Deciphering sex-specific miRNAs as heat-recorders in zebrafish

MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression in a wide variety of physiological processes, including those related to the reproductive system. Although in the last decade a plethora of miRNAs has been reported, the miRNA alterations occurred by environmental cues and their biological functions have not yet been elucidated. With the aim to identify epigenetic regulations mediated by miRNAs in the gonads in a climate change scenario, zebrafish ( Danio rerio ) were subjected to high temperatures during sex differentiation (18-32 days post fertilization, dpf), a treatment that results in male-skewed sex ratios. Once the fish reached adulthood (90 dpf), ovaries and testes were sequenced by high-throughput technologies. About 101 million high-quality reads were obtained from gonadal samples. Analyses of the expression levels of the miRNAs identified a total of 23 and 1 differentially expressed (DE) miRNAs in ovaries and testes, respectively, two months after the heat treatment. Most of the identified miRNAs were involved in human sex-related cancer. After retrieving 3’ UTR regions, ~400 predicted targets of the 24 DE miRNAs were obtained, some with reproduction-related functions. Their synteny in the zebrafish genome was, for more than half of them, in the chromosomes 7, 2, 4, 3 and 11 in the ovaries, chromosome 4 being the place where the predicted sex-associated-region ( sar ) is localized in wild zebrafish. Further, spatial localization in the gonads of two selected miRNAs (miR-122-5p and miR-146-5p) showed exclusive expression in the ovarian germ cells. The present study expands the catalog of sex-specific miRNAs and deciphers, for the first time, thermosensitive miRNAs in the zebrafish gonads that might be used as potential epimarkers to predict environmental past events.