Classical and molecular cytogenetic tools to resolve the Bixa karyotypes

Abstract

The cytogenetic studies performed in Bixa orellana have reported 2n=14 or 2n=16 chromosomes with little karyotypic information. In this context, an improved cytogenetic protocol using enzymatic maceration of meristematic cellular walls and digital image analysis was applied to B. orellana and B. arborea. In B. arborea, the high-resolution technique, Ag-NOR and C banding methods, and FISH using a probe of 45S rDNA genes, were also performed. Prometaphasic and standard C-metaphasic chromosomes presented well-defined primary and secondary constrictions facilitated the pairing of homologues and assembly of the karyogram for the 2 species. These species showed similar karyotypes with 2n=14 chromosomes, being composed of 5 metacentric pairs (1, 2, 3, 4 and 6) and 2 submetacentric pairs (5 and 7). The chromosome 1 is approximately 2 times as long as the others and possesses the secondary constriction adjacent to the centromere, which suggests that Robertsonian translocation between 2 smaller chromosomes may have occurred during the karyotype evolution of Bixa. Additionally, the active NOR, the NOR-adjacent heterochromatic region and 18S, 5.8S and 26S rDNA genes were identified in the secondary constriction of chromosome 1. © 2006 The Japan Mendel Society.