Transposable elements have driven genome evolution and plasticity in many ways across a range of organisms. Different types of biotic and abiotic stresses can stimulate the expression or transposition of these mobile elements. Here, we cytogenetically analyzed natural fish populations of the same species living under different environmental conditions to test the influence and organization of transposable elements in their genome. Differential behavior was observed for the markers Rex 1, Rex 3, and Rex 6 in the chromosomes of individuals of the same species but coming from different environments (polluted and unpolluted). An increase in the number of Rex transposable elements in the chromosomes and their influence on the genome of populations living in a polluted environment indicates that they must be under constant adaptive evolution.
Ctenoluciidae (Characiformes), a family of freshwater fishes, comprises 2 genera, <i>Ctenolucius</i> and <i>Boulengerella</i>, with 7 recognized species. Up to now, only species of the genus <i>Boulengerella</i> have been subjected to cytogenetic studies. Here, we investigated the karyotype and other cytogenetic features of pike characin, <i>Ctenolucius hujeta</i>, using conventional (Giemsa staining, C-banding, Ag-NOR staining) and molecular (rDNA, telomeric sequences, and fiber-FISH mapping) procedures. This species has a diploid chromosome number of 2n = 36, and a karyotype composed of 12m + 20sm + 4a and FN = 68, similar to that found in <i>Boulengerella</i> species. However, differences regarding the number and distribution of several chromosomal markers support a distinct generic status. Colocalization of the 18S and 5S rDNA genes is an exclusive characteristic of the <i>C. hujeta</i> genome, with an interspersed distribution in the chromosomal fiber, an unusual phenomenon among eukaryotes. Additionally, our results support the view that Ctenoluciidae and Lebiasinidae families are closely related.
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