An XX/XY sex chromosome system in a fish species, Hoplias malabaricus, with a polymorphic NOR-bearing x chromosome

Cytogenetic studies were carried out in the fish, Hoplias malabaricus, from the Parque Florestal do Rio Doce (Brazil). This population is characterize d by 2n = 42 chromosomes for both males and females and an XX/XY sex chromo some system, confirmed through several banding methods. Females show 24 met acentric, 16 submetacentric and 2 subtelocentric chromosomes. Males show 24 metacentric, 17 submetacentric and 1 subtelocentric chromosomes. While the X chromosome is easily recognized (the only subtelocentric element), the Y chromosome is somewhat difficult to identify but appears to correspond to the smallest submetacentric in the male karyotype. In-situ hybridization wi th an 18S rDNA probe showed 10 well-labeled chromosomes, including the X ch romosome. The 5S rDNA is interstitially located in a single metacentric pai r independent of the 18S rDNA sites. The NOR on the X chromosome is always active and occurs adjacent to a heterochromatic distal segment on the long arm. Variations in size of the NORs and/or heterochromatic segment correspo nd to a polymorphic size condition observed in the X chromosome. The presen t results confirm the XX/XY sex chromosome system in the population analyze d as well as a new cytotype in the Hoplias malabaricus group.