EVIDENCE THAT 2 TYPES OF 18S RDNA COEXIST IN THE GENOME OF DUGESIA (SCHMIDTEA) MEDITERRANEA (PLATYHELMINTHES, TURBELLARIA, TRICLADIDA)

Sequences of 18S ribosomal DNA (rDNA) are increasingly being used to i nfer phylogenetic relationships among living taxa. Although the 18S rD NA belongs to a multigene family, all its copies are kept homogeneous by concerted evolution (Dover 1982, Hillis and Dixon 1991). To date, t here is only one well-characterized exception to this rule, the protoz oan Plasmodium (Gunderson et al. 1987; Waters, Syin, and McCutchan 198 9; Qari et al. 1994). Here we report the Ist case of 18S rDNA polymorp hism within a metazoan species. Two types (I and II) of 18S rDNA have been found and sequenced in the platyhelminth Dugesia (Schmidtea) medi terranea (Turbellaria, Seriata, Tricladida). Southern blot analysis su ggested that both types of rDNA are present in the genome of this flat worm. This was confirmed through sequence comparisons and phylogenetic analysis using the neighbor-joining method and bootstrap test. Althou gh secondary structure analysis suggests that both types are functiona l, only type I seems to be transcribed to RNA, as demonstrated by Nort hern blot analysis. The finding of different types of 18S rDNAs in a s ingle genome stressess the need for analyzing a large number of clones whenever 18S sequences obtained by PCR amplification and cloning are being used in phylogenetic reconstruction.