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Karyotypic characterization and new geographical record of Salinomys delicatus (Rodentia, Cricetidae, Sigmodontinae).

Key words. Chromosomes. Geographic range. Phyllotini. South American drylands.

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The delicate mouse, Salinomys delicatus, is a small phyllotine rodent with a narrow distribution in the central aridlands of Argentina. The biology of this monotypic genus is basically unknown, and its record of occurrence is associated to salt flats and sand-dune habitats in the temperate Monte desert and Monte-Chaco ecotonal areas (Braun and Mares, 1995; Ojeda et al., 2001). Salinomys delicatus is known from just six localities, and the northernmost record was mentioned for Salinas Grandes and La Antigua salt flats, La Rioja Province (Mares et al., 2000; Ojeda et al., 2001). The delicate mouse shows ecophysiological specializations for a xeric life, such as elongated renal papillae and one of the highest renal indices and urine concentrations among phyllotines and other desert rodents in the world (Diaz and Ojeda, 1999; Diaz, 2001).

The purpose of this note is to report for the first time the karyotype of S. delicatus, and a new record of its distribution.

The study area is a desert shrubland located on the Pipanaco's salt basin (27[degrees] 49' 16.15" S,66[degrees] 14' 34.9" W; 740 m), and dominated by Larrea cuneifolia, Atriplex sp., Heterostachys ritteriana, and Suaeda divaricata. The "Bolson de Pipanaco" is located in the Province of Catamarca, Argentina, and belongs to the northern Monte desert (Morello, 1958). The specimen was collected using Sherman traps baited with oatmeal and peanut butter.

A pregnant female (three embryos) of Salinomys delicatus, was trapped in early spring 2004 (September). Its standard external measurements (in mm) were: body length: 170; tail length: 97; hind foot length: 23; ear length: 17, and a weight of 12 g. The specimen (skin, skull and karyotype) is housed at the Coleccion Mastozoologica IADIZA under number CMI 06815. The present record of S. delicatus extends the known distribution of this species approximately 270 km to the northwest (Fig. 1).

Chromosomes were obtained from bone marrow cells using the conventional in vivo colchicine hypotonic technique (Ford and Hamerton, 1956) with minimal modifications. The diploid number was determined by counting twenty metaphase and prometaphase spreads. Nomenclature for chromosome morphology and fundamental number (FN) followed Patton (1967).

The karyotype of the delicate mouse is composed of 2n = 18 and FN = 32; all chromosome elements are biarmed (Fig. 2). Arranging chromosomes by decreasing size, pair one is metacentric, pairs two and three are submetacentric, and the remaining chromosomes are metacentric. The first three pairs stand out from the rest because of their large size; the following three pairs are medium-sized, and the last three pairs are small sized. As the individual trapped was a female, sexual chromosomes are indistinguishable from the autosomics pairs. Prometaphase stages, where chromosomes are considerably elongated, were relatively frequent. Identification of secondary constrictions in the smallest chromosome pair was possible only at this stage.

South American drylands constitute biomes of particular importance from the standpoint of their mammalian diversity and endemisms (Mares, 1992; Ojeda et al., 2000). Within these areas, salt flats have been like "islands" that favored diversification and specialization in several groups of rodents (Mares et al., 2000; Ojeda et al., 2001). This seems to be the case of S. delicatus, with a high specialization to saline and xeric habitats (Diaz and Ojeda, 1999; Diaz, 2001). Moreover, its narrow geographic range and low records of occupancy (Fig. 1; e.g. only seven sites recorded since its description in late 1995) lead us to characterize Salinomys as a rare species (Gaston, 1994) among phyllotines.

Karyotypically, the tribe Phyllotini exhibits high chromosomal multiformity (Ortells et al., 1989; Tiranti, 1998; Spotorno et al., 2001; Martino et al., 2002; Lanzone and Ojeda, 2005). Diploid numbers range from 2n = 22 in Auliscomys boliviensis, to 2n = 78 in Andalgalomys p. pearsoni (Pearson and Patton, 1976; Olds et al., 1987). The karyotype shown in this study comprises the lowest diploid number known among phyllotines. The delicate mouse, S. delicatus, is karyotypically similar to Auliscomys boliviensis (2n = 22, NF = 30) in its general chromosomal morphology. Both species show a low diploid number and several biarmed chromosomes. Braun and Mares (1995) found Salinomys to be morphologically similar to Graomys, Andalgalomys and Eligmodontia. Later molecular analyses have shown a weak relationship among Salinomys, Andalgalomys, Eligmodontia and Tapecomys (Anderson and Yates, 2000). However, S. delicatus presents a highly divergent karyotype compared with the high diploid numbers reported for Andalgalomys species (the lowest diploid number reported for this genus corresponds to A. olrogi and A. roigi with 2N = 60; Mares and Braun, 1996), and for the monotypic genus Tapecomys (T. primus 2N = 56; Anderson and Yates, 2000). The karyotypic differences with Graomys and Eligmodontia are also marked, even when compared with the lowest diploid numbers recorded for these two genera, which correspond to 2n = 28 in G. domorum and 2n = 32 in E. morgani, respectively (Pearson and Patton, 1976; Kelt et al., 1991; Anderson and Yates, 2000; Spotorno et al., 2001).

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[FIGURE 2 OMITTED]

In accordance with the proposal by Reig (1986) for the evolution of phyllotines, and assuming that chromosomal fusions are much more frequent than fissions, some authors have suggested a hypothesis of chromosomal evolution for this tribe (Pearson and Patton, 1976; Spotorno et al., 2001). According to this hypothesis, species with ancestral karyotypes (mostly distributed in the high Andean plateau) possess a high diploid number in which telocentric chromosomes predominate. Phylogenetically derived species would be distributed outside of this area, and would present low diploid numbers and mostly meta-submetacentric chromosomes. Along with this reduction in the number of chromosomes, a reduction in the number of NORs (Nuclear Organization Regions) would also have occurred in these species (Spotorno et al., 2001). Under this scenario the karyotype of S. delicatus would correspond to a highly derived species among phyllotines. However, later molecular and morphological studies have shown some differences with Reig's view as to which genera must be included in this tribe (Steppan, 1993, 1995; Smith and Patton, 1999; D'Elia, 2003). Moreover, cytogenetic studies have reported high diploid and fundamental number for phyllotine species in the lowlands, such as in the genera Andalgalomys and Calomys (Bonvicino et al., 2003 and literature cited there; Olds et al., 1987), thus, contradicting the hypothesis of reduction of chromosome number by Robertsonian fusion in the taxa distributed outside the Andean plateau.

In conclusion, the conjunction of these approaches has shown no simple associations between cladogenetic patterns and cromosomal changes. Like in other rodents, chromosomal evolution may, in some cases, be too complex to resolve unequivocally the evolutionary history of taxa (Baker et al., 1983). Further studies are necessary to test for the degree of consistency between phylogenetic relationships and karyotypic data in order to understand the dynamics of chromosomal change in this chromosomically diverse group of sigmodontine rodents.

We thank two anonymous reviewers and Solana Tabeni for their critical comments and suggestions made on an earlier version of the manuscript. We appreciate the assistance of Silvia Brengio and Agustina Ojeda with laboratory procedures. We thank Nelly Horak for the English version and Daniel Duenas for drawing the map. This study was partially financed by CONICET (PIP 2884) and SECYT (PICT 11768).

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Cecilia Lanzone, Ricardo A. Ojeda, Soledad Albanese, Daniela Rodriguez, and Mariana Dacar Grupo de Investigaciones de la Biodiversidad, IADIZA, CRICYT, CONICET, CC 507, (5500) Mendoza, Argentina.
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Author:Lanzone, Cecilia; Ojeda, Ricardo A.; Albanese, Soledad; Rodriguez, Daniela; Dacar, Mariana
Publication:Mastozoologia Neotropical
Date:Jul 1, 2005
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