Balanced lethal systems are more than biological curiosities: As theory predicts, they should quickly be eliminated through the joint forces of recombination and selection. That such systems might become fixed in natural populations poses a challenge to evolutionary theory. Here we address the case of a balanced lethal system fixed in crested newts and related species, which makes 50% of offspring die early in development. All adults are heteromorphic for chromosome pair 1. The two homologs (1A and 1B) have different recessive deleterious alleles fixed on a non-recombining segment, so that heterozygotes are viable, while homozygotes are lethal. Given such a strong segregation load, how could autosomes stop recombining? We propose a role for a sex-chromosome turnover from pair 1 (putative ancestral sex chromosome) to pair 4 (currently active sex chromosome). Accordingly, 1A and 1B represent two variants (YA and YB) of the Y chromosome from an ancestral male-heterogametic system. We formalize a scenario where turnovers are driven by sex-ratio selection stemming from gene-environment interactions on sex determination. Individual-based simulations show that a balanced-lethal system can be fixed with significant likelihood, provided the masculinizing allele on chromosome 4 appears after the elimination of the feminizing allele on chromosome 1. Our study illustrates how strikingly maladaptive traits might evolve through natural selection.

InputFilesInput files for simulations run. See "ReadMe.txt" for more information