Molecular characterization of intergenic spacer region of 5S ribosomal RNA genes in subgenus Vigna: extensive hybridization among V. unguiculata subspecies

We report the molecular analysis of the 5S ribosomal RNA intergenic spacer (IGS) region from 57 Vigna species of subgenus Vigna. Sequence analysis revealed that the 5S IGS was highly variable in length (189?C237?bp) and sequence (58% polymorphic sites). Most of the Vigna species analysed harboured a single type of 5S rRNA repeat unit, except V. unguiculata and V. reticulata that showed multiple ??intragenomic?? 5S types. The intragenomic 5S types among the six V. unguiculata subspecies were characterized by PCR-RFLP, genomic RFLP and sequencing. The 5S IGS was phylogenetically informative (comparable to ITS-1 and ITS-2 spacers) in inferring species relationships among the Vigna species analysed. However, due to the presence of multiple intragenomic 5S types and their incomplete homogenization among V. unguiculata subspecies the relationships in section Catiang could not be resolved below species level. The results presented indicate that intraspecies hybridization might have resulted in the ??horizontal transfer?? of 5S types among the V. unguiculata subspecies, while their maintenance could be due to a slow molecular drive.