CYTOGENETICS OF THE GENUS LEPORINUS (PISCES, ANOSTOMIDAE) .1. KARYOTYPE ANALYSIS, HETEROCHROMATIN DISTRIBUTION AND SEX-CHROMOSOMES

Cytogenetic analyses (Giemsa staining, C-banding, of nucleolus organiz er regions with base-specific fluorochromes) were performed on the Sou th American fish species Leporinus friderici, L. obtusidens and L. elo ngatus. The overall karyotypic structure, position of NORs, as well as the amount, distribution and composition of constitutive heterochroma tin were determined. Particular attention was given to the highly diff erentiated ZZ/ZW sex chromosome system of L. obtusidens and L. elongat us. Sharing the apparently ancient macroscopic karyotype of Anostomida e, all three species have 2n=54 meta- or submetacentric chromosomes. N ORs were found exclusively on chromosome pair 2, which may represent t he ancestral NOR-bearing chromosome of the anostomid karyotype. Observ ed differences in the relative position of NORs along chromosome 2 and variations in the amount and distribution of constitutive heterochrom atin throughout the karyotype were most probably caused by heterochrom atin-mediated chromosome rearrangements. Detailed analysis of the morp hologically similar heteromorphic ZZ/ZW sex chromosomes of L. obtuside ns and L. elongatus allowed detection of differences in the DNA compos ition of the largely heterochromatic W chromosomes. However, since the se and the W chromosomes of three other Leporinus species exhibit homo logies with respect to their relative size, centromere position and am ount and distribution of heterochromatin, it is concluded that they ev olved from the same ancestral W chromosome.