Molecular systematics of the genus analysis of restriction polymorphisms in ribosomal DNA.

Part of the nuclear ribosomal DNA unit of , including most of the intergenic sequences, was used as a probe to hybridize with DNA from eleven taxa (29 isolates), digested with nine restriction endonucleases. For all digests, a high level of rDNA heterogeneity between and within species was detected, which is in agreement with the phenetic variability already reported in previous studies on taxa. Numerical analysis of the results, performed by use of two different tree-making methods, clearly distinguished among well-defined species, i.e. and , which presented large phenetic distances with the rest of the taxa tested. , despite morphological similarity and interfertility with , appears to be a discrete taxon. Although they showed relative affinity, and presented large distances among their representative isolates, supporting the idea that they correspond to distinct taxa. All strains of and having small phenetic distances were closely positioned on dendrograms; in relation to results from recent interfertility and isozyme studies, these results support the common genetic background of and ; the position of remains controversial. Furthermore, the rDNA analysis identified ten fragments suitable for species identification: eight allowed the characterization of five taxa ( and ) and two distinguished among clusters including related taxa. By a combination of just two restriction enzymes, the rDNA probe used seems to be the appropriate tool for both varietal characterization and protection of commercially valuable strains.