CYTOTAXONOMIC STUDIES IN VIGNA - IV - VARIATION OF THE NUMBER OF ACTIVE AND SILENT RDNA SITES IN VIGNA-UNGUICULATA POPULATIONS

Fluorescent in situ hybridization (FISH) using the pTa71 probe (18S-5. 8S-25S rDNA) allowed the physical localization of the rRNA gene cluste rs on five chromosome pairs in mitotic metaphases of the reference lin e Tvx3236. Four pairs of these sites, showing high hybridization signa l, colocalised with the chromosomal domains identified after Chromomyc in A3 (CMA) fluorochrome staining, in telomeric regions. After stainin g with AgNO3, it was possible to ascertain that these sites had transc riptional activity. The fifth site, showing a lesser signal, had centr omeric location and no transcriptional activity. In wild populations o f Vigna unguiculata subsp. dekindtiana three major sites were detected with telomeric location and transcriptional activity. In two lines fr om Southern Italy six sites were observed: five were located telomeric ally and were genetically active, while the sixth was centromeric and had non activity. No variation was observed for the localization of th e 5S rRNA gene clusters. In all lines analysed it was possible to obse rve 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 de position of silver grains after AgNO3 staining. These results indicate that in cowpea the number of active r-DNA clusters is variable and mi ght suggest that during the domestication/evolution of this crop an in crease of these sites occurred.