Abstracts (first author)
Fitness effects of a transposable element insertion depend on environmental conditions in Drosophila melanogaster
Understanding how adaptation works is one of the major questions in Evolutionary Biology. Towards answering this question, we identified recent Transposable Element (TE)-induced adaptations in Drosophila melanogaster and we are currently connecting them to their molecular mechanisms and fitness effects. Specifically, the TE insertion named Bari-Jheh, is present at high frequency, but not fixed, in all the populations analyzed and shows signatures of a selective sweep. Bari-Jheh is inserted in the intergenic region between Jheh2 and Jheh3 genes encoding for Juvenile Hormone Epoxy Hydrolase. Previous results show that Bari-Jheh is associated with a down-regulation of the expression of its nearby Jheh2 and Jheh3 genes. Furthermore, Bari-Jheh is also associated with reduced viability and extended developmental time that probably represent the cost of selection of this insertion. In order to identify the adaptive effect of this insertion, we analyzed the sequence of Bari-Jheh and we found that it contains two Antioxidant Response Elements. This suggests that the presence of the insertion could lead to an increased level of expression of Jheh genes under Oxidative Stress (OS) conditions and to increase resistance to this stress. We indeed found that under OS conditions the TE is associated with an increased expression of its nearby genes Jheh1 and Jheh2 (t-test p< 0.05) and that flies with the insertion show a higher survival rate compared to flies without the insertion (Kaplan-Meier logrank test: p<< 0.001). Our results show that the fitness effects of TE insertions could depend on the environmental conditions. This would explain why adaptive TEs such as Bari-Jheh are present at high frequencies but not fixed in any population. Moreover, this study shows that we need to explore several phenotypes in order to fully characterize the effects of an adaptive mutation.
Adaptive TE insertions in Drosophila: NATs, miRNAs, and piRNAs
Recent Transposable Element (TE) insertions in Drosophila melanogaster are a useful tool to identify adaptive mutations. The objective of this study is moving from the identification of a putatively adaptive TE insertion to the molecular mechanisms and associated fitness effects. We have focused on a TE insertion located in the 3'UTR region of Kmn1 gene. The 3´UTR of Kmn1 overlaps with the 3´UTR of its nearby gene, CG11699, giving rise to a cis-natural antisense pair of transcripts (cis-NATs). We found that besides being incorporated into the 3’UTR of Kmn1, the TE insertion also affects the transcript length of CG11699. Specifically, the TE disrupts the GU-rich downstream element of the distal polyA signal of CG11699 and as a consequence a shorter transcript is produced. These structural changes are very likely to have functional consequences since the presence of the TE adds (Kmn1) and eliminates (CG11699) miRNA binding sites, introduces piRNA binding sites (Kmn1) and affects the length of the overlapping region between these two genes. Indeed, we detected that this TE insertion is associated with an under-expression of Kmn1 and an over-expression of CG11699. In order to identify the phenotypic effect of this insertion, we first focused on CG11699 since this gene is involved in the activation of Aldh-III, an enzyme involved in xenobiotic metabolism. We found that the TE insertion is associated with an increased survival rate after an acute exposure to benzaldehyde, a prototypical drug classically used to assess Aldh-III activity, strongly suggesting that this TE confers resistance to xenobiotic stress. Our results show that a single mutational event has broad molecular consequences that can be translated into ecologically relevant phenotypic effects. This example reinforces the idea that TEs are a powerful natural tool for genome evolution.