Chromosomal NOR Karyotypes and Genome Size Variation among Squawfishes of the Genus Ptychocheilus (Teleostei: Cyprinidae)

Abstract

Nucleolus organizer region (NOR) chromosomes from species of the North American cyprinid genera Ptychocheilus, Mylopharodon, and Lavinia were documented and tested for presumed homology using trypsin G-banding. Four species of Ptychocheilus (P. lucius, P. grandis, P. umpquae, and P. oregonensis), M. conocephalus, and L. exilicauda possessed two pair of NOR-bearing chromosomes with the NORs situated terminally on the short arms of medium-sized acrocentric chromosomes (NOR phenotype A). Both pair of NOR chromosomes in all three genera were identical in trypsin G-banding pattern and hence were uninformative for inference of phylogenetic relationships within Ptychocheilus. The apparent conservatism of chromosomal NOR evolution in these cyprinid genera from western North America is in striking contrast to that documented among cyprinids from eastern North America. Genome sizes (DNA contents) of the six species also were documented using flow cytometry. Heterogeneity tests revealed three homogeneous groups of samples: (1) L. exilicauda, P. lucius, P. grandis, and P. umpquae from the Siuslaw River with low genome sizes; (2) M. conocephalus with an intermediate genome size; and (3) P. oregonensis and P. umpquae from the Umpqua River drainage with high genome sizes. The difference in genome size between P. umpquae from the Siuslaw and Umpqua river drainages is greater than that found, on average, between any two North American cyprinid species drawn at random. The difference in genome size, coupled with morphological differences documented previously, suggests that P. umpquae from the two drainages may represent distinct species. Using an optimization procedure, genome sizes were partitioned among branches of two contrasting hypotheses of relationships in Ptychocheilus. The hypothesis of {P. lucius [P. grandis (P. umpquae, P. oregonensis)]} required about 20% less change in genome size (i.e., was more parsimonious) than the hypothesis of {P. oregonensis [P. umpquae (P. grandis, P. lucius)]}.