Orphan and <i>de novo</i> Genes in Fungi and Animals: Identification, Origins and Functions

Genes that lack identifiable homologs in other species have been an intriguing and interesting topic of research for many years. These so-called orphan genes were first studied in yeast and since then, they have been found in many other species. This has fostered a whole field of research aiming at tracing back their evolutionary origin and functional significance. Orphan genes represent an important part of protein-coding genes in many species. Their presence was initially mainly hypothesized to result from high divergence from a pre-existing gene, with duplications or horizontal gene transfer facilitating their accelerated evolution. More recently, their possible de novo emergence from nongenic regions has gained particular interest. Several orphan genes are predicted to be involved in reproduction, while others are involved in specific developmental stages, in adaptation mechanisms such as freeze protection or even human disease. However, there is currently no unified resource or synthesis that brings together existing knowledge about how prevalent orphan genes are across different species and what their roles might be. In this review, we focus on orphan genes in animals and fungi. We provide a detailed summary of discoveries over time in terms of orphan gene prevalence in genomes, their origins as well as their roles in different biological contexts.