Final Report Summary - WATER WALKING (Developmental genetics and adaptive bases of a major ecological transition - How to walk on water!) The origin of novel phenotypic characters is a key component in organismal diversification, yet the mechanisms and selective forces underlying the emergence of such novelties are largely unknown. In Rhagovelia sp. (genus of water-walking insects) the evolution of a highly elaborate swimming fan on the tarsus of the mid-legs increases their propelling function. This novel adaptive trait allowed Rhagovelia to invade and diversify on running water surfaces; a niche not accessible for most other water-walking insects. We used transcriptomics combined with gene expression and function screens (RNAi) to find genes potentially associated with the evolution of this strikingly novel phenotype. We show that a Rhagovelia specific duplication of a hemiptera-restricted gene is required for fan development and is involved in the evolution of this trait. The knock-down of these duplicates (named geisha and mother of geisha) resulted in the absence or reduction of the fan, which in turn decreased the performance of these individuals in strong current environments. By studying the genetic and adaptive bases of this novel phenotype we showed that a lineage specific duplication of a taxonomically restricted gene is involved in the emergence of a spectacular morphological novelty and consequently the invasion of a novel habitat.