Sex determination candidate loci in two anuran species groups

SémIdeev
 25/09/2025
 12:00:00
 Matthias Stöck, Leibniz Institute of Freshwater Ecology and Inland Fisheries - IGB, Berlin, Germany
 IDEEV - Salle Rachel Carlson

Vertebrate sex is determined by repeatedly evolved environmental or genetic triggers. In amphibians, undifferentiated sex chromosomes and large genomes have caused major knowledge gaps. Only a single master sex-determining gene, dm-w in clawed frogs (Xenopus; ZW♀/ZZ♂), is known across over 8770 amphibian species. Combining chromosome-scale female and male genomes (3.8 Gbp) of the European green toad, Bufo(tes) viridis, with ddRAD- and pool-sequencing, we identified a candidate master locus, governing a male-heterogametic system (XX♀/XY♂). Targeted sequencing across taxa showed structural X/Y-variation in the bod1l 5′-regulatory region, where a Y-specific non-coding RNA (ncRNA-Y), only expressed in males, suggests that this locus initiates sex-specific differentiation. Developmental transcriptomes and RNA in-situ hybridization show sex-specific ncRNA-Y and bod1l expression in primordial gonads, coinciding with differential H3K4me-methylation in pre-granulosa/pre-Sertoli cells, suggesting a new mechanism of amphibian sex determination (Kuhl, Tan et al., https://doi.org/10.1038/s41467-024-49025-2)   .

Diminished coverage of single-end-sequenced ddRAD-markers in males of a family of fire-bellied toads (Bombina bombina) coincided with hemizygous long-read coverage in a male reference genome (9 Gbp, GCF_027579735.1). Therein, we discovered a Y-specific extra-copy of a steroid hormone receptor, present in XY-males of B. bombina, B. variegata, and B. orientalis. I will present work-in-progress on this model of amphibian hybridization and speciation.