Abstract
Basic and molecular cytogenetic analyses were performed in specimens of Characidium cf. zebra from five collection sites located throughout the Tietê, Paranapanema and Paraguay river basins. The diploid number in specimens from all samples was 2n = 50 with a karyotype composed of 32 metacentric and 18 submetacentric chromosomes in both males and females. Constitutive heterochromatin was present at the centromeric regions of all chromosomes and pair 23, had additional interstitial heterochromatic blocks on its long arms. The nucleolar organizer regions (NORs) were located on the long arms of pair 23, while the 5S rDNA sites were detected in different chromosomes among the studied samples. One specimen from the Alambari river was a natural triploid and had two extra chromosomes, resulting in 2n = 77. The remarkable karyotypic similarity among the specimens of C. cf. zebra suggests a close evolutionary relationship. On the other hand, the distinct patterns of 5S rDNA distribution may be the result of gene flow constraints during their evolutionary history.
Highlights
Characidium is the richest genus in number of species within the subfamily Characidiinae (Crenuchidae)
Due to its phylogenetic position, the cytogenetic analysis of species related to C. zebra may represent an interesting starting point for understanding events related to the karyotypic evolution within the genus
The 5S rDNA sites were localized at interstitial regions of five chromosome pairs (1, 6, 9, 17 and 18) in the samples from the Juba river (Figure 3c)
Summary
Characidium is the richest genus in number of species within the subfamily Characidiinae (Crenuchidae). It comprises about 50 valid species widespread throughout the Neotropical region, from Panama to Argentina (Buckup, 2003). Studies based on morphological data using cladistic methods indicated that Characidiinae along with Crenuchinae formed the family Crenuchidae (Buckup, 1998). Characidium zebra represents a dispersed and polymorphic species, often misidentified as C. fasciatum (Buckup and Reis, 1997). This species is characterized by the presence of a completely scaled isthmus. Due to its phylogenetic position, the cytogenetic analysis of species related to C. zebra may represent an interesting starting point for understanding events related to the karyotypic evolution within the genus
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