From morphology and karyology to molecules. New methods for taxonomical identification of asexual populations of freshwater planarians. A tribute to Professor Mario Benazzi

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. Show more

Sequences of the internal transcribed spacers (ITS1 and ITS2) of the mosquito Aedes aegypti, and the ITS2 of six related species, A. simpsoni, A. albopictus, A. vexans, A. triseriatus, Haemagogus mesodentatus, and Psorophora ferox are reported. Intraspecific variation in A. aegypti ITS1 is 1.07% among four clones from three individuals, and in the ITS2 is 1.17% among 15 clones from four individuals. In A. simpsoni, intraspecific ITS2 variation is 0.46% among 10 clones from a single individual. Alignment of the ITS2 sequence of the seven species reveals several homologous domains. Secondary structure predictions for the ITS2 region indicate that these domains base pair to form a core region central to several stem features. The sequence outside the ITS2 homologous domains tends to be GC-rich and characteristically slippage generated; these areas preserve or add to the stem length of the predicted secondary structures. These ITS2 intraspacer variable regions resemble previously described expansion segments of the 28S gene region. Evolutionary analysis of the ITS2 of these species, using both sequence and secondary structure information, leads to the prediction of divergence in the mosquito tribe Aedini that is not clearly reflected in current taxonomic designations. Show more