Molecular cytogenetic organization of 5S and 18S-26S rDNA loci in white clover (Trifolium repens L.) and related species

The genus Trifolium (Leguminosae), which includes several important forage legumes of temperate and subtropical regions, is characterized by small chr omosomes of uniform size. Cytogenetic mapping of the two multigene families coding for 18S-5.8S-26S rRNA and SS rRNA in white clover (T. repens) and s even closely related species/subspecies was carried out using fluorescence in situ hybridization (FISH). The 18S-26S rDNA, generally confined to nucle olus organizer regions (NORs), was consistently located proximally on the s horter arm of one or two metacentric or submetacentric chromosome pairs. DA PI-negative, diffused and highly stretched 18S-26S FISH signals, representi ng previously transcriptionally active NORs, were observed up to late metap hase. The Banking condensed ends of the diffused 18S-26S rDNA signals were DAPI-positive and represented perinucleolar knobs which are presumed to be transcriptionally inactive. In interphase cells, the decondensed NOR chroma tin passed through the nucleolar domain. In one species(T. isthmocarpum), i n addition to two NOR chromosome pairs, 18S-26S signals were also located o n a third pair, with no apparent NOR association. The 5S rDNA signals were located proximally on a long arm in six species and were syntenic to the NO Rs in all these species except T. ambiguum and T. nigrescens sap. petrisavi i. In two species (T, hybridum, T. isthmocarpum), SS sequences occurred in the chromosomal satellites aligned distally to the NOR sequences. The relat ive distribution patterns of the two types of tandemly repeated DNA sequenc es were species-specific except for T. nigrescens ssp. petrisavii and T. am biguum, both of which showed identical hybridization patterns. The two type s of rDNA sequences have provided molecular markers for individual identifi cation of a set of clover chromosomes. Present findings support the allotet raploid origin of white clover (T. repens) and suggest that T. nigrescens s sp. petrisavii may be one of the present day diploid ancestors. (C) 1999 An nals of Botany Company.