Cytogenetics description in Batrachoides surinamensis, (Batrachoididae: Batrachoidiformes): What does the estuary have to say?

The Batrachoididae (Batrachoidiformes) is a diverse fish family (84 species) of considerable medical interest, responsible for large numbers of injuries in fishermen and bathers in northern Brazil. Batrachoides surinamensis (the Pacuma toadfish) is the most common Batrachoididae species in the Amazon coastal zone. The capacity of this species to adapt to the dynamic environment of estuaries, its sedentary behavior, and benthic spawning all contribute to the interest in this fish as a model for the study of patterns of chromosomal diversification. The present study investigated the chromosomal features of this species through conventional (Giemsa, C-banding, and Ag-NOR) and molecular (FISH mapping of the 18S and 5S rRNA, and the telomeric sequences) cytogenetic approaches. This is the first description of the karyotype of this species, which has a diploid number (2n) of 46 chromosomes (6m+8sm + 20st+12a) and a fundamental number (FN) of 80. All the chromosomes presented C-banding in the pericentromeric region. The AgNOR/18S rRNA sites were observed exclusively on the short arms of pair 13, whereas multiple 5S rRNA sites were found, on pairs 7 (submetacentric) and 20 (acrocentric). The telomeric probes revealed interstitial telomeric sequences (ITSs) in metacentric pair 3, indicating the occurrence of chromosome fusion. The karyotype of B. surinamensis presents a number of derived karyotypic features, and is differentiated primarily by its pericentric inversions and the apparent occurrence of at least one chromosome fusion event. The collection of additional cytogenetic data on other Batrachoididae species, and other populations of B. surinamensis, will provide further insights into the role of estuarine environments in the chromosomal diversification of this fish group.