**3. Conclusion**

*Cytogenetics - Classical and Molecular Strategies for Analysing Heredity Material*

and several chromosomes appeared stained.

element mobilization.

Examining the relationship between retroelements and heterochromatin polymorphism, Silva et al. [36], found a significant increase in heterochromatin in *Colossoma macropomum* exposed to cupric sulfate at 30% of the species LC50, compared to the group control. Changes in heterochromatin levels were evident after 48 h of stress, and after 72 h, several chromosomes appeared marked with heterochromatin. In addition, there were significantly more copies of the Rex1 retroelement in individuals exposed to CuSO4 for 72 h, when compared to the control. This element was mapped by FISH in animals exposed for 48 h to CuSO4,

A study of the parental species and hybrid offspring (commonly known as *tambacu*) of crosses between female tambaqui (*Colossoma macropomum*) and male pacu (*Piaractus mesopotamicus*), Ribeiro et al. [38] found variation in the heterochromatin pattern of hybrids as well as conspicuous patches of Rex3 and Rex6 retrotransposable elements. This result was interpreted as arising from the need for adjustments in cell division and introgressive hybridization, since the joining of two different genomes frequently leads to changes in the control of gene expression, DNA methylation, chromosomal rearrangements, and, consequently, transposable

Ferreira et al. [39] exposed *Colossoma macropomum* to three climate change scenarios of the Intergovernmental Panel on Climate Change (IPCC). They reported chromosomal heterochromatinization in individuals exposed to the A2 climate scenario, in which terminal and interstitial bands of bands were observed in several chromosomes. These correlated with the presence of Rex3 retrotransposon elements. In another study involving tambaqui, Costa et al. [40] detected different patterns in heterochromatin distribution associated with increased Rex3 expression, following exposure to the parasiticide Trichlorfon. The authors found that,

*The experimental data obtained in* Colossoma macropomum *indicate that in all conditions of chemical stress, change in environmental temperature or shock between interspecific genomes, there was an increase in the amount of heterochromatin in the pericentromeric portion of all chromosomes of the complement or in the telomere portion, followed by an increase in the number of copies of the transposable elements. From left to right: Heterochromatinization and amplification of Rex1 in exposure to CuSO4 (yellow arrow); heterochromatinization and amplification of Rex3 in contact with antiparasitic Triclorfon and climate change provided for in the IPCC (green arrow); finally, amplification of Rex3 (green arrow) and Rex6 (red arrow) accompanied by heterochromatinization in a situation of interspecific artificial hybridization (more details* 

*see: [36–39]). Yellow arrow = Rex1, green arrow = Rex3 and red arrow = Rex6.*

**38**

**Figure 3.**

In conclusion, the species of Amazonian fish studied for heterochromatin assembly and retroelement dispersion (especially Rex 3) seem to respond dynamically and with remarkable similarity to a range of stressing stimuli.
