Molecular Biology and Evolution, Vol 13, 824-832, Copyright © 1996 by Society for Molecular Biology and Evolution
S Carranza, G Giribet, C Ribera, and M Riutort
Sequences of 18S ribosomal DNA (rDNA) are increasingly being used to infer
phylogenetic relationships among living taxa. Although the 18S rDNA belongs
to a multigene family, all its copies are kept homogeneous by concerted
evolution (Dover 1982; Hillis and Dixon 1991). To date, there is only one
well-characterized exception to this rule, the protozoan Plasmodium
(Gunderson et al. 1987; Waters, Syin, and McCutchan 1989; Qari et al.
1994). Here we report the 1st case of 18S rDNA polymorphism within a
metazoan species. Two types (I and II) of 18S rDNA have been found and
sequenced in the platyhelminth Dugesia (Schmidtea) mediterranea
(Turbellaria, Seriata, Tricladida). Southern blot analysis suggested that
both types of rDNA are present in the genome of this flatworm. This was
confirmed through sequence comparisons and phylogenetic analysis using the
neighbor-joining method and bootstrap test. Although secondary structure
analysis suggests that both types are functional, only type I seems to be
transcribed to RNA, as demonstrated by Northern blot analysis. The finding
of different types of 18S rDNAs in a single genome stresses the need for
analyzing a large number of clones whenever 18S sequences obtained by PCR
amplification and cloning are being used in phylogenetic reconstruction.
ORIGINAL ARTICLE
Evidence that two types of 18S rDNA coexist in the genome of Dugesia (Schmidtea) mediterranea (Platyhelminthes, Turbellaria, Tricladida)
Departament de Genetica, Facultat de Biologia, Universitat de Barcelona, Spain. carranza@porthos.bio.ub.es
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