Transposable Element Tissue-Specific Response to Temperature Stress in the Stenothermal Fish Puntius tetrazona

SIMPLE SUMMARY: Teleosts are one of the most diversified group of vertebrates, as they colonized different aquatic environments worldwide. Stenothermal fish are characterized by a limited ability to tolerate temperature changes, therefore variations in this abiotic factor represent a threat for their survival. In the present study, we selected the tiger barb Puntius tetrazona, for which RNA-Seq data were available for brain, gill, and liver, to investigate the possible role of the transcriptional activity of transposable elements (TEs) in the rapid adaptation of this species. Our findings highlighted a tissue-specific response of TEs with a remarkable increase at 13 °C recorded in liver, strengthening the view of TEs as source of genetic variability acting positively in species resilience. ABSTRACT: Ray-finned fish represent a very interesting group of vertebrates comprising a variety of organisms living in different aquatic environments worldwide. In the case of stenothermal fish, thermal fluctuations are poorly tolerated, thus ambient temperature represents a critical factor. In this paper, we considered the tiger barb Puntius tetrazona, a freshwater fish belonging to the family Cyprinidae, living at 21–28 °C. We analyzed the available RNA-Seq data obtained from specimens exposed at 27 °C and 13 °C to investigate the transcriptional activity of transposable elements (TEs) and genes encoding for proteins involved in their silencing in the brain, gill, and liver. TEs are one of the tools generating genetic variability that underlies biological evolution, useful for organisms to adapt to environmental changes. Our findings highlighted a different response of TEs in the three analyzed tissues. While in the brain and gill, no variation in TE transcriptional activity was observed, a remarkable increase at 13 °C was recorded in the liver. Moreover, the transcriptional analysis of genes encoding proteins involved in TE silencing such as heterochromatin formation, the NuRD complex, and the RISC complex (e.g., AGO and GW182 proteins) highlighted their activity in the hepatic tissue. Overall, our findings suggested that this tissue is a target organ for this kind of stress, since TE activation might regulate the expression of stress-induced genes, leading to a better response of the organism to temperature changes. Therefore, this view corroborates once again the idea of a potential role of TEs in organism rapid adaptation, hence representing a promising molecular tool for species resilience.