**Abstract**

Transcriptionally inactive portions of genomic DNA, condensed with histones and architectural proteins, are known as heterochromatic regions, often positive C band. The advent of epigenetics and new methodological approaches, showed that these regions are extremely dynamic and responsive to different types of environmental stress. The relationship of the constitutive heterochromatin with the transposable elements inactivation, especially from the Rex family, seems to be a frequent condition in fish. In this manuscript we review the existing knowledge of the nature and function of these genomic regions, based on species-based studies, with a focus on species of fish from the Amazon region.

**Keywords:** environmental stressors, transposable elements, adaptive response, *Colossoma*, *Hypancistrus*

#### **1. Introduction**

The genomic DNA of eukaryotic organisms combines with histone proteins to form chromatin. Chromatin is classified into two forms: euchromatin (de-condensed region, rich in genes and transcriptionally active) and heterochromatin (condensed, transcriptionally silent) [1–3]. This early classification was based on differing dyeresponses and condensation profiles [4]. Heterochromatin, in turn, can be classified into constitutive heterochromatin and facultative heterochromatin, the former being preferably assembled in regions that house repetitive elements, such as satellite DNA and transposable elements [2, 5, 6]. The latter is preferentially assembled in genes related to the regulation of organismal development. The idea that the material is strongly related to the heterochromatinization of one of the X sex chromosomes in female mammals is known as the Lyon hypothesis [7].

Recent studies have shown that both constitutive and facultative heterochromatin are regulated dynamically and are responsive to various stressful stimuli. It is also known that while these changes in chromatin structure can potentially help organisms adapt to new environments, they can also produce aberrant phenotypes [8, 9] and diseases in humans [10, 11]. They are also related to the aging process [12]. In this review, we will discuss the dynamics of heterochromatin localization, obtained from different studies in fish from the Amazon region (**Figure 1**).

#### **Figure 1.**

*General distribution of chromatin in eukaryotos. This schematization compiles information from several studies. Here, euchromatin is represented as the portion of chromatin with transcriptionally active genes. Meanwhile, heterochromatin can be divided into two groups depending on constitution and activity. The constitutive herochromatin is composed of repetitive sequences and transposable elements, distributed in pericentromeric and terminal region of chromosomes that occasionally participate in phenotypic plasticity, through the regulation of chromatin and dispersion of transposable elements, uneven crossing-over or rearrangements. On the other hand, the portion of facultative heterochromatin, regulated during ontogenetic development, is related to the response of environmental stress or laboratory simulated, allowing plasticity and environmental adaptation.*
