Hoplias malabaricus, a widely distributed neotropical freshwater fish, shows a conspicuous karyotypic diversification. An overview of this diversity is presented here comprising several Brazilian populations, and some others from Argentina, Uruguay and Surinam. Seven general cytotypes are clearly identified on the basis of their diploid number (2n = 39 to 2n = 42), chromosomal morphology and sex chromosome systems, which can be clustered into two major karyotypic groups. This clustering suggests that karyotype structure would be more informative than the diploid number regarding cytotype relationships in this fish group. While some cytotypes show a wide geographical distribution, some others appear to be endemic to specific hydrographic basins. Sympatric cytotypes can occur without detection of hybrid forms; this situation points to a lack of gene flow, a fact that is also reinforced by studies with genomic markers. The karyotypic data support the view that the nominal taxon H. malabaricus corresponds to a species complex comprising distinct evolutionary units, each with well-established chromosomal differences.

Cytogenetic studies were carried out in the fish, Hoplias malabaricus, from the Parque Florestal do Rio Doce (Brazil). This population is characterized by 2n = 42 chromosomes for both males and females and an XX/XY sex chromosome system, confirmed through several banding methods. Females show 24 metacentric, 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 with an 18S rDNA probe showed 10 well-labeled chromosomes, including the X chromosome. The 5S rDNA is interstitially located in a single metacentric pair 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 correspond to a polymorphic size condition observed in the X chromosome. The present results confirm the XX/XY sex chromosome system in the population analyzed as well as a new cytotype in the Hoplias malabaricus group.

Specimens of Hoplias malabaricus from Lagoa Carioca, an isolated lake of the Rio Doce State Park (state of Minas Gerais, Brazil), were cytogenetically studied. The diploid number was found to be constant, i.e., 2n = 42 chromosomes, although two karyotypic forms were found: karyotype A, characterized by 22M + 20SM chromosomes, observed only in a male specimen, and karyotype B, characterized by 24M + 16SM + 2ST and 24M + 17SM + 1ST chromosomes in female and male specimens, respectively. This sex difference found in karyotype B is related to an XX/XY sex chromosome system. Another female specimen of H. malabaricus, also carrying karyotype A, had previously been found in the same lake. The available data indicate that two sympatric cytotypes of H. malabaricus exist in the Lagoa Carioca, with cytotype A occurring at a lower frequency and differing from cytotype B by undifferentiated sex chromosomes.Keywords: Hoplias malabaricus, fish, karyotypes, sympatric forms. RESUMO Uma nova região de simpatria para formas cariotípicas distintas deHoplias malabaricus (Pisces, Erythrinidae)Foram analisados, cromossomicamente, espécimes de Hoplias malabaricus provenientes da Lagoa Carioca, localizada no Parque Estadual do Rio Doce (Minas Gerais, Brasil). O número diplóide de cromossomos foi constante, 2n = 42, mas duas formas cariotípicas distintas foram encontradas: cariótipo A, caracterizado por 22M + 20SM, presente em apenas um dos exemplares machos e cariótipo B, caracterizado por 24M + 16SM + 2ST e 24M + 17SM + 1ST em fêmeas e machos, respectivamente, diferença esta devida a um sistema de cromossomos sexuais do tipo XX/XY. Contudo, um outro exemplar fêmea, apresentando também o cariótipo A, foi previamente detectado nessa mesma lagoa, o que permite caracterizar o cariótipo A como portador de 2n = 42 cromossomos, mas sem um sistema diferenciado de cromossomos sexuais. Os dados disponíveis evidenciam que dois citótipos distintos (A e B) são encontrados em simpatria e sintopia na Lagoa Carioca, tendo o citótipo A uma freqüência reduzida.Palavras-chave: Hoplias malabaricus, peixes, cariótipos, formas simpátricas.

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