CHROMOSOMAL NUCLEOLAR ORGANIZER REGION (NOR) PHENOTYPES IN 9 SPECIES OF THE GENUS OPHRYOTROCHA (POLYCHAETA, DORVILLEIDAE)

Chromosomal nucleolar organizer region (NOR) phenotypes have been char acterized in nine species of the genus Ophryotrocha (Polychaeta: Dorvi lleidae), namely O. notoglandulata, O. sp. macrovifera, O. sp. labroni ca pacifica, O. labronica labronica, O. puerilis puerilis, O. diadema, O. sp. robusta, O. gracilis and O. hartmanni. Irrespective of chromos ome number and morphology, Ag positive regions were terminally located in all but one species, O. diadema, where the NORs were pericentromer ical in a metacentric pair. The presence of a single chromosome pair b earing NOR in invertebrates is considered an ancestral trait. Accordin g to this assumption, O. sp, robusta, O. diadema, and perhaps O. p. pu erilis, appear to be more ancestral than the other species. On the con trary, O. notoglandulata, O. sp. macrovifera, O. sp. labronica pacific a, with two chromosomal pairs bearing NOR sites, seem to represent exa mples of further evolution within the genus Ophryotrocha.