GENE SILENCING OF SOME NUCLEAR rDNA UNITS IN V. UNGUICULATA SUBSPECIES TENUIS

Transcriptional status of two intra-genomic ITS variants (Type-I and Type-II) identified in Vigna unguiculata ssp. tenuis (NI 1637) was analysed by RT-PCR. Total RNA was isolated from leaves, stem and roots of 15-day old seedlings and was reverse transcribed using specific primers for amplifying ITS-1 and ITS-2. Gel electrophoresis on a high-resolution agarose revealed that only Type-II variant was transcriptionally active in all the tissues analysed, while Type-I variant has been silenced. Sequence analysis revealed that Type-I variant in NI 1637 was of ssp. pubescens origin, whereas Type-II variant was of ssp. tenuis origin. These results indicate that NI 1637 is a hybrid between ssp. tenuis and ssp. pubescens; and further showed that either the hybridization is of recent origin or subspecies tenuis has a slow rate of homogenization. Our results suggest that although the rRNA genes of ssp. pubescens origin are present in NI 1637 genome, they have been rendered transcriptionally silent probably by epigenetic mechanisms.