Zool., 66. 239-244 (1999) A molecular cytogenetic comparison of planarians from the 'Dugesia gonocephala group' (Platyhelminthes, Tricladida) RENATA BATISTONI LEONARDO ROSSI ALESSANDRA SALVETTI PAOLO DERI Laboratorio di Biologia Cellulare e dello Sviluppo, Dipartimento di Fisiologia e Biochimica, Università di Pisa, via Carducci 13, I-56010 Ghezzano, Pisa (Italy) ABSTRACT Planarians belonging to the Dugesia gonocephala group share extensive karyotype similarities when compared using classical cytological techniques. However, after in situ hybridization with rDNA, differences in localization and number of rDNA loci were identifiable among the species. Species-specific features in the rDNA structure have been observed by restriction pattern analysis. A family of long interspersed repeated elements (Del), isolated in Dugesia etrusca, and conserved only in some species of the group, was also informative in establishing specific genomic relationships among the species. We suggest that an understanding of the organization and evolution of repeated DNA sequences can be a useful tool for studying molecular mechanisms of evolutionary significance in planarians.