Chromosome analysis in Saccodon wagneri (Characiformes) and insights into the karyotype evolution of Parodontidae
Parodontidae is a relatively small group of Neotropical characiform fishes
consisting of three genera (Apareiodon, Parodon, and Saccodon) with 32 valid
species. A vast cytogenetic literature is available on Apareiodon and Parodon, but
to date, there is no cytogenetic data about Saccodon, a genus that contains only
three species with a trans-Andean distribution. In the present study the karyotype
of S. wagneri was described, based on both conventional (Giemsa staining, Ag-
NOR, C-bands) and molecular (repetitive DNA mapping by fluorescent in situ
hybridization) methods. A diploid chromosome number of 2n = 54 was observed
in both sexes, and the presence of heteromorphic sex chromosomes of the ZZ/
ZW type was detected. The W chromosome has a terminal heterochromatin band
that occupies approximately half of the long arm, being this band approximately
half the size of the Z chromosome. The FISH assay showed a synteny of the
18S-rDNA and 5S-rDNA genes in the chromosome pair 14, and the absence
of interstitial telomeric sites. Our data reinforce the hypothesis of a conservative
karyotype structure in Parodontidae and suggest an ancient origin of the sex
chromosomes in the fishes of this family.