C 1996 The Japan Mendel Society Cytologia 61: 247-252, 1996 Karyotypic Studies and Cytotaxonomic Considerations on Callichthys callichthys (Pisces, Siluroidei) from Argentina Sanchez, S. and Fenocchio, A. S. Facultad de Ciencias Exactas, Quimicas y Naturales. UNaM. Felix de Azara 1552, 3300- Posadas, Misiones. Argentina Accepted May 10, 1996 The Callichthyidae are an interesting Neotropical armored catfish and at now are re- presented by eight genera (Aspidoras, Brochis, Callichthys, Cascadura, Cataphractops, Coryd - oras, Dianema and Hoplosternum) (Gosline 1940). Some cytogenetic studies have been performed in this family, exclusively in brazilian river basins (Venere and Galetti Jr. 1986, Oliveira et al 1988, Oliveira et al. 1993). Callichthys callichthys Linee, 1758 is theoretically a monotypic species but they are largely distributed throughout South-America and the available karyotypic data evidenced at least three different chromosome numbers 2n = 52 (Rebelo Porto and Feldberg 1992), 2n = 54 (Rebelo Porto and Feldberg 1988, Martins Dos Santos et al. 1990) and 2n = 58 (Oliveira et al. 1988, Oliveira et al. 1990, Rebelo Porto and Feldberg 1992). In several populations was also observed the presence of 1 to 16 accesory minute chromosomes (Erdtmann 1986, Venere and Galetti Jr. 1986, Oliveira et al. 1988, Oliveira et al. 1993). In the Table 1 are summarized all the chromosome data about C. callichthys. This species has single NORs except in one population which has two nucleolar chromo - some pairs (Rebelo Porto and Feldberg 1988). The C-banding have evidenced the ocurrence of heteromorphisms or polymorphisms not related to sex. They were characterized by the presence of biarmed chromosomes with one euchromatic arm and another heterochromatic (Oliveira et al. 1988, Oliveira et al. 1993). Oliveira et al. (1993) have proposed that 2n =52-58 are the basic number for Callichthyi - dae and C. callichthys. These ancestral karyotype could to origin all the karyotypes of the family either by fission, fusion and polyploidization events. In the present paper was analized cytogenetically a population of Callichthys callichthys from Argentina. Material and methods Ten specimens of Callichthys callichthys (7,•‰ and •Š) collected in lagoons near to Reconquista city (Santa Fe, Argentina) were cytogenetically analized (Fig. 1). Chromosome preparations were obtained from kidney cells (cephalic portion) utilizing direct methods (Foresti et al. 1993). Conventional Giemsa coloration, C (Sumner 1972) and NOR (Howell and Black 1980) banding were realized. The chromosomes were classified according to Levan et al. (1964) in metacentrics (M) , submetacentrics ( SM ) and subtelo-acrocentrics (ST-A). Results The population studied presents three diploid chromosome numbers. In five individuals (3•‰ and •Š) we have confirmed 2n = 56 with a chromosome formulae 14M, 10SM, 32ST-A 248 Sanchez, S. and Fenocchio, A. S. Cytologia 61 Fig . 1. Map showing the sampling location. (Fig. 2a). Three specimens (2•‰ and 1 •Š ) have 57 chromosomes (15M, 10SM, 32ST-A) (Fig. 2b) and two (•‰) 2n = 58 (16M, 10SM, 32ST-A) (Fig. 2c). Each of these cytotypes is resultant of the addition of one or two small metacentric chromosomes respectively. The accesory element not shows intraindividual variability. It was not observed microchromosomes in the population analized. NORs were observed associated to secondary constrictions on the short arm in one chromosome pair (ST) (Fig. 3). Frequently, these constrictions appear decondensed and shows very marked heteromorphysm between the NOR homologues (Figs. 2a, 3, 4). In cases the nucleolar regions are associated (Fig. 3b). The constitutive heterochromatin were detected in pericentromeric and centromeric regions in several chromosomes (Fig. 3a). In some individuals were observed chromosomes with one arm euchromatic and another heterochromatic (Fig. 3a). This fact was not related to sex. Successive C-banding and silver staining shows coincidence between NORs and C-positive regions (Fig. 3). Discussion The available cytogenetic data on Callichthys callichthys (Table 1) evidence a great karyotypic variability ranged between 2n = 52 to 2n = 74. The greater number include accesory minute chromosomes. The populations studied are geographically isolated and show peculiar chromosomal characteristics. This fact allows to consider it as single cytotaxonomic groups. In the present work it was described for the first time Callichthys callichthys with 2n = 56 , which represents the basic chromosome number for the population analized. It is described also the occurrence of a numerical polymorphism due to accesory chromo- somes. These supernumerary elements originate the cytotypes 2n = 57 (56 + 1) and 2n = 58 (56 + 2). The size and shape of the additional chromosome are different to them detected in other studies on C. callichthys. Another important observation was the variability evidenced by the C-banding that stains entirely only one arm in a metacentric chromosome pair. The NORs seems to be conservative in all the populations studied at now. Le Grande (1985) has suggested that the ancestral karyotype of the Siluroidei is composed 1996 Cytogenetics of Callichthys callichthys from Argentina 249 a b C Fig. 2. Callichthys callichthys. a. Karyotype 2n = 56. In the inset is showed the chromosome pair with NORs (successive staining Giemsa-NOR). b. Karyotype 2n = 57. The accesory meta - centric chromosome is underlined. c. Karyotype 2n = 58. The accesory metacentric chromosome pair is underlined. 250 Sanchez , S. and Fenocchio, A. S. Cytologia 61 a b Fig . 3 . Callichthys callichthys. a-b. C-NOR successive banding. The arrowheads show NOR regions and positive heterochromatic blocks. a b C d Fig . 4 . Callichthys callichthys. a-b. Successive Giemsa-NOR staining. The arrows show the NORs. c-d. The arrowheads show different NOR configurations. 1996 Cytogenetics of Callichthys callichthys from Argentina 251 Table 1. Available cytogenetic data about C. callichthys by 2n = 56 •} 2 chromosomes and high NF (100). Oliveira et al. (1993) has proposed that 2n = 52-58 (one or two NORs, M-SM chromosomes, DNA 1.92 pg/nucleus) might represent the most primitive karyotype condition for Callichthyidae and Callichthys. According these authors the low chromosome numbers detected in the family, was derived forms. The increase in chromosome number might be due to polyploidy and fusion events and the reduction could be due to fusion end to end events. The cytotypes with higher chromosome numbers due to the presence of microchromo- somes could be originated independently as result of karyotypic reordering. In spite of the suggestion of Oliveira et al. (1993) it is interesting the fact that the smallest M chromosome pair presents in C. callichthys 2n = 58 studied by them are very similar in shape and size to accesory elements detected by us in the present study. On these basis we suggest that 2n = 56 could be the basic chromosome number for C. callichthys. It is possible that the karyotype 2n = 58 observed in other populations be derived from 2n = 56 by fixation of two accesory chromosomes (2n = 56 + 2M). The cytotypes 2n =52 and 2n = 54 may be also derived forms from 2n = 56. These hypothesis need to be tested using additional techniques and methods. However this work shows clearly that C. callichthys is not a monotypic species. Summary We studied cytogenetically a population of Callichthys callichthys from Argentina. It showed three different cytotypes. The basic chromosome number observed in this population was 2n = 56. The presence in some individuals of one or two metacentric accesory chromo - somes originated polymorphic forms with 2n = 57 (56 +1) and 2n = 58 (56 + 2). C-bands were observed in centromeric and pericentromeric regions. The NORs were located in terminal position in the short arms on a ST chromosome pair. The successive aplication of these techniques evidence the correlation between NORs and constitutive heterochromatic regions. On the basis of available data we suggest that 2n = 56 may be the basic chromosome number for Callichthys callichthys. All the different cytotypes described at the present could be derivated forms. Key words : Pisces, Supernumerary chromosomes, Callichthys callichthys (Siluroidei). Acknowledgments The authors are grateful to MSc G. M. Duarte for translation to English. This research was supported by Prog. Incen. Inv. (S. P. U.) and Prog. Coop. Ctfca. con Iberoamerica (Esparia). 252 Sanchez, S. and Fenocchio, A. S. Cytologia 61 References Erdtmann, B. 1986. Microcromossomos em Callichthys callichthys L. (Siluriformes, Pisces). I. Simposio de Cito- genetica Evolutiva e Aplicada de Peixes Neotropicais: p. 37. Foresti, F., Almeida, C. and Foresti de Almeida Toledo, L. A. 1993. A method for chromosome preparations from large fish specimens using "in vitro" short-term treatment with colchicine. Experientia 49: 810-814. Gosline W. A. 1940. A revision of the Neotropical catfishes of the family Callichthyidae. Stan. Lchth. Bull. 2: 1-29. Howell, W. M. and Black, D. A. 1980. Controlled silver-staining of nucleolus organizer regions with a protective colloidal developper: a 1-step method. Experentia 36: 1014-1015. Le Grande, W. H. 1981. Chromosomal evolution in North American cat-fishes (Siluriformes, Ictaluuridae) with particular enfasis on the madtons, Noturus. Copeia 1: 33-52. Levan, A., Fredga, K. and Sandberg, A. A. 1964. Nomenclature for centromeric position on chromosomes. Hereditas 52: 201 - 220. Martins Dos Santos, I. C., Ferreira