NOR regions of polychaete worms of the genus Ophryotrocha studied by chromosome banding techniques and FISH

This article reports the results of cytogenetic analyses carried out on 10 species of polychaete worms belonging to the genus Ophryotrocha (Dorvilleid ae). Nucleolar organizer regions (NORs) were characterized by Ag staining, C-banding, CMA, staining, and ribosomal fluorescent in situ hybridization ( rDNA FISH). Extensive intraspecific variation in NOR number and distributio n were observed in O. costlowi, O. sp, macrovifera, O. notoglandulata, O. I . labronica, O. I, pacifica (2n = 6), O. p. puerilis, O. diadema (2n = 8), O. hartmanni, O. gracilis (2n = 10), In O. sp. robusta (2n = 10), Ag-NORs w ere always located on a single chromosome pair, CMA(3) staining suggests a possible trend toward a GC-rich rDNA compartmentalization. In O, I. labroni ca, O. p. puerilis, O. diadema, and O. sp, robusta rDNA FISH shows that Ag and FISH signals coincide. Results from C-banding seem to indicate that the increased genome size (GS) observed in O. sp. macrovifera (0.8 pg) and O. hartmanni (1.16 pg) compared to the base GS value of the genus (0.4 pg) can not be attributed to variation in the heterochromatin content.