The balanced-lethal system of crested newts: a ghost of sex chromosomes past?

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 forma...