Lengths and nucleotide sequences of the internal spacers of nuclear ribosomal DNA in gymnosperms and pteridophytes

The nucleotide sequences of the internal transcribed spacers (ITS1 and ITS2) of the nuclear ribosomal DNA were analyzed in species belonging to gymnosperms and pteridophytes. The lengths of the ITSs of sixteen species of gymnosperms and seven species of pteridophytes were estimated. The gymnosperms have ITS1 regions larger than those observed in the pteridophytes and angiosperms (ca. 610-3100 bp versus 159-360 bp). On the other hand, the ITS2 regions appear to be of a conserved length (182-370 bp). We have determined the complete nucleotide sequences of ITS regions from four gymnosperm species and five pteridophyte species by cloning the PCR products. Sequence analysis showed the presence of three short tandem arranged subrepeats of about 70 bp in the 1112 bp ITS1 of Ephedra fragilis. Pyrimidine rich (about 90%) DNA segments of 40-50 bp were observed in the ITS1 of Ginkgo biloba. A highly conserved 16 bp long sequence known to be present in the ITS1 of the angiosperm species has been also found in the ITS1 of Cycas revoluta, Taxus baccata and Ephedra fragilis.