XX:XY sex chromosome system with X heterochromatinization: an early stage of sex chromosome differentiation in the neotropic electric eel Eigenmannia virescens - Almeida-Toledo, Lurdes Foresti de; Foresti, Fausto; Pequignot, E V; Daniel-Silva, Maria de Fátima Zambelli (Cytogenetics and Cell Genetics, 2001)