Chromosome polymorphism in Astyanax fasciatus (Teleostei, Characidae). 1. Karyotype analysis, Ag-NORs and mapping of the 18S and 5S ribosomal genes in sympatric karyotypes and their possible hybrid forms.
Astyanax fasciatus may be characterized as a chromosomally diversified 'species' presenting distinct cytotypes, each with its specific variants. The sympatric and syntopic occurrence of different cytotypes reinforces the hypothesis in which A. fasciatus may represent a group of species currently placed under a single common designation. Specimens from three collection points spread along the Mogi-Guaçu River in southeast Brazil were examined in the present work: (1) near its headwaters (Ouro Fino--MG), (2) in the middle region of the river (Cachoeira de Emas, Pirassunun ga--SP) and (3) close to its confluence with the Pardo River (Barrinha--SP). The 2n = 48 chromosomes cytotype was found in all sampling points, while cytotype 2n = 46 was only encountered in Barrinha and Cachoeira de Emas. In the latter locality, cytotype 2n = 46 predominated; nevertheless, other karyotype forms with 2n = 45 and 47 chromosomes also occurred, besides a structural variant of cytotype 2n = 46. One specimen with 2n = 47 chromosomes was also found in Ouro Fino. The Ag-NOR analysis, as well as the location of the 18S and 5S ribosomal genes, were conserved in all cytotypes. The data indicate that the variant karyotypes are a consequence of interbreeding between the standard cytotypes (2n = 46 and 48) and/or its descendants. This suggests a karyotype plasticity for this species, where at least a few variant karyotypes would not have deleterious effects on their bearers.
Departamento de Genética e Evolução, Universidade Federal de São Carlos, São Carlos, SP, Brazil. pazza@biociencia.org
R
R
Pazza
K F
KF
Kavalco
L A C
LA
Bertollo
D000818:Animals; D034961:Antigens, Nuclear / Q000235:genetics*; D002874:Chromosome Mapping; D004275:DNA, Ribosomal; D005399:Fishes / Q000235:genetics*; D017404:In Situ Hybridization, Fluorescence; D007621:Karyotyping; D009687:Nuclear Proteins / Q000235:genetics*; D009693:Nucleic Acid Hybridization; D011110:Polymorphism, Genetic; D012337:RNA, Ribosomal, 18S / Q000235:genetics*; D012341:RNA, Ribosomal, 5S / Q000235:genetics*
D034961:Antigens, Nuclear; D004275:DNA, Ribosomal; D009687:Nuclear Proteins; D012337:RNA, Ribosomal, 18S; D012341:RNA, Ribosomal, 5S; C051963:nucleolar organizer region associated proteins