a REVIEWS Transcriptional Evolution Underlying Vertebrate Sexual Development

Background: Explaining the diversity of vertebrate sex-determining mechanisms ranging from geno-typic (GSD) to temperature-dependent (TSD) remains a developmental and evolutionary conundrum. Using a phylogenetic framework, we explore the transcriptional evolution during gonadogenesis of sev-eral genes involved in sexual development, combining novel data from Chrysemys picta turtles (TSD) and published data from other TSD and GSD vertebrates. Results: Our novel C. picta dataset under-scores Sf1 and Wt1 as potential activators of the thermosensitive period and uncovered the first evi-dence of Dax1 involvement in male development in a TSD vertebrate. Contrasting transcriptional profiles revealed male-biased Wt1 expression in fish while monomorphic expression is found in tetra-pods but absent in turtles. Sf1 expression appears highly labile with transitions among testicular, ovar-ian, and non-sex-specific gonadal formation patterns among and within lineages. Dax1’s dual role in ovarian and testicular formation is found in fish and mammals but is dosage-sensitive exclusively in eutherian mammals due to its X-linkage in this group. Contrastingly, Sox9 male-biased and Aromatase female-biased expression appear ancestral and virtually conserved throughout vertebrates despite sig-nificant heterochronic changes in expression as other elements likely replaced their function in early gonadogenesis. Conclusions: Finally, research avenues are highlighted to further study the evolution o