Abstract
Astyanax mexicanus is popularly known as the blind Mexican tetra or blind cave tetra and has been extensively studied regarding various aspects of its biology and genetics. Despite the identification of linkage maps of genes related to quantitative trait loci by many recent studies, only its diploid number was known from a cytogenetical point of view. With the purpose of providing a base for comparative studies and for the elucidation of physical maps for the species, cytogenetical studies were performed in a group of 10 blind specimens from Mexico. All the individuals presented 2n = 50 chromosomes and a karyotypic formula composed of 8M + 18SM + 12ST + 12A. A few specimens presented one or two B microchromosomes of the acrocentric type. Although simple argyrophilic nucleolar organizer regions (Ag-NORs) were evidenced, fluorescence in situ hybridization (FISH) with an 18S rDNA probe evidenced eight sites, and six sites were observed with a 5S rDNA probe. Little constitutive heterochromatin was observed, mainly related with the Ag-NORs and located close to the centromeres, including those from the B microchromosomes. A few pericentromeric heterochromatin regions were mainly constituted by GC, including the one from the Ag-NOR. Very subtle markings were observed by FISH with an As-51 satellite DNA probe. The B microchromosome did not present ribosomal genes or satellite DNA. Chromosomal aspects of the genus Astyanax are discussed.
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