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Abstract
The genus Schmidtea includes four planarian species characterized by karyological differentiation. Different levels of ploidy also define various biotypes, typically associated with sexual or pseudogamic forms of reproduction. The present paper gives a karyological survey of somatic cells and unfertilized oocytes of specimens collected from natural populations. With the aim of integrating classical karyological analyses with molecular data, the molecular structure of ribosomal DNA (rDNA), in individuals of different species and biotypes is also compared, using restriction enzyme analysis. The results reveal the presence of species‐specific rDNA restriction patterns. In spite of the occurrence of poly‐ploid forms, a high level of structural intraspecific homogeneity of these genes was also observed among different populations.