chromosome banding persists and contributed for elucidating these variations, being that G and C-banding and Ag-NORs are the commonest and cheapest banding techniques.

In fact, the distribution of constitutive heterochromatin and Ag-NORs can be markers in some species. For example, large blocks of constitutive heterochromatin were detected in Clyomys laticeps (family Echimyidae) (Souza and Yonenaga-Yassuda 1984, Bezerra et al. 2012) and a huge heterochromatic arm in Pseudoryzomys simplex (family Cricetidae, subfamily Sigmodontinae, tribe Oryzomyini) (Moreira et al. 2013). C-band pattern is also an important technique for recognizing sex chromosomes, especially within the subfamily Sigmodontinae (Silva 1994, Di-Nizo 2013). Regarding the nucleolus organizer region, it seems that secondary constriction is a characteristic feature of the family Echimyidae and, as with other vertebrates, may be an important marker. However, chromosomal comparison is now passing from banding patterns to the use of higher resolution innovation of molecular cytogenetics using FISH.

FISH using chromosome painting allows a comparison in a wide genomic scale, revealing a greater number of chromosome changes, unrevealed by the commonest banding techniques, especially in the tribes Akodontini and Oryzomyini of the Subfamily Sigmodontinae. For instance, G-banding pattern showed several rearrangements between Akodon species (Tribe Akodontini) (Geise et al. 1998, Silva et al. 2006), but much more complex rearrangements within this genus were observed after cross-species chromosome painting (Ventura et al. 2009).

Extensive chromosomal rearrangements such as Robertsonian, in tandem fusion/fission and pericentric inversion, were also observed within the genus Oligoryzomys (Tribe Oryzomyini), after chromosome painting. Using a molecular phylogeny as a reference, it was also possible to detect the direction of the rearrangements and to infer that fission events were as common as fusion events (Di-Nizo et al. 2015). Moreover, Robertsonian rearrangement between Oryzomys rupestris Weksler & Bonvicino, 2005 (referred as Oligoryzomys sp. 1), 2n = 46, FN = 52, and Oligoryzomys sp. 2, 2n = 46, FN = 52 was firstly detected by using classic cytogenetic and FISH with telomeric probes (Silva and Yonenaga-Yassuda 1997) and later corroborated by chromosome painting (Di-Nizo et al. 2015). However further studies with molecular phylogeny and morphology are necessary to clarify if both entities represent a single species (with a polymorphism spread in the population) or two different species (in the case of this rearrangement resulted in reproductive incompatibilities leading to the speciation of ancestral population).

The advent of chromosome painting made it possible to compare not only related species but also distant ones, something which is difficult to achieve with banding patterns. Hass et al. (2008) compared Mus musculus (family Muridae) to Akodon species (family Cricetidae); Nagamachi et al. (2013) compared two different, unrelated genera of the tribe Oryzomiyni (Cerradomys and Hylaeamys) and Suárez et al. (2015) and Pereira et al. (2016) compared homologies between the tribes Akodontini and Oryzomyini.

Despite the ‘modern cytogenetics era’, chromosome banding is still an important tool for animal cytogenetic studies, not only because FISH cannot reveal chromosome inversions, but also because it is still a difficult and expensive technique to use.

Rodents proved to be a good model for chromosome evolution studies. Cytogenetics associated with molecular or morphological phylogenetic reconstruction broke cytogeneticist paradigms that fusion rearrangement is more common than fission, and that the reduction in 2n is the expected pattern (e.g. Di-Nizo et al. 2015).

Chromosomal rearrangement could possibly be the cause of reproductive isolation in many Brazilian rodents’ species, leading to speciation. The main rearrangements that lead to species formation are Robertsonian, in tandem fusion/fission and pericentric inversion, while the variability in constitutive heterochromatin does not seem to create a reproductive barrier and consequent speciation (King 1993, Romanenko and Voloboeuv 2012).

For a long time, it was thought that chromosomal structural rearrangements promoted speciation by generating gametes with duplications and deficiencies, therefore, causing less adaptability of the heterozygotes, but this model was rejected because it lacked theoretical support (Rieseberg 2001, Patton 2004, Jackson 2011). Recently, a different model of chromosome speciation was proposed in which the gene flow is reduced because of recombination-suppression in rearranged regions (Noor et al. 2001, Rieseberg 2001).

In fact, normal meiotic behavior with suppression of crossing over in inverted segments of heteromorphic chromosomes caused by pericentric inversions of Akodon cursor and Oligoryzomys nigripes was observed, with non-selective disadvantages in heterozygous carries (Fagundes et al. 1998, Bonvicino et al. 2001a). Some genetic mechanisms seem to be responsible for overcoming meiotic errors in heterozygous individuals, such as the occurrence of heterosynapsis and the low frequency of chiasm between the inverted segments.

A remarkable chromosome variation can be found in the semi- and fossorial Brazilian rodents Blarinomys breviceps (in which molecular phylogeny demonstrated two structured clades – see Ventura et al. 2012), Clyomys laticeps and Ctenomys minutus. Their species status, and whether their chromosome variation is adaptative and correlated with ecological patterns should be evaluated.

For example, a very well-known case of chromosome speciation due to population adaptation to climatic stress and ecological unpredictability was described in the subterranean rodent Spalax ehrenbergi (Family Spalacidae) found in Israel, in which diploid numbers increase coincidently with geographic regions of high aridity (Wahrman et al. 1969). The weak dispersion pattern of this fossorial rodent may have contributed to the fixation of adaptative chromosome change (Árnason 1972).

Cytotaxonomy is the use of chromosome data as the first clue in the identification of species. Since many Brazilian rodent species present species-specific karyotype and show morphological similarities, chromosome information showed to be useful in the diagnosis of species.

The present revision showed that the delimitation of species based on chromosome data (cytotaxonomy) is essential for recognizing some species of the genera Akodon, Calomys, Cerradomys, Euryoryzomys, Delomys, Hylaeamys, Juliomys, Neacomys, Oecomys, Oligoryzomys (family Cricetidae, subfamily Sigmodontinae), Ctenomys (family Ctenomyidae), and Thrichomys and Trinomys (family Echimyidae).

On the other hand, since rates of karyotype evolution differ in distinct branches of the rodents’ phylogeny, some species present identical diploid and fundamental numbers, and they cannot be identified solely through chromosome data. This is the case of the following species: (i) Cavia aperea, Cavia fulgida and Cavia magna; (ii) Kerodon acrobata and Kerodon rupestris (Family Caviidae); (iii) Akodon lindberghi and A. mystax; (iv) Akodon paranaensis and A. reigi; (v) Brucepattersonius griserufescens, B. iheringi, B. soricinus and Thaptomys nigrita; (vi) Oxymycterus caparoae, Oxymycterus dasytrichus, Oxymycterus nasutus and Oxymycterus roberti (the other four species of Oxymycterus also have the same diploid number but lacks information on FN) (Family Cricetidae, Subfamily Sigmodontinae, Tribe Akodontini); (vii) Cerradomys marinhus and Pseudoryzomys simplex; (viii) Drymoreomys albimaculatus and Oecomys sp. 4; (vix) Euryoryzomys emmonsae, E. nitidus and E. russatus (despite E. nitidus and E. russatus have disjunction distribution); (x) Holochilus brasiliensis and Nectomys squamipes; (xi) Hylaeamys laticeps and Hylaeamys seuanezi; (xii) Hylaeamys oniscus and H. perenensis; (xiii) Oecomys bahiensis, Oecomys concolor, Oecomys sp. 2 and sp. 3; (xiv) Neacomys dubosti and N. amoenus (family Cricetidae, Subfamily Sigmodontinae, tribe Oryzomyini); (xv) Rhipidomys cariri, R. gardneri, R. tribei, R. itoan and R. macconnelli (family Cricetidae, Subfamily Sigmodontinae, Tribe Thomasomyini); (xvi) Dasyprocta azarae, D. iacki, D. fuliginosa, D. leporina, D. prymnolopha, D. variegata and Dasyprocta sp. (family Dasyproctidae); (xvii) Isothrix bistriata, Mesomys hispidus, M. stimulax, Trinomys albispinus and T. dimidiatus; (xviii) Proechimys brevicauda and Proechimys cuvieri; (xix) Proechimys gardneri and Proechimys pattoni (family Echimyidae) and (xx) Guerlinguetus brasiliensis and Hadrosciurus spadiceus (family Sciuridae) (Table 1).

Furthermore, some unrelated species, that belong to different tribes, or even families, present the same diploid and fundamental number, suggesting a homoplastic character: (i) Hylaeamys megacephalus and Oxymycterus delator; (ii) Juliomys pictipes and Thalpomys cerradensis; (iii) Calomys laucha and Neacomys amoenus (although there are differences in the size of the biarmed chromosomes); (iv) Oecomys franciscorum and Delomys sublineatus (despite the first acrocentric pair in D. sublineatus is bigger than in Oryzomys franciscorum as well as the biarmed pair in the last species); (v) Coendou melanurus and Oligoryzomys utiaritensis; (vi) Ctenomys ibicuiensis and Scolomys ucayalensis and (vii) Callistomys pictus, Coendou spinosus and Myocastor coypus.

Since the beginning of the cytogenetic studies in Brazilian rodents, there have been cases in which different karyotypes were assigned to one species or the same karyotype was referred to in different species. In fact, many of these cases were solved after the integration of different disciplines. For instance, for many years cytogenetic information indicated that the previous “Oryzomys subflavus” could, in fact, be more than one species, since nine different karyotypes were attributed to a single taxonomic entity (Maia and Hulak 1981, Almeida and Yonenaga-Yassuda 1985, Svartman and Almeida 1992, Silva 1994). Nowadays, after interdisciplinary studies with morphology and molecular phylogeny, it is possible to recognize eight species (Weksler et al. 2006, Percequillo et al. 2008, Tavares et al. 2011, Bonvicino et al. 2014). Moreover, for a long time Nectomys was represented by only one species in Brazil, with two diploid numbers (2n = 52 + 1 to 3 Bs and 2n = 56 + 1 to 3 Bs). Nevertheless analyses of the spermatogenesis in hybrids and the sterility of crosses between both cytotypes indicated that Nectomys should be considered two distinct species: Nectomys rattus (2n = 52) and Nectomys squamipes (2n = 56) (Bonvicino et al. 1996).

The opposite occurred in the genus Oligoryzomys since the same karyotype (2n = 62, FN = 80-82) was attributed to different names (Oryzomys nigripes, Oryzomys delticola, and Oryzomys eliurus). After molecular and morphology integration, Oryzomys delticola and Oryzomys eliurus were considered as a junior synonym of Oryzomys nigripes (Bonvicino and Weksler 1998).

Some of these cases persist until today, for instance, more than one karyotype was described for Euryoryzomys macconnelli and E. lamia (Table 1). Molecular phylogeny and morphology corroborate the species complex status of both entities (Almeida 2014, Percequillo 2015a). Similarly, Oecomys roberti, Oryzomys paricola, and Oryzomys catherinae are probably species complexes, not only because of their variability in diploid number, but also because of phylogenetic reconstruction and morphological studies (Suárez-Villota et al. 2017). Ctenomys minutus, C. torquatus, Hylaeamys yunganus, Rhipidomys nitela, Sigmodon alstoni and Zygodontomys brevicauda also deserve taxonomic attention because they may represent cases in which different diploid numbers are attributed to the same names. Similarly, Blarinomys breviceps has a variable diploid number and two geographic structured clades were recovered in the molecular phylogeny (Ventura et al. 2012), indicating that a morphological revision is needed.

Remarkably, such examples can also be found in the family Echimyidae. The need to use different approaches for taxonomic revision is clear in order to investigate whether Phyllomys blainvillii, Phyllomys pattoni, and Proechimys guyannensis represent species complexes, given the fact that they have more than one karyotype associated.

Interdisciplinary approaches, including cytogenetic, molecular phylogeny, morphology and geographic distribution are essential for accessing the limits of Brazilian rodents’ species. One of the best-known examples was the old genera Oryzomys, considered the most complex and composing almost half of the species of the tribe Oryzomyini (Musser and Carleton 1993). The current genera Melanomys, Microryzomys, Nesoryzomys, Oecomys, and Oligoryzomys, were first considered a subgenus of Oryzomys and later elevated to the category of genus after morphology, chromosomal and molecular analyses (Myers et al. 1995, Smith and Patton 1999, Bonvicino and Moreira 2001). Another outstanding example of an integrative approach was the study in which ten new genera were described for species that were previously referred to as Oryzomys (Weksler et al. 2006), corroborating the cryptic diversity in Oryzomyini previously indicated by cytogenetic data.

Within the Family Echimyidae, the association of morphology and molecular analysis was essential for elevating Trinomys (considered subgenus of Proechimys) to the genus category (Lara et al. 1996, Leite and Patton 2002).

Despite the new technological approaches, chromosome characterization with conventional staining and banding pattern is still important, mainly because 38 species lack any karyotype information (Table 1). From this amount, 16 are distributed in the Amazonian biome, evidencing the lack of knowledge for this region. The fieldwork is very important and must be encouraged not only because new species and even genera are constantly being described but also because cytogenetic and distribution information of several species are poorly known.

Concerning the family Echimyidae, it is noteworthy that cytogenetic information is lacking for more than 20% of its species. Eleven out of 17 echimyid genera which occur in Brazil are arboreal (Galewski et al. 2005, Emmons et al. 2015). The issues for sampling small arboreal mammals and the consequent low number of studies with this approach have already been highlighted in the literature (Malcolm 1991, Taylor and Lowman 1996, Graipel et al. 2003). In this sense, it can be inferred that this deficiency in echimyid cytogenetic knowledge may be related to sampling scarcity.

The future of molecular biology is promising, with next-generation sequencing (NGS) technology and mitogenomics hopefully providing more robust phylogenetic studies. This new approach was performed with the Family Echymyidae, revealing new supported nodes and clarifying some aspects of the group’s taxonomy (Fabre et al. 2016).

However, it is important to reiterate the heterogeneity of characters since DNA, chromosomes, morphology, and behavior are not evolving at the same rate. This particularity may imply in different taxonomic interpretations, with a population being identified as a unique species by one character and two or more species by another, especially in the cases of recent or ongoing speciation. The consequences can be taxonomic inflation or underestimation of the biodiversity, and that is why interdisciplinary approaches are crucial to better understand the biological diversity of rodents.