title: The balanced lethal system of crested newts: a ghost of sex chromosomes past?
abstract: 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 homologues (1A and 1B) have different recessive deleterious alleles fixed on a nonrecombining 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 (Y(A) and Y(B)) of the Y chromosome from an ancestral male-heterogametic system. We formalize a scenario in which 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.
authors.affiliation: Department of Ecology and Evolution, University of Lausanne, Switzerland. christine.grossen@uzh.ch
authors.forename: Christine
authors.initials: C
authors.lastname: Grossen
authors.forename: Samuel
authors.initials: S
authors.lastname: Neuenschwander
authors.forename: Nicolas
authors.initials: N
authors.lastname: Perrin
mesh_terms: D000483:Alleles; D000818:Animals; D019143:Evolution, Molecular; D005260:Female; D059647:Gene-Environment Interaction; D008297:Male; D008957:Models, Genetic; D012641:Selection, Genetic; D012730:Sex Chromosomes / Q000235:genetics*; D019849:Sex Determination Processes; D014319:Triturus / Q000235:genetics*