Cytogenetic and cytosystematic studies of the nucleous organizer regions of North American cyprinid fishes

Abstract

Chromosomal NOR banding was used to assay the variability of chromosomal NORs within and among 49 species (380 specimens) of North American Cyprinidae. Considerable intra- and interspecific NOR variability was documented. Intraspecifically, 13% of all specimens examined, regardless of species, were heteromorphic. The two types of variant NORs predominantly observed were (1) an enlarged or amplified NOR site, and (2) a deleted or transcriptionally inactive NOR site. Collectively, cyprinid species thought to possess multiple NOR pairs were more heteromorphic than those thought to possess only a single NOR pair. The different intraspecific NOR variants observed in this study could possibly represent true genetic polymorphisms. Lastly, the patterns of NOR variability observed intraspecifically could account for some of the interspecific NOR differences seen in the North American Cyprinidae. Interspecifically, 48 of the 49 cyprinid taxa examined in this study were found to possess species-specific NOR conditions. Among these 48 species, there was a minimum total of 19 differences with respect to chromosomal NORs. Interspecific variations in chromosomal NORs included (1) the number of NOR-bearing chromosome pairs per genome, and (2) NOR chromosome phenotypes, i.e., the chromosomal positions and types of chromosomes upon which the NORs were located. The 19 NOR differences exhibited by the 48 species indicate that a minimum of 18 chromosomal rearrangements have occurred since the 48 species last shared a common ancestor, and that the rate of chromosomal evolution within the group may have been higher than previously thought. The interspecific chromosomal NOR data for the North American Cyprinidae were found to be taxonomically informative. Chromosomal NORs also were found to be phylogenetically informative. Treatment of chromosomal NOR phenotypes as character states allowed the formulation of phylogenetic hypotheses relative to the taxa examined. Chromosomal C- (and R-) banding was employed to establish certain NOR chromosome homologies and thereby test the validity of some of the proposed relationships. Finally, the phylogenetic inferences drawn from this study provide a framework for which future chromosomal studies in the North American cyprinids can be based.