Cytogenetic studies in North American minnows (Cyprinidae). XII. Patterns of chromosomal nucleolus organizer region variation among 14 species

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Abstract
The chromosomal nucleolus organizer region (NOR) phenotypes of eight species of North American cyprinid fish are documented. This brings the total number of cyprinid species examined for chromosomal NORs to 14. At least 10 different NOR chromosome phenotypes are identifiable among the 14 species. These interspecific variations include differences in the (haploid) number of chromosomal NORs, the chromosomal location(s) of the NORs, and the type(s) of chromosomes upon which the NOR is located. Intraspecific variations or heteromorphisms of NOR chromosomes also occur, but are of a qualitatively different nature than the NOR variants observed between species. Arrangement of the interspecies NOR differences into phylogenetic hypotheses yields results which are not discordant with present concepts of North American cyprinid taxonomy, and in fact support the hypothesis of a close relationship between the cyprinid genera Notropis and Pimephales. These data suggest that NOR chromosome phenotypes will be useful in resolving problems in cyprinid systematics. The data also show that at least one or more chromosomal changes involving the NOR separate most of the species examined, and that at least nine different chromosomal rearrangements have occurred since the 14 species last shared a common ancestor. This suggests that chromosomal changes in cyprinids have been much more frequent than previously thought.