1.1. Genomic DNAs were isolated from 15 species of teleostean fishes and examined for % GC (guanine—cytosine base pair) values and nucleotide distributions using thermal denaturation.2.2. The % GC values among the species ranged from 37.3 to 41.6; compositional heterogeneity values were relatively uniform and ranged from 8.8 to 13.5; asymmetry values were generally low.3.3. Discrete melting components, indicative of blocks of repeated DNA sequences and identified by the presence of shoulders or minor peaks in differential melting rate profiles, were found in only two species.4.4. Based on comparable data from other major vertebrate groups, it appears that fish genomes may be organized differently from other vertebrates and that genome evolution in fishes may have proceeded at a slower rate.

The standard karyotype, genome size (DNA content), and genomic DNA base composition and distribution of the relict paracanthopterygian fish,Aphredoderus sayanus, were investigated. Several features of theA. sayanus genome appear to be derived rather than primitive conditions. These include a large number (at least 10 pairs) of bi-armed chromosomes, a low genome size, and high DNA asymmetry. This may indicate thatA. sayanus is not a typical paracanthopterygian fish in terms of its genome structure.

Base compositions and differential melting rate profiles of genomic DNAs from twenty species of North American cyprinid fishes were generated via thermal denaturation. Base pair composition expressed as % GC values ranged among the twenty species from 36.1–41.3%. This range is considerably broader than that observed at comparable taxonomic levels in other vertebrate groups. Both the range and average difference in base pair composition between species in the diverse and rapidly evolving genus Notropis were considerably greater than those between species in other North American cyprinid genera. This may indicate that genomic changes at the level of base pair composition are frequent and possibly important events in cyprinid evolution. Compositional heterogeneity and asymmetry values among the twenty species were uniform and low, respectively, suggesting that most of the species lacked DNA components in their genomes which differed substantially from their main-band DNAs in base pair composition. The melting rate profiles revealed a prominent and distinct heavy or GC-rich DNA component in the genomes of three species belonging to the subgenus Cyprinella of Notropis. These and other data suggest that the heavy melting component may reflect a large, comparatively GC-rich family of highly repeated or satellite DNA sequences common to all three genomes.

Chromosome formulae (diploid chromosome and chromosome arm number) are listed for 309 North American fish species, including representatives from 145 genera in 54 families. This updates earlier lists by more than 130 entries. In addition the list displays the presence of microelements or sex chromosomes in the complements of several species. The source of taxa was the Third Edition (1970) of the list of common and scientific(names of fishes prepared by the American Fisheries Society's Committee on Names of Fishes.

We have examined karyotypes from thirteen species of the North American cyprinid genus Notropis: N. chrysocephalus, N. fumeus, N. longirostris, N. lutrensis, N. oxyrhynchus, N. roseipinnis, N. sabinae, N. shumardi, N. signipinnis, N. texanus, N. umbratilis, N. venustus, and N. volucellus. All thirteen species have 2n=50 chromosomes. Estimated haploid arm numbers among the thirteen species ranged from 46-50; one-armed chromosomes (centromeres subterminal to terminal) comprised only a small portion of the karyotype of each species. Chromosomal evolution in Notropis, and in North American Cyprinidae, has been remarkably conservative. Almost all species karyotyped have 2n=50 chromosomes, and with one exception, cyprinid species with 50 chromosomes vary in estimated haploid chromosome arm number from only 46-50. In contrast, speciation in these fishes appears to have been rapid. Thus, the present data suggest that gross chromosomal rearrangement has played only a very minor role in the speciation and evolution of these fishes.