Genetica 119: 147–153, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 147 B chromosome polymorphism in the fish, Astyanax scabripinnis Dagmar Aparecida de Marco Ferro, Orlando Moreira-Filho & Luiz Antonio Carlos Bertollo ∗ Departamento de Gen´ etica e Evolução, Universidade Federal de São Carlos, Rodovia Washington Luiz Km 235, Caixa Postal 676, CEP 13565-905 São Carlos, SP, Brazil; ∗ Author for correspondence (E-mail: bertollo@power.ufscar.br) Received 2 August 2002 Accepted 3 March 2003 Key words: Astyanax scabripinnis, B chromosomes, cytogenetic analyses, variant forms Abstract Four populations of Astyanax scabripinnis (Pisces, Characidae) were analyzed for B chromosome features. This species contains a metacentric macrochromosome (B M ), similar in size to the first chromosome of the karyotype, besides two variant forms, a large submetacentric (B SM ) and a small metacentric (B m ), both showing a reduced frequency. These variant forms were observed only in the populations from the Campos do Jordão region (São Paulo State, Brazil), although not present in all the populations sampled. The three B chromosome forms are heterochromatic and at least the B M and B SM are also GC-rich, as evidenced by C-banding and chromomycin A 3 staining. In spite of the morphological similarity between the B M form and the first chromosome pair, they differ in the G-banding pattern, which does not favor a possible relationship among these chromosomes. FISH with a telomeric DNA probe evidenced signals on the terminal region of all chromosomes, including the Bs. Interstitial telomeric bands, indicative of some chromosomal rearrangements such as translocation or tandem fusion in the origin of the B chromosomes, were not observed. B SM and B m are probable derivative B chromosome forms from an ancestral B M chromosome, showing a restricted occurrence and low frequency in the populations. Introduction Supernumerary B chromosomes have been found in several animal and plant species (Jones & Rees, 1982; Camacho, Sharbel & Beukeboom, 2000). Their first report in neotropical fish referred to Prochilodus lin- eatus (cited as P. scrofa), appearing as small hetero- chromatic, predominantly metacentric chromosomes (Pauls & Bertollo, 1983). However, many other neo- tropical fish species belonging to different genera, families and orders, have evidenced supernumerary chromosomes in the karyotype (Moreira-Filho et al., 2001). B chromosomes of Astyanax scabripinnis have been more intensively studied in recent years, since the initial investigations of Salvador and Moreira-Filho (1992) and Maistro et al. (1992). Most of this fish populations have shown 0–1 large sized, usually a metacentric B chromosome, the occurrence of two Bs being a rare condition (Vicente, Moreira-Filho & Camacho, 1996). Exceptionally they may occur as microchromosomes, which were found in only two populations (Rocon-Stange & Almeida-Toledo, 1993; Mizoguchi & Martins-Santos, 1997). A. scabripinnis populations from Campos do Jordão region (São Paulo State, Brazil) have drawn special attention in view of the geographical con- ditions. Indeed, this mountainous region allows the establishment of several isolated populations in the headwaters of small streams. In some well-studied cases the large metacentric B chromosome showed a significant difference related to sex, with a pre- dominance of males without B chromosomes and females with one B chromosome (Vicente, Moreira- Filho & Camacho, 1996). The frequency of B chro- mosomes was also correlated with different altitudes (1920, 1800 and 700 m) in three populations be- longing to a single stream (Néo, Moreira-Filho &