Evolutionary process of sex chromosomes and sex determination systems in reptiles

Abstract

Sex-determining systems are highly divergent between different lineages of reptiles. Birds and snakes have genotypic sex determination (GSD), whereas all crocodilians and tuataras exhibit temperature-dependent sex determination (TSD). Sex-determining mechanisms in turtles and squamates include GSD with male heterogamety, GSD with female heterogamety, and TSD. Comparing different sex determination systems with molecular cytogenetics and comparative genomics thus provides unparalleled opportunities to capture the evolution of sex chromosomes and determination systems in action. The direct or indirect comparison of linkage homologies of sex chromosomes among various GSD species highlighted that the evolutionary process of sex determination systems in reptiles is highly complex and includes transitions between sex determination systems in closely related species and cryptic transitions (ZW system to new ZW system, XY system to new XY system). This approach also revealed the convergent evolution of sex chromosomes, in which ancestral autosomes suitable for the role of sex determination evolved into new sex chromosomes multiple times during reptile evolution. We discuss the mechanisms and rules underlying the evolution of the sex chromosomes and sex determination systems, such as the association of simple short repeat amplification with the heterochromatinization of sex-specific chromosomes and temperature-induced transition from GSD to TSD.