Exposing Selection and Genetic Linkage in the Evolutionary Enigmatic Balanced Lethal System in Triturus Newts

A balanced lethal system is a genetic disease that results in the loss of half of the reproductive output. The best-known balanced lethal system is found in newts of the genus Triturus. In these newts, two different versions of chromosome 1, named 1A and 1B, do not recombine along a particular region. Only individuals with both versions are viable, whereas those that possess the same version twice die. We cross two Triturus species to generate F1 and F2 hybrid offspring. This allows us to follow 30 species-specific single nucleotide polymorphisms (SNPs) from across the genome, including the non-recombining region involved in the balanced lethal system, over three generations. We confirm that individuals carrying the same chromosome version twice die, whereas those that possess both versions survive. Recombination is observed outside, but not within, the region associated with the balanced lethal system. Our experiment provides a clear-cut example of Mendelian inheritance with a deadly twist, well suited to teach basic principles of natural selection and genetic linkage.