Silver-Staining and Heteromorphism of Chromosomal Nucleolus Organizer Regions in North American Cyprinid Fishes

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Abstract
Silver-stained karyotypes and patterns of variation in chromosomal nucleolus organizer regions (NOR) within and between 6 spp. of cyprinid fishes were studied [Notropis lutrensis, N. venustus, Pimephales vigilax, Campostoma anomalum, Notemigonus crysoleucas and Notropis emiliae]. Interspecies heteromorphisms included the number of silver-stained NOR/genome and their chromosomal location. Intraspecies heteromorphisms included differences in sizes of silver-stained NOR and in apparent number of active NOR sites. The patterns of silver-stained NOR in these fishes are no different from those found in other vertebrates, including the few fishes already studied. The interspecies differences in NOR may be of use in cyprinid phyletics or systematics, although confirmation must await sampling of additional species in concert with other types of chromosome banding. The intraspecific heteromorphisms in NOR were relatively frequent and raise the question of whether the variations are true genetic polymorphisms. The observed variations in chromosomal NOR among the 6 spp. suggest that major chromosome structural changes in North American cyprinid fishes may not be as infrequent as previously thought.