The first description of the karyotype of Dasyprocta azarae Lichtenstein, 1823 (Rodentia, Dasyproctidae) from Brazil

Ana L. G. Souza¹^, ², Margaret M. O. Corrêa² and Leila M. Pessôa²

¹ Programa de Pós-graduação em Zoologia, Museu Nacional, Quinta da Boa Vista, São Cristóvão, CEP: 20940-040, Rio de Janeiro, Brasil <ana.lgs@gmail.com>. ² Lab. Mastozoologia, sala A1-121, Instituto de Biologia, CCS, Universidade Federal do Rio de Janeiro, Av. Brigadeiro Trompowski s/nº, Ilha do Fundão, CEP: 21940-590, Rio de Janeiro, Brasil.

ABSTRACT: The karyotype of Dasyprocta azarae has not been described previously. In this article we describe the karyotype of D. azarae from Northern Pantanal, Mato Grosso, Central Brazil. D. azarae has a diploid number 2n=64 and a number of autosomal arms NA=122. The 2n and NA are the same as those reported for other Brazilian species of Dasyprocta, but differ in chromosome morphology. Differences in the sex chromosomes were also found. The Y chromosome is acrocentric whereas in other Dasyprocta species is metacentric or submetacentric. The C-banding pattern coincides with that described for other species, except for pair six, having two bands of heterochromatin. The NOR patterns revealed intraindividual variation, with 1 to 6 NOR chromosomes. In the other species of Dasyprocta the NOR is present in only 1 pair. Although few data about morphological variation is available and restricted to color pattern, the small chromosomal differences observed among species of Dasyprocta are however, capable to differentiate taxa of the genus.

RESUMEN: Primera descripción del cariotipo de Dasyprocta azarae Lichtenstein, 1823 (Rodentia, Dasyproctidae) de Brasil. En este artículo se describe por primera vez el cariotipo de D. azarae procedente del norte del Pantanal, Mato Grosso, Brasil. La especie posee un número diploide 2n=64 y un número de brazos autosómicos NA=122. Los resultados para 2n y NA son los mismos que los encontrados para otras especies brasileñas de Dasyprocta, pero difieren en la morfología cromosómica. También fueron identificadas diferencias en los cromosomas sexuales. El cromosoma Y es acrocéntrico en ésta, mientras que en otras especies del género es metacéntrico o submetacéntrico. El patrón de bandas C es igual al descrito para otras especies, con la excepción del par seis, que presenta dos bandas de heterocromatina. Los patrones NOR demuestran variación interespecífica, con uno a seis cromosomas marcados con NORs. En otras especies de Dasyprocta el NOR está presente en apenas un par. Aunque hay poca información disponible sobre variación morfológica, restringida al patrón de color, las pequeñas diferencias cromosómicas entre especies de Dasyprocta son, sin embargo, capaces de diferenciar taxas del género.

Key words. Caviomorpha. C-banding. Conventional staining. Dasyprocta azarae. NOR.

Palabras clave. Bandas C. Caviomorpha. Coloración convencional. Dasyprocta azarae. NOR.

INTRODUCTION

   The family Dasyproctidae Bonaparte, 1838, belongs to the suborder Hystricognathi and contains two genera: Dasyprocta Illiger, 1811, with 11 species, and Myoprocta Thomas, 1903, with two species (Woods, 1993). The species-level recognition within Dasyprocta is problematic because of morphologically intermediate forms and high intra-specific variation (Matson and Shump, 1980; Lima and Langguth, 1998; Ximenes, 1999).
   The genus is distributed from southern Mexico to northern Argentina. In Brazil, there are five species (Woods, 1993): D. azarae Lichtenstein, 1823; D. fuliginosa Wagler, 1832; D. leporina (Linnaeus, 1758); D. prymnolopha Wagler, 1831 and D. punctata Gray, 1842. The karyotypes of all Brazilian species, with exception of D. azarae, are known and their diploid and autosomal numbers have been described (George and Weir, 1974; Kasahara and Yonenaga-Yassuda, 1984; Lima and Langguth, 1998; Ramos et al., 2003). Literature on D. azarae includes only the diploid number (Lima and Langguth, 1998), neither autosomal number nor a complete description of the karyotype has been published.
   Here, we provide the first complete description of the karyotype, the C-banding and NOR chromosomes of D. azarae and compare them with data on other Brazilian species within the genus.

MATERIALS AND METHODS

   Specimens were collected with Tomahawk field traps in a private reserve (Reserva Particular do Patrimônio Natural SESC-Pantanal) in Barão de Melgaço, Mato Grosso State, Central Brazil (16 o 28' 24.9" S and 16 o 48' 43.6" S; 56 o 01' 37.6" W and 56 o 28' 49.4" W). Fur color was used to identify specimens according to Ximenes (1999). Voucher specimens (two males and one female) were deposited in the mammal collection of the Museu Nacional, Rio de Janeiro, Brazil: MN64090, MN64487, and MN64616.
   Mitotic chromosomes were obtained by the method of Ford and Hamerton (1956). In the laboratory, 287 metaphases of the three specimens were analyzed under a light microscope using conventional Giemsa staining. The chromosomes were measured and classified according to Levan et al. (1964) as follows: metacentric (M), submetacentric (SM) and subtelocentric (ST) chromosomes were considered biarmed and acrocentric chromosomes uniarmed. Silver nitrate staining, following the procedures of Howell and Black (1980), was used to detect the nucleolus organizer regions (NORs). C-bands were revealed by Sumner's (1972) techniques.

RESULTS

   The three specimens of D. azarae had a diploid number 2n=64 and number of autosomal arms NA=122. The chromosome complement (Fig. 1) consisted of 30 pairs of biarmed chromosomes and one pair of uniarmed chromosomes. Pairs 1 to 15 and 19 to 29 were metacentric chromosomes, pairs 16, 17 and 18 were submetacentric, pair 30 was subtelocentric, and pair 31 was acrocentric. The X chromosome was a medium-sized metacentric similar in size to pair 6. The Y chromosome was a small acrocentric, the smallest chromosome of the complement.

Fig. 1. Conventionally stained male karyotype of D. azarae (2n=64, NA=122) (M - metacentric; SM - submetacentric; ST - subtelocentric; A - acrocentric).

   C-bands of varying sizes were observed in the pericentric region of all autosomes (Fig. 2). Heterochromatin was also observed in the pericentric region of the X chromosome. No heterochromatin was observed in the Y chromosome. Pair 6 showed two bands of constitutive heterochromatin in the pericentromeric region and in the interstitial region of the long arm.

Fig. 2. C-banding karyotype of a male specimen of D. azarae.

   NORs were always observed in the telomeric region of the autosomes. There was intraindividual variation in the number of NOR-bearing chromosomes in all specimens analyzed (Fig. 3). We found from one to six NORs, with a modal number of three.

Fig. 3. Metaphase cells of D. azarae with Ag-NOR a) with one NOR-bearing chromosome; b) with two NOR-bearing chromosome; c) with three NOR-bearing chromosome; d) with four NOR-bearing chromosome; e) with five NOR-bearing chromosome; f) with six NOR-bearing chromosome. Scale bars = 10µm.

DISCUSSION

   The diploid number (2n=64) and the number of autosomal arms (NA=122) found for D. azarae coincided with that of the other four Brazilian species within the genus (George and Weir, 1974; Kasahara and Yonenaga-Yassuda, 1984; Lima and Langguth, 1998). In a recent study, including four species from the Brazilian Amazon, Ramos et al. (2003) found numerical variation from 2n=64 to 65 due to the presence of one supernumerary chromosome. The chromosome composition in D. azarae was different from that of the other species: the latter have a larger number of submetacentrics (five to nine pairs) and subtelocentrics (three to four pairs) (George and Weir, 1974; Lima and Langguth, 1998; Ramos et al., 2003), whereas D. azarae has a higher number of metacentric chromosomes (26 pairs). The sex chromosomes also differ in D. azarae. The X chromosome is a medium metacentric, as in D. aguti, but the other species presents a large metacentric or submetacentric X chromosome. The Y chromosome is a small acrocentric chromosome, but the Y chromosome of the other species is always metacentric or submetacentric. Summary karyotype data for the species of Dasyprocta are shown in Table 1.

   The C-bands in D. azarae are the same as in other species (Table 1). Nevertheless, the other species of Dasyprocta presents heterochromatin in the Y chromosome whereas in D. azarae it is absent (Lima and Langguth, 1998; Ramos et al., 2003). Pair 6 in D. prymnolopha, D. leporina and Dasyprocta sp. has only one block with an increased amount of heterochromatin (Lima and Langguth, 1998; Ramos et al., 2003). This result on pair 6 indicates that a pericentric inversion has probably occurred in this region in D. azarae. According to Ramos et al. (2003), the basic pattern for the distribution of constitutive heterochromatin in the genus includes a small block in pair six, as occurs in the karyotype of D. fuliginosa from Brazilian Amazon.
   Nucleolus organizer regions (NORs), were observed in the telomeric region of chromosomes in D. azarae, as in the other species of Dasyprocta. However, we observed intraindividual variation from one to six NOR-bearing chromosomes in all specimens here analyzed (Fig. 3). This variation included the acrocentric pair, the only pair marked for NORs in the other species of the genus so far (Ramos et al., 2003) (Table 1). The acrocentric pair was the pair most frequently marked with silver nitrate in this study.
   These techniques stain only NORs activated during the previous interphase, so that the differences in the number of stained NORs per cell may reflect differences in the number of active NORs (Thiriot-Quiévreux and Insua, 1992; Galleti et al., 1995). Although this polymorphism in the number of active NOR sites per cell commonly occurs in other groups of rodents (Yonenaga-Yassuda et al., 1992; Svartman and Almeida, 1993; Lima et al., 2003), in the suborder Hystricognathi the pattern has until now been constant (Yonenaga-Yassuda et al., 1985; Leal-Mesquita et al., 1992; Pessôa et al., 2004).
   The diploid number (2n=64) and the number of autosomal arms (NA=122) of Dasyprocta are considered high for the suborder Hystricognathi (George and Weir, 1974). Dasyproctidae and Caviidae are the most homogeneous groups of the suborder Hystricognathi, with a high number of metacentric chromosomes (George and Weir, 1974). The difference in the number of metacentric chromosomes in D. azarae is probably due to a rearrangement, with deletion or duplication of genetic material. The same has probably occurred to the X chromosome. However, in the Y-chromosome, besides deletion or duplication, pericentric inversions are also involved. These questions might be resolved if new samples were available for G-band analysis.
   Despite the differences observed in chromosome composition between D. azarae and the other Brazilian species, the general pattern found for 2n and NA is the same. The fact that they share the same diploid number and autosomal number corroborates the hypothesis that the species of Dasyprocta have a conservative karyotype.
   Although few data about morphological variation is available and restricted to color pattern, the small chromosomal differences observed among species of Dasyprocta (C-banding, NOR, and the Y chromosome) are however, capable to differentiate taxa of the genus.

ACKNOWLEDGMENTS

   The authors would like to thank L. Brandão, and staff of R.P.P.N. SESC Pantanal. This work has been funded by grants from SESC (Serviço Social do Comércio), Universidade Federal do Rio de Janeiro. L. M. Pessôa has been partially supported by a fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq- 301386-927). We thank Dr. Guilherme Muricy for a critical review of a preliminary version of the manuscript; Dr. C. J. Tribe from Cambridge University, for reviewing the English of the manuscript; Dr. João Alves de Oliveira and Dr. Luiz Flamarion B. Oliveira and Stella Franco (Museu Nacional) helped with specimens in the collection. A collecting permit was provided by the Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis (IBAMA).

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Recibido 12 abril 2006.
Aceptación final 12 abril 2007.