Three divergent rDNA clusters predate the species divergence in Quercus petraea (matt.) liebl. and Quercus robur L.

Quercus petraea and Quercus robur are two closely related oak species that frequently hybridize. We sequenced 70 clones containing the 5.8S and ITS2 r egions of ribosomal DNA (rDNA) from these two species and did not detect a species-specific difference. Surprisingly, three divergent (up to 12.6%) rD NA families were identified in both species, indicating that they predate t he speciation event. Despite a large between-rDNA-families divergence, rDNA sequences were very similar within families, suggesting ongoing concerted evolution. Expression analysis, relative-rate tests, and mutation spectrum analyses indicated that only a single rDNA family is functional. We propose that past hybridization events, combined with nucleolar dominance, were th e evolutionary processes underlying the contemporary rDNA variability in Q. petraea and Q. robur.