High intraspecific karyological conservation in four species of Gymnotus (Pisces: Gymnotiformes) from Southeastern Brazilian basins - Fernandes-Matioli, Flora Maria de Campos; Marchetto, Maria Carolina Nasser; Almeida-Toledo, Lurdes Foresti de; Toledo Filho, Sílvio de Almeida (Caryologia, 1998)