Cytotaxonomic studies in Vigna. IV. Variation of the number of active and silent rDNA sites in Vigna unguiculata populations

Abstract

Fluorescent in situ hybridization (FISH) using the pTa71 probe (18S5.8S-25S rDNA) allowed the physical localization of the rRNA gene clusters on five chromosome pairs in mitotic metaphases of the reference line Tvx3236. Four pairs of these sites, showing high hybridization signal, colocalised with the chromosomal domains identified after Chromomycin A3 (CMA) fluorochrome staining, in telomeric regions. After staining with AgNO3, it was possible to ascertain that these sites had transcriptional activity. The fifth site, showing a lesser signal, had centromeric location and no transcriptional activity. In wild populations of Vigna unguiculata subsp. dekindtiana three major sites were detected with telomeric location and transcriptional activity. In two lines from Southern Italy six sites were observed: five were located telomerically and were genetically active, while the sixth was centromeric and had non activity. No variation was observed for the localization of the 5S rRNA gene clusteys. In all lines analysed it was possible to observe a strict topological correspondence between major rRNA genes sites and CMA stained heterocromatin. Moreover in all cases the regions so identified demonstrated transcriptional activity as assessed by the deposition of silver grains after AgNO3 staining. These results indicate that in cowpea the number of active r-DNA clusters is variable and might suggest that during the domestication/evolution of this crop an increase of these sites occurred. © 1998 Taylor & Francis Group, LLC.