© 2004 The Japan Mendel Society Cytologia 69(4): 475–479, 2004 Karyology of Apareiodon affinis from Paraná River (Argentina). I. Chromosome Polymorphism Marcos Román Calgaro1,2, Alberto Sergio Fenocchio1,3, María Cristina Pastori1 and Héctor Alberto Roncati1 1 Laboratorio de Citogenética General, Facultad de Ciencias Exactas, Químicas y Naturales, Universidad Nacional de Misiones, Félix de Azara 1552; 3300 Posadas, Argentina 2 Universidad Federal de São Carlos (SP) Brasil 3 Universidade Estadual de Londrina. (PR) Brasil Received September 10, 2004; accepted October 30, 2004 Summary Karyotypic studies on a population of Apareiodon affinis from Paraná river (Argentina) were carried out. Although the modal diploid number of the population was 54 chromosomes, some structural and numerical polymorphisms were detected. In the first case these variations deal with the number of mono- and biarmed chromosomes and in the second with the presence of an additional unpaired chromosome that appeared only in female individuals, suggesting some kind of linkage with a chromosomal system of sex determination. These data were compared with the available cyto- genetic information and the geographic distribution of A. affinis. Key words Chromosome polymorphism, Apareiodon affinis, Neotropical fishes, Paraná river. Parondontidae is a relatively small family of neotropical fishes with only three genera, Apareiodon, Parodon and Saccodon (Garavello 1977) distributed from Panamá to the La Plata basin in Argentina. Apareiodon and Parodon are widely distributed in South America, their tooth being the principal diagnostic characters (Travassos 1955, Britski 1972, Roberts 1974). This family constitute a major group of Characiformes with Anostomidae, Curimatidae, Prochilodontidae and Chilodontidae showing a general cytogenetic uniformity with a diploid num- ber of 54 biarmed chromosomes (FNϭ108) in almost all the studied species (Galetti Jr. et al. 1994). However, in some cases in Parodontidae and Anostomidae the occurrence of sexual chromosome systems with female heterogamety (ZZ/ZW) has been reported (Galetti Jr. and Foresti 1986, Galetti Jr. et al. 1995, Moreira 1983, Centofante et al. 2002). Additionally, a special multiple sexual chro- mosome system was described in Apareiodon affinis from the Upper Parana river basin, also with female heterogamety, but with multiple W chromosomes (Moreira et al. 1980). In this case males ϭ ϭ ϭ ϭ have 2n 54 (FN 108) (ZZ) and females have 2n 55 (FN 110) (ZW1W2) (Moreira et al. 1980). A comparative study among A. affinis from Paraná and Paraguay rivers reports that in the last popu- lation males and females show 2nϭ54 without sexual chromosome heteromorphisms but with a FNϭ98, due to the presence of acrocentric elements in the karyotype (Jesus et al. 1999). Jorge and Moreira (2000) comparing populations of this species from the Paraná river at Brazil and Argentina locations corroborate that the first ones present well established multiple sex chromosomes that are not present in Argentinean population. In the former, however, a wide chromosomal polymorphism was observed due to the presence of variable number of acrocentric chromosomes, performing 10 different cytotypes. In this paper was cytogenetically studied A. affinis from an Argentinean population geographi- cally located between the both analyzed by Jorge and Moreira (2000). * Corresponding author, e-mail: [email protected] 476 M. R. Calgaro et al. Cytologia 69(4) Material and methods Were cytogenetically studied 34 individuals (14 females, 16 males and 4 of undetermined sex) of Apareiodon affinis from the Paraná river at Posadas (Misiones Province, Argentina). Chromo- some preparations were obtained from kidney cells by means of direct and culture techniques (Bertollo et al. 1978, Fenocchio et al. 1991). C and NOR bands were obtained following the meth- ods of Sumner (1972) and Howell and Black (1980) respectively. Chromosomes in karyotypes were arranged in the order of meta-submetacentrics, subtelocentrics and acrocentrics. Results and discussion All the analyzed males showed 2nϭ54 chromosomes but were found females with 2nϭ54 and 2nϭ55 chromosomes. An accentuated structural polymorphism were detected, with at least seven different cytotypes: a) female 2nϭ55 (47 M-SM; 8 ST–NFϭ110); b) female 2nϭ55 (43 M-SM; 10 ST and 2 A–NFϭ108); c) female 2nϭ55 (43 M-SM; 10 ST-A and 2 A–NFϭ108); d) female 2nϭ54 (45 M-SM; 8 ST and 1 A–NFϭ107) (Fig. 1); a) male 2nϭ54 (40 M-SM; 12 ST and 2 A–NFϭ106); b) male 2nϭ54 (40 M-SM; 9 ST and 5 A–NFϭ103) and c) male 2nϭ54 (42 M-SM; 8 ST and 4 A–NFϭ104) (Fig. 2). These results show that only females have 2nϭ55 chromosomes and the structural polymor- phism is related to the number of the diverse chromosome types, varying principally the number of acrocentrics, changing, in consequence, the FN. NORs were observed distally on the long arm of a subtelocentric chromosome pair, as was re- ported in the literature. C banding shows heterochromatic blocks in centromeric regions, few telom- eric ones and a single pair with the short arms entirely stained (results not showed). Present results disagree with the first cytogenetic data reported for Parodontidae and specially for A. affinis studied by Moreira et al. (1980, 1985) in Brazil. Differences raise first, on the absence of a well established sex chromosome system and second, on the presence of acrocentric chromo- somes into the complement. Occurrence of polymorphism and monoarmed chromosomes were re- cently reported by Jorge and Moreira (2000) in A. affinis from Paraná river at Corrientes (Argenti- na) and the authors suggested that acrocentrics could be originated from pericentric inversions in- volving more than one chromosome. For the population of A. affinis studied here belonging also to the Paraná river at Posadas (Argentina), it is possible to suggest that the same mechanism could have originated the complex structural polymorphism now reported. Additionally, in the present study females with 55 chromosomes resembling those from Brazil were observed, having ZZ/ZW1W2 sex chromosome system. This fact suggests that, in addition to the occurrence of chromosomal rearrangements into the populations, actually a hybridization process among the brazilian sexually well differentiated populations and Argentinean ones could have occurred. The populations of A. affinis studied now throughout the Parana river have actually a disjunct distribution, being separate by an important geographical barrier (Sete Quedas or Saltos del Guairá) that should have contributed to establish in Brazil a more conservative karyotype, showing a differ- entiated sex chromosome system without conspicuous chromosome polymorphisms (Moreira et al. 1980, 1985). In contrast, populations from Argentina, Corrientes (Jorge and Moreira 2000) and Posadas (present study), show variable karyotypes, that could be due to specially favorable ecologi- cal conditions of the habitat, leading to numerically large population and maintaining a wide chro- mosomal polymorphism. Present report reinforces former data about structural polymorphisms in A. affinis from Ar- gentina additionally showing not previously observed numerical variation in females that could be related to sexual determination and needs further carefully investigation. 2004 Chromosome Polymorphism of Apareiodon affinis 477 Fig. 1. Cytotypes of females of Apareiodon affinis a) 2nϭ55 (M-SM-ST), b) 2nϭ55 (M-SM-ST-A), c) 2nϭ55 (M-SM-ST-A), d) 2nϭ54 (M-SM-ST-A). 478 M. R. Calgaro et al. Cytologia 69(4) Fig. 2. Cytotypes of males of Apareiodon affinis a) 2nϭ54 (M-SM-ST-A), b) 2nϭ54 (M-SM-ST-A), c) 2nϭ54 (M-SM-ST-A). Acknowledgements Authors are grateful to the Secretaria de Políticas Universitarias (Programa Incentivos a Do- centes—Investigadores) and to Mr. O. Moreira Filho for valuable suggestions. References Bertollo, L. A. C., Takahashi, C. S. and Moreira Filho, O. 1978. Cytotaxonomy considerations on Hoplias lacerdae (Pisces: Erythrynidae) Brazil. J. Genet., 2: 17–37. Britski, H. A. 1972. 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