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Rex retroelements are the best-known transposable elements class and are broadly distributed through fish and also individual genomes, playing an important role in their evolutionary dynamics. Several agents can stress these elements; among them, there are some parasitic compounds such as the organochlorophosphate Trichlorfon. Consequently, knowing that the organochlorophosphate Trichlorfon is indiscriminately used as an antiparasitic in aquaculture, the current study aimed to analyze the effects of this compound on the activation of the Transposable Elements (TEs) Rex1, Rex3, and Rex6 and the structure of heterochromatin in the mitotic chromosomes of the tambaqui (Colossoma macropomum). For this, two concentrations of the pesticide were used: 30% (0.261 mg/L) and 50% (0.435 mg/L) of the recommended LC50–96 h concentration (0.87 mg/L) for this fish species. The results revealed a dispersed distribution for Rex1 and Rex6 retroelements. Rex3 showed an increase in both marking intensity and distribution, as well as enhanced chromosomal heterochromatinization. This probably happened by the mediation of epigenetic adaptive mechanisms, causing the retroelement mobilization to be repressed. However, this behavior was most evident when Trichlorfon concentrations and exposure times were the greatest, reflecting the genetic flexibility necessary for this species to successfully adapt to environmental changes.

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The transposable elements are known by their ability to move and integrate into the genome of the host organism. They are classified in retrotransposons class I, which has, as intermediate of the transposition, the RNA and retrotransposons class II, which is composed of the DNA transposons, whose DNA migrates directly or is copied and inserted into the genome. The retrotransposable element Rex1 is a non-LTR retrotransposon found in several types of organisms. Many studies indicate that the Rex retrotransposons have the capacity to respond to environmental stress. The results found in this study corroborate the hypothesis that this retrotransposon possesses a response to environmental stress, since the chromosomal mapping, obtained through FISH, showed a higher number of markings on animals that were submitted to stress in 48h by the copper sulfate action. Furthermore, we can identify an increase in heterochromatic regions in the chromosomes. In the absolute quantification by Real-Time PCR, we found HIGHLIGHTS• Retrotransposable elements can respond to environmental stress.• Cooper sulfate can activate retrotransposon Rex1 within 72h of exposure.• Rex1 copy number is higher in exposed animals.• The heterochromatin profile was different in exposed animals.