Trabajo presentado en el 14th Congress of the European Society for Evolutionary Biology (ESEB 2013), celebrado en Lisboa del 19 al 24 de agosto de 2013. 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. No