Chromosomal characteristics of ribosomal DNA in the primitive semionotiform fish, longnose gar Lepisosteus osseus

The chromosomes of longnose gar, Lepisosteus osseus, an extant representati ve of early radiation of actinopterygian fishes, were studied using convent ional Giemsa-staining, Ag-staining, CMA(3)-fluorescence and fluorescence in -situ hybridization (FISH). The diploid chromosome number was 2n = 56 and t he karyotype contained 11 pairs of metacentric, 6 pairs of submetacentric, 3 pairs of subtelocentric macrochromosomes and 16 microchromosomes. Nearly all macrochromosomes showed large CMA(3)-positive regions resembling the R- bands of higher vertebrates, indicating extensive distribution of GC-rich D NA along chromosomes. The nucleolar organizer regions (NORs) were located o n the end of the short arm of a single small metacentric macrochromosomal p air. These sites were strongly CMA(3)-positive, suggesting that ribosomal s ites are associated with GC-rich DNA. In-situ hybridization (FISH) with a r DNA probe gave consistently positive signals in the same regions detected b y Ag-staining and CMA(3)-fluorescence. The evolutionary conservation of pos itive CMA(3)-fluorescence of ribosomal sites in 'holostean' and teleostean fishes is discussed.