Molecular cytogenetics in planarians. I. Fluorescence in situ hybridization (FISH) of ribosomal DNA to mitotic chromosomes of Dugesia sicula Lepori (Turbellaria: Tricladida)

The results of the isolation of repetitive DNA elements in the genome of Dugesia etrusca, a species of the Dugesia gonocephala group, are reported. These sequences, about 1.4 kb long, represent only a part of longer interspersed genomic units (De1 family) and appear to be limited to the genome of some planarians of this group, as indicated by a Southern blot analysis performed in different species and populations. The genomic relationships among different species and populations of the D. gonocephala group are discussed in relation to the results obtained in the present work. 40-Caryologia 1998 Freshwater planarians show a wide range of karyological phenomena such as polyploidy, aneuploidy as well as occurrence of B-chromosomes. In the past, this karyological variation was extensively investigated from a cytogenetic and cytotaxonomic point of view by classical karyometric analyses (Benazzi 1982). Recently, a more reliable identification of the individual chromosomes was obtained by the application of different banding methods. In particular, the c-banding technique has produced species-specific patterns of heterochromatin distribution on planarian chromosomes, that are informative for understanding the karyological events that have accompanied speciation in these organisms (Batistoni et al. 1995; Canovai et al. 1995). With the aim of bridging morphological and molecular information, the techniques of non radioactive in situ hybridization have matured in recent years to procedures that have been successfully applied in a number of different organisms. These methods, together with the availability of obtaining cloned DNA fragments, have significantly improved the resolution of cytogenetic analysis as well (cf. Buckle and Kearney 1994; Joos et al. 1994). In this framework, we have localized the nucleolus organizing regions (NORs) on the chromosomes of Dugesia sicula Lepori (1948), by using an improved in situ fluorescence hybridization procedure.