Cytogenetic Studies in North American Minnows (Cyprinidae)

XXVI. Chromosomal NOR phenotypes of 21 species from the Western United States

Abstract

Karyotypes and chromosomal nucleolus organizer region or NOR phenotypes of 19 described and two undescribed species of western North American cyprinids are documented. All 21 species examined possessed 2N=50 chromosomes and at least two pair of NOR-bearing chromosomes. The high incidence of multiple NOR chromosomes among western cyprinids is unusual. One pair of A NOR chromosomes was found in all 21 species, and 17 species possessed a second pair of A NOR chromosomes. An AA NOR state was hypothesized to represent a chromosomal synapomorphy uniting western cyprinids (including the genus Richardsonius) into a monophyletic assemblage. Within this clade, a transition from an A NOR to a D NOR was hypothesized to represent a chromosomal synapomorphy uniting three speices of the subgenus Gila, and an addition of an F NOR was hypothesized to represent a chromosomal synapomorphy uniting two species of the genus Rhinichthys. Chromosomal NORs in western cyprinids appear conserved relative to eastern cyprinids and may be the result of (i) a process of karyotypic canalization, and/or (ii) character state reversal in, or introgressive hybridization among, western cyprinid genera.