Five major nuclear ribosomal repeats represent a large and variable fraction of the genomic DNA of Picea rubens and P. mariana.

The nuclear ribosomal repeats for the 18S, 5.8S, and 26S RNAs of two closely related Picea (spruce) species were characterized by restriction mapping and Southern blot hybridization. Restriction polymorphisms were identified in the IGS and ITS sequences; however, no polymorphism was species specific. As many as five different rDNA repeat units were observed in individual genomes. The repeat size for these gymnosperms ranged from a minimum of 32 kbp to greater than 40 kbp, two- to threefold larger than the typical angiosperm rDNA unit. Slot-blot hybridizations were used to determine the nuclear rDNA copy concentration. Among P. rubens individuals threefold variation was observed in the rDNA copy concentration, and among P. mariana individuals such variation was as much as sixfold. At a size greater than 32 kbp and at a concentration averaging 1.2-1.3 x 10(4) copies/pg, the rDNA constitutes approximately 4% of the total genome. Regression analysis revealed a significant relationship between copy concentration of the rDNA repeat unit in P. rubens and geographic origins. Differences in the rDNA content in Picea could contribute to the variation, in overall genome size, that has been observed within conifer species.