XY or ZW? Gecko chromosomes reveal evolution's hidden playbook

Geckos offer a unique opportunity for scientists to study the evolution of sex determination systems. Unlike many reptiles, geckos can exhibit temperature-dependent sex determination (TSD) as well as genotypic sex determination (GSD) like humans. By analyzing 22 gecko species, researchers traced the origins of sex chromosomes back to just 17 ancestral chromosomes.

This discovery suggests that sex determination systems in geckos have evolved independently multiple times from different ancestral chromosomes. The timing of the emergence of sex chromosomes within gecko species ranges from under 6 million years to over 54 million years, with a cluster of five species differentiating their sex chromosomes around 9.6 million years ago.

This clustering coincides with the Middle Miocene Climatic Transition, a period of global cooling and habitat fragmentation, indicating that climatic disruption may have driven gecko lineages towards genetically fixed GSD. The sex chromosomes themselves follow a pattern: ancestral chromosomes with a high number of testis-expressed genes are more likely to evolve into Z chromosomes, while those with fewer such genes become X chromosomes.

This pattern is consistent across 39 vertebrate species, suggesting that intense sexual selection on sperm may influence the outcome when a new sex determination system is born. Once a sex chromosome pair stops recombining, the Y or W chromosome begins to lose genes, leading to an imbalance that can be compensated by dosage compensation.

This four-stage model proposes that climatic disruption, ancestral gene content, recombination suppression, and gene loss with dosage compensation drive the birth and stabilization of new sex determination systems across vertebrates.