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Chromosomal variation in the scrub mouse Akodon molinae (Rodentia: Sigmodontinae) in central Argentina.

Abstract. -- A cytogenetic study of 34 specimens of the scrub mouse Akodon molinae from nine localities in La Pampa and San Luis provinces of central Argentina revealed 2n = 43 in 18 specimens, 2n = 44 in eight and 2n = 42 in eight individuals. This variation results from a Robertsonian polymorphism involving chromosome pair 1. Information on the distribution and biogeography of Akodon molinae is provided for central Argentina.

Resomen. -- En un estudio citogenetico de 34 ejemplares del rat n pajizo Akodon molinae provenientes de nueve localidades de las provincias de La Pampa y San Luis en el centro de Argentina, se encontr 2n = 43 en 18 especimenes, 2n = 44 en ocho y 2n = 42 en ocho individuos. Esta variaci n se debe a un polimorfismo Robertsoniano en el par 1. Se incluye informati n sobre la distribuci n y biogeografia de Akodon molinae para el centro de Argentina.


The scrub mouse, Akodon molinae Contreras 1968, is one of the most studied sigmodontine rodents in South America (Myers 1988). Molecular (Apfelbaum & Blanco 1984; Apfelbaum & Reig 1989), ecological (Ojeda 1989) and cytogenetical (Bianchi et al. 1969; 1973; 1976; 1979a; Bianchi & Merani 1980; Wittouck et al. 1995) information has been described. In central Argentina, this widespread species inhabits Espinal forests dominated by calden, Prosopis caldenia. It also occurs in scrub vegetation of halophytic shrub communities, Monte Desert habitats, borders of grasslands and cultivated fields, and deforested areas where there is adequate shrub cover.

Akodon molinae is difficult to distinguish morphologically from other members in the A. varius species group to which it belongs (Myers 1989), and cytogenetics can provide key characters for identification. Akodon molinae has a variable karyotype with a diploid number ranging from 2n = 42 to 44. In the 2n = 42 form the first pair of autosomes is represented by two large metacentrics, which are represented by four large subtelocentrics in the 2n = 44 cytotype. The 2n = 43 is the most common variant and the first pair of chromosomes consists of two submetacentrics and a large metacentric. The rest of the autosomes are acrocentric except for the smallest pair which is metacentric. The X chromosome is a large acrocentric and the Y chromosome is a small acrocentric (Bianchi et al. 1973). This polymorphism was first reported by Bianchi et al. (1969), and subsequently described in greater detail (Bianchi et al. 1971; 1973; 1979a; Wittouck et al. 1995).

Akodon dolores, described by Thomas (1916), a related if not conspecific form, shares chromosomal banding patterns and morphology with A. molinae and both have high values of genetic similarity, based on protein electrophoresis (Bianchi et al. 1979b; Apfelbaum & Blanco 1984; Myers 1989). The basic karyotype of Akodon dolores (2n = 34) consists of five pairs of large metacentric or submetacentric chromosomes, eleven pairs of acrocentric chromosomes of decreasing size, and a pair of small metacentrics. The X chromosome is an acrocentric of medium size and the Y chromosome a small acrocentric. Additionally, the first five pairs of biarmed chromosomes can present polymorphisms producing individuals with 2n = 34-40 diploid numbers (Kibliski et al. 1976; Wittouck et al. 1995). The geographic distribution of both, based on published records, do not overlap (Apfelbaum & Blanco 1984). Moreover, at the type locality of Akodon dolores specimens had the karyotype originally described as belonging to A. molinae (cf. Wittouck et al. 1995). In this report the specific name (epitat) "molinae" is applied to the 2n = 42-43-44 forms. Herein, additional cytogenetic data for Akodon molinae are reported from other areas of central Argentina for which this information has not been published.


Thirty-four specimens of Akodon molinae were collected from nine localities in San Luis and La Pampa provinces of central Argentina. In most cases, the yeast stress method (Lee & Elder 1980) was used to obtain a higher mitotic index. Specimens were subjected to the standard procedure of in-vivo colchicine induced mitotic arrest for obtaining chromosomes from bone marrow. Slides were produced by dropping the cell suspension from a 50-60 cm height into a large drop of distilled water on the surface of the slide (Baker et al. 1982). Chromosome slides were Giemsa stained, observed and photographed and the diploid number and chromosomal morphology were determined for each specimen based on ten spreads.

Material examined. -- All voucher specimens were prepared as standard study skins and skulls or fluid-preserved and are housed in the collections of Texas Tech University Museum (TTU), Lubbock, Texas; the mammal collection of the Universidad Nacional de Rio Cuarto (UNRC), Rio Cuarto, Cordoba, Argentina; and the collection of La Pampa Vertebrate Survey (RVP, Plan de Relevamiento de los Vertebrados de la Provincia de La Pampa), deposited in the Museo Provincial de Historia Natural, Santa Rosa, La Pampa, Argentina. Vegetation information for habitats where Akodon molinae specimens were collected in La Pampa province follows Cano et al. (1980). Collection localities with numbers in parenthesis are referenced to Figure 1.


San Luis Province: Chacabuco Department: Rincon de Papagayos (Site 1): One female (RVP 245). General vegetation of the area are Chaco Serrano transition forests with palms.

La Pampa Province: Rancul Department: 15 km SW Chamaico, Loma Loncovaca (Site 2): One female (RVP 246), open Espinal forests of calden (Prosopis caldenia). Toay Department: ca. 45 km NW of Santa Rosa, Estancia El Pincen (Site 3): One female (TTU 64404), open Espinal forests of calden; 10 km SW of Santa Rosa, Chacra La Lomita (Site 5): One male (TTU 66518) and one female (TTU 66517), crops with pastures and linear habitats along fences with piquillin (Condalia microphylla), molle (Schinus sp.), and calden; 12 km NNE of Naico, Estancia Los Toros (Site 6): Eight males (TTU 64393, TTU 64395, TTU 64397, TTU 64399, TTU 64401, TTU 64402, TTU 64403, RVP247) and eight females (TTU 64394, TTU 64392, TTU 64396, TTU 64398, TTU 64400, TTU 66519, UNRC 163, UNRC 164), young ("renoval") calden forests and pastures; Parque Luro (Site 7): Four males (RVP 248, RVP 249, UNRC 165, UNRC 166) and two females (RVP 250, UNRC 167), open Espinal forests of calden. Capital Department: Laguna Don Tomas, Santa Rosa (Site 4): Two females (TTU 64391, UNRC 168), floodable area with semihalophytic shrubs. Utracan Department: 10 km W Quehue, Estancia Los Molinos (Site 8): Two males (TTU 64405, TTU 64406), calden forests and mixed shrublands of Larrea divaricata, Condalia microphylla and Chuquiraga erinacea. Caleu Caleu Department: 40 km N Anzoategui, Almacen El 52 (Site 9): Two males (TTU 64388, TTU 64390) and one female (TTU 64389), mixed shrubland composed mainly of Condalia microphylla, Larrea divaricata and Acantholippia seriphioides with Prosopis caldenia forests.



Eight of the specimens examined had the 2n = 42 chromosomic variant (Fig. 2). The majority (n = 18) possessed the heterozygous 2n = 43 karyomorph (Fig. 3a), and the 2n = 44 cytotype was present in eight individuals (Fig. 3b). These were distributed in the localities sampled as follows: Rincon de Papagayos (1 = 42), Loma Loncovaca (1 = 42), Estancia El Pincen (1 = 43), Laguna Don Tomas (2 = 43), Chacra La Lomita (1 = 42; 1 = 43), Estancia Los Toros (2 = 42; 7 = 43; 7 = 44), Parque Luro (2 = 42; 4 = 43), Estancia Los Molinos (1 = 42; 1 = 43), Almacen El 52 (2 = 43; 1 = 44). Previously, different proportions of these karyomorphs have been reported. For Chasico, Buenos Aires Province, the type locality of A. molinae, a total of 18 specimens possessed the 2n = 42 karyotype, 14 had the 2n = 43, and one had the 2n = 44 form (Bianchi et al. 1969; 1971; 1973). More recently Wittouck et al. (1995) reported three animals with 42, nine with 43, and nine with 44 chromosomes for Yacanto and Villa Dolores, Cordoba Province.


At Estancia Los Toros, all three cytotypes were present in proportions expected of a random mating population ([x.sup.2] = 0.015, p = 0.01 for accordance with Hardy-Weinberg equilibrium).

Akodon molinae is restricted to the south and west part of the range of the A. dolores/molinae complex, which is located in the Espinal forests and with some localities (not karyologically studied) inside the Monte Desert. The distribution of the A. dolores karyomorphs is restricted to the east of the Cordobean Sierras, except for specimens reported for Catamarca Province (Bianchi et al. 1979b). A careful survey may show the extent of the geographic distribution of these forms.


Work in La Pampa Province was supported by the Subsecretaria de Cultura, where N. Durango, G. Siegenthaler and E. Fiorucci contributed in many ways to the accomplishment of this report. My stay at TTU was supported in part by the Direccion Nacional de Cooperacion Internacional, Ministerio de Cultura y Educacion, Argentina, and the Universidad Nacional de La Pampa, Argentina. Localities Loma Loncovaca, Almacen El 52, Estancia Los Molinos and Estancia El Pincen were sampled as part of La Pampa Province Vertebrate Survey. R. J. Baker, R. D. Bradley, M. J. Hamilton and L. E. Wiggins critically reviewed and provided helpful suggestions on the first drafts of the manuscript. A later version benefited from the criticisms of four anonymous reviewers.


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Sergio I. Tiranti

Department of Biological Sciences, Texas Tech University

Lubbock, Texas 79409-3131, USA

Present address:

Facultad de Ciencias Exactas y Naturales

Universidad Nacional de La Pampa

Uruguay 151, 6300 Santa Rosa

La Pampa, Argentina

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Author:Tiranti, Sergio I.
Publication:The Texas Journal of Science
Geographic Code:3ARGE
Date:Aug 1, 1998
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