siamese fighting fish predatorsOrnamental betta individuals have variable contributions from other Betta species and have also introgressed into wild populations of these species. We determine dmrt1 as the main sex decision gene in ornamental betta but not in wild B. splendens, and find proof for recent directional option at the X allele of the locus. Furthermore, we discover genes with signatures of contemporary, strong option which have large results on color in exact parts of the body, or the shape of individual fins, and are almost all unlinked. Our results exhibit how simple genetic architectures paired with anatomical modularity may end up in vast phenotypic diversity generated during animal domestication, and set the stage for using betta as a trendy system for evolutionary genetics. The Siamese combating fish Betta splendens, commonly called the betta, is a well-liked fish in the aquarium trade.