Chromosomal nucleolar organizer region (NOR) phenotypes have been characterized in nine species of the genus Ophryotrocha (Polychaeta: Dorvilleidae), namely O. notoglandulata, O. sp. macrovifera, O. sp. labronica pacifica, O. labronica labronica, O. puerilis puerilis, O. diadema, O. sp. robusta, O. gracilis and O. hartmanni. Irrespective of chromosome number and morphology. Ag positive regions were terminally located in all but one species, O. diadema, where the NORs were pericentromerical in a metacentric pair. The presence of a single chromosome pair bearing NOR in invertebrates is considered an ancestral trait. According to this assumption, O. sp. robusta, O. diadema, and perhaps O. p. puerilis, appear to be more ancestral than the other species. On the contrary, O. notoglandulata, O. sp. macrovifera, O. sp. labronica pacifica, with two chromosomal pairs bearing NOR sites seem to represent examples of further evolution within the genus Ophryotrocha.
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