Molecular cytogenetic analysis and genomic organization of major DNA repeats in castor bean (Ricinus communis L.)

This article addresses the bioinformatic, molecular genetic, and cytogenetic study of castor bean (Ricinus communis, 2n = 20), which belongs to the monotypic Ricinus genus within the Euphorbiaceae family. Because castor bean chromosomes are small, karyotypic studies are difficult. However, the use of DNA repeats has yielded new prospects for karyotypic research and genome characterization. In the present study, major DNA repeat sequences were identified, characterized and localized on mitotic metaphase and meiotic pachytene chromosomes. Analyses of the nucleotide composition, curvature models, and FISH localization of the rcsat39 repeat suggest that this repeat plays a key role in building heterochromatic arrays in castor bean. Additionally, the rcsat390 sequences were determined to be chromosome-specific repeats located in the pericentromeric region of mitotic chromosome A (pachytene chromosome 1). The localization of rcsat39, rcsat390, 45S and 5S rDNA genes allowed for the development of cytogenetic landmarks for chromosome identification. General questions linked to heterochromatin formation, DNA repeat distribution, and the evolutionary emergence of the genome are discussed. The article may be of interest to biologists studying small genome organization and short monomer DNA repeats.