Incompatibilities between emerging species

Natural hybridization in swordtail fish uncovers cancer genes involved in speciation One defining characteristic of species is reproductive incompatibility; hybrids between two species either do not form or have low fitness. The general explanation is the development of genetic incompatibilities that reduce fitness in hybrids. Such incompatibilities could occur if there is a deleterious interaction between two genetic variants that have previously not occurred in the same genetic background, commonly called Dobzhansky-Muller incompatibilities (1, 2). Identifying the genes underlying these incompatibilities is challenging; the more reproductively isolated two species are, the more difficult it is to cross them and map the incompatibility. As a result, very few such interactions have been identified (3, 4). On page 731 of this issue, Powell et al. (5) identify the genes underlying hybrid incompatibilities using a natural experiment, ongoing hybridization between two species of swordtail fish. What they find is surprising: The same cancer gene that causes speciation in a different set of fish is at play here as well.