Cytogenetic Studies in North American Minnows (Cyprinidae). XX. Chromosomal NOR Variation in the Genus Luxilus

Chromosomal nucleolus organizer region (NOR) phenotypes are documented for all extant taxa of the North American cyprinid genus Luxilus. All nine species, including two subspecies of L. chrysocephalus (L. c. chrysocephalus and L. c. isolepis), possess 2n = 50 chromosomes. Five species and one population of L. c. isolepis from the Blue River in Oklahoma possess a single pair of NORbearing chromosomes. These included L. cerasinus and L. c. isolepis from Oklahoma (NOR phenotype C), L. zonistius (NOR phenotype D,), and all three species (L. cardinalis, L. pilsbryi, and L. zonatus) of the L. zonatus group (NOR phenotype Dj). Four taxa (L. albeolus, L. coccogenis, L. c. chrysocephalus, and L. cornutus) and populations of L. c. isolepis from Alabama, Mississippi, and Louisiana possess two pairs of NOR-bearing chromosomes (NOR phenotype C, D,). A serial banding procedure (Z-banding) was used to test homology of C, D,, and D, NOR chromosomes. The NOR chromosome data support monophyly of the L. cornutus group (L. albeolus, both subspecies of L. chrysocephalus, and L. cornutus) and L. coccogenis. Assuming the L. coccogenis group (L. coccogenis and L. zonistius) is monophyletic, as suggested by morphological, allozymic, and mtDNA data, the chromosomal data support the hypothesis that the L. coccogenis and L. cornutus groups are sister taxa and that L. cerasinus is sister to the L. coccogenis groupL. cornutus group clade. Because the D, NOR chromosome of all three species of the L. zonatus group does not occur elsewhere in Luxilus, there is no chromosomal evidence for a phyletic relationship between the L. zonatus group and any other species in Luxilus. The hypothesis that the L. coccogenis group is sister to the L. cornutus group suggests that the "missing" species of Luxilus in the Ohio River-northern Atlantic slope drainages may have been a member (or ancestor) of the L. cornutus group.