A Heteromorphic ZZ/ZW Sex Chromosome System in Fish, Genus Hypostomus (Loricariidae)

Ten specimens of Hypostomus sp. from the Araguaia basin (Barra do Garcas, MT, Brazil) were cytogenetically studied, with characterization of the constitutive heterochromatin and positive segments for the fluorochrome Mithramycin. It was detected a chromosomal heteromorphism related to sex, of the ZZ/ZW type, not observed yet in Hypostomus catfish. Thus, in male karyotype can be found one pair of subtelo-acrocentric Z chromosomes, represented by a unique component in female karyotype. However, this latter has an exclusive metacentric the W chromosome. The available data indicate that the differentiation of this sex chromosome system may have occurred by a pericentric inversion, followed by length reduction in an ancestral chromosome similar to the Z one, originating a W chromosome like that now observed in the female karyotype of this fish species.