Cytogenetic studies in North American minnows (Cyprinidae). XXII. Chromosomal nucleolar organizer regions in the genus Pimephales

Chromosomal nucleolar organizer region (NOR) phenotypes are documented for all four extant species in the North American cyprinid fish genus Pimephales. All four species (P. notatus, P. promelas, P. tenellus, and P. vigilax) possess 2n = 50 chromosomes and a pair of NOR-bearing chromosomes with the NOR situated terminally on the short arm of a medium-sized to large submetacentric chromosome (NOR phenotype C). Trypsin G-banding demonstrated that the C NOR chromosome in all four species is homologous. Two of the species (P. tenellus and P. promelas) also possess a C′ NOR chromosome, which is defined as an NOR situated terminally on the short arm of a large submetacentric chromosome that is also the largest chromosome in the complement. The C′ NOR chromosome occurs infrequently in P. promelas, being found in only 8% or so of all metaphases examined. Trypsin G-banding demonstrated that the C′ NOR chromosomes in the two Pimephales species are homologous to one another and to the C′ NOR chromosomes found in the cyprinid genus Cyprinella. A presumed derivative of the C′ NOR chromosome occurs in the monotypic cyprinid genus Opsopoeodus. The NOR chromosomal data support monophyly of the four extant species of Pimephales, and further suggest that the genus Pimephales belongs in a monophyletic assemblage with, among others, the cyprinid genera Cyprinella and Opsopoeodus. The data do not support the previous hypothesis that Pimephales is a basal clade outside of a larger assemblage of "Notropis"-like, shiners.