Genomic Organization Under Different Environmental Conditions:Hoplosternum Littoraleas a Model

Abstract

The Amazon has abundant rivers, streams, and floodplains in both polluted and nonpolluted environments, which show great adaptability. Thus, the goal of this study was to map repetitive DNA sequences in both mitotic chromosomes and erythrocyte micronuclei of tamoatás from polluted and nonpolluted environments and to assess the possible genotoxic effects of these environments. Individuals were collected in Manaus, Amazonas (AM), and submitted to classical and molecular cytogenetic techniques, as well as to a blood micronucleus test. Diploid number equal to 60 chromosomes are present in all individuals, with 18S ribosomal DNA sites present in one chromosome pair and no interstitial telomeric sites on chromosomes. The micronucleus test showed no significant differences in pairwise comparisons between environments or collection sites, but the Rex3 retroelement was dispersed on the chromosomes of individuals from unpolluted environments and compartmentalized in individuals from polluted environments. Divergent numbers of 5S rDNA sites are present in individuals from unpolluted and polluted environments. The mapping of repetitive sequences revealed that micronuclei have different compositions both intra-and interindividually that suggests different regions are lost in the formation of micronuclei, and no single fragile region undergoes breaks, although repetitive DNA elements are involved in this process.

Transposable DNA Elements in Amazonian Fish: From Genome Enlargement to Genetic Adaptation to Stressful Environments

“…Thus, TE insertion contributed to a selective advantage for the species. Another example was the differential genomic instability, mediated by the accumulation/increase of the Rex 3 retroelement, in a species of fish (Hoplosternum littorale) that survives in polluted environments [Silva et al, 2016]. This study showed distinct patterns of classical and molecular cytogenetic markers interspersed with the retroelement, indicating that such a sequence might play a functional role in processes associated with the adaptation of this species to polluted environments [Silva et al, 2016], conclusions corroborated by the current study.…”

“…Another example was the differential genomic instability, mediated by the accumulation/increase of the Rex 3 retroelement, in a species of fish (Hoplosternum littorale) that survives in polluted environments [Silva et al, 2016]. This study showed distinct patterns of classical and molecular cytogenetic markers interspersed with the retroelement, indicating that such a sequence might play a functional role in processes associated with the adaptation of this species to polluted environments [Silva et al, 2016], conclusions corroborated by the current study. Ferreira et al [2019], in a study with Colossoma macropomum (tambaqui) individuals exposed to IPCC scenarios B1 (mild), A1B (intermediate), and A2 (extreme) for 45 days found chromosome heterochromatinization and an increase of Rex 1 and Rex 3 in individuals exposed to the A2 scenario.…”

“…However, to what extent can they be involved in the origin of adaptive characteristics? A number of studies have claimed that, after invasion events, adaptation occurs in response to selective pressures in the new environment [Lee, 2002;Brawand et al, 2014] and that such organisms may respond with changes within their biological characteristics [Frederico et al, 2016] mainly as a result of genomic plasticity [Silva et al, 2016;Heinrichs-Caldas et al, 2019].…”

“…They are likely to play a role in adaptation due to their ability to generate mutations as well as responding and being susceptible to envi-ronmental changes [Hua- Van et al, 2011;Casacuberta and Gonzalez, 2013;Stapley et al, 2015]. As a result, different types of biotic and abiotic stressors can stimulate TE expression or transposition in a wide range of organisms [Silva et al, 2016;Suh et al, 2018].…”

“…A study of 3 populations of Hoplias malabaricus from different aquatic environments of the Amazon basin (whitewater rivers, clearwater rivers, and blackwater rivers) found distinct, environmentally specific patterns for the Rex 3 retrotransposon, indicating the possible existence of local adaptive processes for populations living under different environmental conditions [Santos et al, 2016]. For Hoplosternum littorale , chromosomal mapping of the Rex 3 retrotransposon revealed distinct patterns of element distribution on chromosomes from individuals from polluted and unpolluted environments [Silva et al, 2016].…”