RESTRICTION SITE AND LENGTH POLYMORPHISM OF THE RDNA UNIT IN THE CULTIVATED BASIDIOMYCETE PLEUROTUS-CORNUCOPIAE

In the ribosomal DNA unit of Pleurotus cornucopiae, the rDNA coding regions are in the order 5', 5S-18S-5.8S-25S, 3', with the 5' location of the 5S gene differing from its 3' location found in other basidiomycetes. The most discriminating probe used to study the rDNA polymorphism consisted of a fragment that included the 5S, 18S and part of the 5.8S and 25S genes flanking three intergenic sequences. A high degree of rDNA polymorphism was observed in the seven P. cornucopiae dikaryons studied. For the first time within a basidiomycete species, the restrictions maps distinguished two types of rDNA units (I and II). In each rDNA type, length variations in the external intergenic sequence IGS 1 located between the 25S and 5S genes allowed characterization of two different rDNA units in type I and four rDNA units in type II. This suggested that the P. cornucopiae rDNA units were derived from two kinds of ancestors (type I and II) by insertion or deletion events (100 - 700 bp) in the IGS 1. In four dikaryotic strains, two rDNA units of the same type (I or II) differing only by the IGS 1 length, were found in a similar number of copies, and presented a meiotic segregation in homokaryotic progeny. In one progeny, some homokaryotic strains possessed two different rDNA units: one with a high copy number and another with a lower one, showing that two different rDNA units could coexist in a single nucleus.