Evolutionary Genomics of Unconventional Sex Chromosomes in Frogs

In sharp contrast to model organisms with degenerated Y/W chromosomes, many reptiles, fish and amphibians have mostly undifferentiated sex chromosomes and exhibit dynamics of birth and death of sex chromosomes. Why and how early-stage sex chromosomes and frequent turnovers are so prevalent across eukaryotes remain mysterious. This also raises pressing questions on alternative mechanisms leading to sex chromosome recombination arrest, their dynamics and stability. Frogs are ideal systems because they have mostly early-stage sex chromosomes and frequent turnovers. In this project, I will use the robber frogs to study unconventional sex chromosome evolution, challenging the paradigm predicted by the canonical model of ?degenerate Y/W?:1. Does genome-wide reduced recombination always associate with the heterogametic sex? I will study an alternative mechanism driving sex chromosome recombination arrest, sex-specific telomere-restricted recombination, in 12 robber frogs with XY, ZW systems in a phylogenetic framework. 2. Do sex-determination turnovers repeatedly occur in frogs with heteromorphic sex chromosomes? I will identify sex chromosomes for each species and analyse the rate of turnovers in XY and ZW systems in a phylogenetic framework. I anticipate several independent transitions between sex-determination mechanisms, and more transitions within the XY or ZW system. 3. What are the genomic consequences of giant Ys and Ws, how do they form and differ? I will perform chromosome-level genome assembly to identify gene content on the Ys/Ws. I will detect inversions, evolutionary strata, compare transposable elements dynamics and dosage compensation mechanisms between XY and ZW systems.If successful, this project will uncover alternative mechanisms driving sex chromosome recombination arrest, their dynamics and genomic features. It will reveal the mysterious formation of giant Ys/Ws. Together, this will conceptually extend boundaries of the canonical model.