SciELO - Scientific Electronic Library Online

vol.11 número1Aportes al conocimiento de Notiomys Edwardsii (THOMAS, 1890) en el noroeste del Chubut, ArgentinaNuevos datos sobre la distribución de Irenomys Tarsalis (Philippi, 1900) (Rodentia: Sigmodontinae) en Argentina índice de autoresíndice de materiabúsqueda de artículos
Home Pagelista alfabética de revistas  

Servicios Personalizados




  • No hay articulos citadosCitado por SciELO

Links relacionados


Mastozoología neotropical

versión impresa ISSN 0327-9383versión On-line ISSN 1666-0536

Mastozool. neotrop. v.11 n.1 Mendoza ene./jun. 2004


Chromosomal characterization of Irenomys Tarsalis (Rodentia, Cricetidae, Sigmodontinae)

Agustina A. Ojeda1, Carolina A. Ríos2, and Milton H. Gallardo2

1Grupo de Investigaciones de la Biodiversidad, IADIZA, CRICYT, Casilla 507, 5500 Mendoza, Argentina.<>.
2Instituto de Ecología y Evolución, Universidad Austral de Chile. Casilla 567, Valdivia, Chile. Telephone: 56 (63) 221469.

ABSTRACT. The systematics of I. tarsalis is far from being understood. Morphological and molecular analyses have given contradictory results, pointing towards its inclusion or exclusion from the Phyllotini. The karyotype of Irenomys tarsalis, as well as its C-bands are reported for the first time. The species has 2n = 64 chromosomes (FN = 98) and a C-banding pattern characterized by small amounts of centromeric heterochromatin. Although the high diploid number and the C-bands of I. tarsalis support the notion of the species´ ancestral karyotypic condition, its affiliation within the sigmodontines cannot be discerned by the data.

Key words: Irenomys tarsalis, karyotype, Sigmodontinae, Phyllotini.

RESUMEN. Caracterización cromosómica de Irenomys tarsalis (Rodentia, Cricetidae, Sigmodontinae). Las relaciones sistemáticas de I. tarsalis están poco comprendidas. Los análisis morfológicos y moleculares han dado resultados contradictorios, que indican su pertenencia o exclusión de la tribu Phyllotini. Se describe por primera vez el cariotipo de Irenomys tarsalis, así como la distribución de la heterocromatina C-positiva. La especie tiene 2n = 64 cromosomas (FN = 98) y un patrón de bandas C pequeñas y centroméricas. Si bien el alto número diploide y la distribución de la heterocromatina apoyan la condición cariotípica ancestral de I. tarsalis, su afiliación dentro de los sigmodontinos no puede ser discernida solamente por estos datos.

Palabras clave: Irenomys tarsalis, cariotipo, Sigmodontinae, Phyllotini.


The Neotropical sigmodontine rodents have been extensively studied cytogenetically (Gardner and Patton, 1976; Pearson and Patton, 1976; Spotorno et al., 2001). The Sigmodontinae is composed of eight tribes (Akodontini, “Abrothricines”, Ichthyomyini, Oryzomyini, Phyllotini, Sigmodontini, Thomasomyini, Wiedomyini) and several genera without clear phylogenetic relationships among them (Smith and Patton, 1999; D´Elía, 2003). The Chilean tree mouse or “laucha arbórea”, Irenomys tarsalis (Philippi, 1900) is the only representative of this monotypic genus, mainly restricted to temperate forests of South-western South America (Kelt, 1993). The phylogenetic relationships of Irenomys to other sigmodontines are controversial. Irenomys was excluded from the phyllotines due to the lack of morphological characters common to the group (Hershkovitz, 1962). Nevertheless, it was included in such a tribe in a later revision by Olds and Anderson (1989) as well as by Braun (1993) and Steppan (1993, 1995). Subsequent phylogenetic analyses of mitochondrial sequences depicted Irenomys outside the phyllotine clade but associated to a variety of divergent taxa depending on the algorithm used (Smith and Patton, 1999). Phylogenetic analyses based on mitochondrial and nuclear genes depict Irenomys as an isolated taxon outside the Phyllotini, close to Scolomys juruaense and Rhipidomys macconnelli (D´Elía, 2003).
   In the present paper, the karyotype and the C-bands of I. tarsalis are described for the first time. This data set is compared with that of the phyllotines, aiming to shed some light into the phylogenetic relationships of Irenomys.


Four specimens of I. tarsalis (three males and one female) were collected from the San Martín woods, Valdivia province, Chile (39º38´S; 73º07´W). Chromosome preparations were obtained from bone marrow following the conventional colchicine-hypotonic solution technique (Lee and Elder, 1980; Baker et al., 1982). C-bands were induced by the barium hydroxide technique (Sumner, 1971). Ten to fifteen metaphase spreads were counted for each specimen. Nomenclature for chromosome morphology and fundamental arm number (FN) follows Patton (1967). The specimens analysed were deposited in the Collection of Mammals of Instituto de Ecología y Evolución, Universidad Austral de Chile.


Conventional analysis indicates 2n = 64, FN = 98 for I. tarsalis. The autosomal complement consists of 5 pairs of metacentrics (2 large, 2 medium-sized and 1 small), 13 pairs of subtelocentrics (medium to small-sized) and 13 pairs of medium-sized acrocentrics (Fig. 1A). The X chromosome is the largest acrocentric present in pairs in the female complement, whereas only one element is observed in males. The Y chromosome is a medium-sized metacentric (Fig. 1A).

Fig. 1.A) Standard Giemsa-stained karyotype of Irenomys tarsalis (2n = 64, FN = 98). Biarmed chromosomes are arranged first, acrocentric chromosomes, last. Sex chromosomes (XY) are also indicated. B) C-banded karyotype of Irenomys tarsalis (2n = 64, FN = 98). Note the centromeric blocks of heterochromatin in most chromosomes, except pair 2.

A) Cariotipo de Irenomys tarsalis con tinción estándar (2n = 64, FN = 98). Los cromosomas biarmadados se ubican al principio y los comosomas acrocentricos al final. También se indican los cromosomas sexuales (XY). B) Banda C de Irenomys tarsalis. Nótense los bloques de heterocromatina centromérica en la mayoría de los cromosomas, excepto en el par 2.

   The C-bands of I. tarsalis are small, centromeric and vary in size across the karyotype (Fig. 1B). Discrete blocks of heterochromatin including part of the arms are observed in pairs 7, 8, 19, 20, 22 and 26. The rest of the autosomes show small centromeric C-bands whereas no banding pattern is observed in pair 2 (Fig. 1B). The X chromosomes exhibit clear C-bands in the centromeric region. The Y chromosome is totally heterochromatic (Fig 1B)


The systematics of Irenomys is far from being understood. The lack of diagnostic morphological characters have been the defining criterium for excluding Irenomys from the Phyllotini (Hershkovitz, 1962). Nevertheless, phylogenetic analyses of morphological traits depict Irenomys as an independent and enigmatic stem clade of phyllotines (Braun, 1993). This affiliation to the phyllotines was further emphasized by Steppan (1995) although a close relationship to Andinomys edax and Chinchillula sahamae is implicated by the data.
   The extent and composition of the Phyllotini (formed by Andalgalomys, Andinomys, Auliscomys, Calomys, Chinchillula, Eligmodontia, Galenomys, Graomys, Neotomys, Phyllotis and Salinomys) has been recently studied by mitochondrial and nuclear DNA markers (D´Elía, 2003). Mitochondrial sequencing data excludes Irenomys from the phyllotines and suggest a relationship with Scolomys juruaense or Sigmodon hispidus (Smith and Patton, 1999). Nevertheless, the position of Irenomys within the Sigmodontinae is uncertain and depends on the algorithm used for phylogenetic inference. A recent revision based on nuclear and mitochondrial markers coincides with Smith and Patton (1999) in excluding Irenomys from the Phyllotini, and considers I. tarsalis as a Sigmodontinae incertae sedis (D´Elía, 2003).
   The karyotypic studies conducted on the Neotropical sigmodontines have indicated extensive chromosomal multiformity and a chromosomal trend towards reduction in diploid number (Gardner and Patton, 1976; Pearson and Patton, 1976). The phyllotines also display an exceedingly broad range of karyotypic variation. In fact, diploid number ranges from 22 to 70 chromosomes, and the fundamental arm number, from 30 to 76 (Pearson and Patton, 1976). Assuming that chromosomal evolution within the phyllotines has proceeded through Robertsonian fusions, the high diploid number of Neotomys ebriosus (2n = 70, FN = 68) and Phyllotis osilae (2n= 70; FN = 68) represent karyotypic ancestrality whereas the totally-biarmed karyotypes of Phyllotis darwini and P. caprinus (2n = 38; FN = 72) represent the derived condition (Pearson and Patton, 1976).
   The diploid number of Irenomys´ (although from a single locality) suggests an association with the phyllotines, whereas its high FN is consistent with sequence-based cladograms that exclude it from such a group. These characteristics (2n and FN) show high variation within the phyllotines to infer detailed relationships on their basis. Further analyses are necessary to clarify the relationship of Irenomys. The characteristic distribution and amount of heterochromatin in the karyotype contributes to genome size differences especially in rodents (Redi et al., 2001; Gallardo et al., 2003). Thus, the increase in heterochromatin content through the selfish replicative properties of short repetitive DNA sequences (Wallrath, 1998; Hennig, 1999) represents a recently evolved trait in rodents (Gamperl, 1982; Gallardo, 1992), insects (Baimai, 1998) and other taxa. On the other hand, the centromeric location and small blocks of heterochromatin of I. tarsalis and its high diploid number are likely ancestral conditions (Gallardo, 1992), and are therefore consistent with the proposed early divergence of Irenomys within the sigmodontine radiation (Smith and Patton, 1999). Further molecular analyses are needed to substantiate this view, since the karyotypic variation displayed by the phyllotines prevents chromosomal data from clarifying this issue.


We thank Fredy Mondaca and Claudio González for all their assistance. Guillermo D´Elía provided valuable comments on a first draft of the manuscript. An anonymus reviewer and Enrique Lessa made helpfull suggestions in the final draft.


BAIMAI, V. 1998. Heterochromatin accumulation and karyotypic evolution in some dipteran insects. Zoological Studies, 37:75-88.         [ Links ]

BAKER, R.J., M.W. HAIDUK, L.W. ROBBINS, A. CADENA, and B.F. KOOP. 1982. Chromosomal studies of South American bats and their systematic implications. Pp.303-327. In: Mammalian Biology of South America. (Mares, M.A. and H.H. Genoways, eds.). Special Publication Series, Pymatuning Laboratory of Ecology, University of Pittsburgh, 6:1-539.         [ Links ]

BRAUN, J.K. 1993. Systematic relationships of the tribe Phyllotini (Muridae: Sigmodontinae) of South America. Oklahoma Museum of Natural History. Special Publications, 1-50.         [ Links ]

D´ELÍA, G. 2003. Phylogenetics of Sigmodontinae (Rodentia, Muroidea, Cricetidae), with special reference to the akodont group, and with additional comments on historical biogeography. Cladistics, 19:307-323.         [ Links ]

GALLARDO, M.H. 1992. Karyotypic evolution in octodontid rodents based on C-band analysis. Journal of Mammalogy, 73:89-98.         [ Links ]

GALLARDO, M.H., J.W. BICKHAM, G. KAUSEL, N. KÖHLER, and R.L. HONEYCUTT. 2003. Gradual and quantum genome size shifts in the hystricognath rodents. Journal of Evolutionary Biology, 16:163-169.         [ Links ]

GAMPERL, R. 1982. Chromosomal evolution in the genus Clethrionomys. Genetica, 57:193-197.         [ Links ]

GARDNER, A.L. and J.L. PATTON. 1976. Karyotypic variation in oryzomyine rodents (Cricetinae) with comments on chromosomal evolution in the Neotropical Cricetinae complex. Occasional Papers, Museum of Zoology, University of Michigan, 49:1-48.         [ Links ]

HENNIG, W. 1999. Heterochromatin. Chromosoma, 108:1-9.         [ Links ]

HERSHKOVITZ, P. 1962. Evolution of Neotropical cricetinae rodents (Muridae) with special reference to the Phyllotine Group. Fieldiana, Zoology, 46:1-524.         [ Links ]

KELT, D.A. 1993. Irenomys tarsalis. Mammalian Species, American Society of Mammalogists, 447:1-3.         [ Links ]

LEE, M.R. and F.B. ELDER. 1980. Yeast estimulation of bone marrow mitoses for cytogenetic investigation. Cytogenetics and Cell Genetics, 26:36-40.         [ Links ]

OLDS, N. and S. ANDERSON. 1989. A diagnosis of the Tribe Phyllotini (Rodentia, Muridae). Pp. 55-74. In: Advances in Neotropical Mammalogy (Redford, K.H. and J.F. Eisenberg, eds.). Sandhill Crane Press, Gainesville, 614 pp.         [ Links ]

PATTON, J.L. 1967. Chromosome studies of certain pocket mice, genus Perognathus (Rodentia, Heteromyidae). Journal of Mammalogy, 48:27-37.         [ Links ]

PEARSON, O.P. and J.P. PATTON. 1976. Relationships among South American Phyllotine rodents based on chromosome analysis. Journal of Mammalogy, 57:339-350.         [ Links ]

REDI, C.A., S. GARAGNA, H. ZACHARIAS, M. ZUCCOTTI, and E. CAPANNA. 2001. The other chromatin. Chromosoma, 110:136-147.         [ Links ]

SMITH, M.F. and J.L. PATTON. 1999. Phylogenetic relationships and the radiation of sigmodontinae rodents in South America: evidence from cytochrome b. Journal of Mammalian Evolution, 6:89-127.         [ Links ]

SPOTORNO, A.E., L.I. WALKER, S.V. FLORES, M. YEVENES, J.C. MARÍN, and C. ZULETA. 2001. Evolution of phyllotines (Rodentia, Muridae) in the Southern Andes. Revista Chilena de Historia Natural, 74:151-166.         [ Links ]

STEPPAN, S.J. 1993. Phylogenetic relationships among the Phyllotini (Rodentia: Sigmodontinae) using morphological characters. Journal of Mammalian Evolution, 1:187-213.         [ Links ]

STEPPAN, S.J. 1995. Revision of the tribe Phyllotini (Rodentia: Sigmodontinae), with a phylogenetic hypothesis for the Sigmodontinae. Fieldiana, Zoology, 80:1-112.         [ Links ]

SUMNER, A.T. 1971. A simple technique for demonstrating centromeric heterochromatin. Experimental Cell Research, 75:304-306.         [ Links ]

WALLRATH, L. 1998. Unfolding the mysteries of heterochromatin. Current Opinion in Genetics and Development, 8:147-153.         [ Links ]

Recibido 4 abril 2004.
Aceptación final 20 julio 2004.

Creative Commons License Todo el contenido de esta revista, excepto dónde está identificado, está bajo una Licencia Creative Commons