Sex-specific functional evolution of Dmrt1 in African clawed frogs (Xenopus), and the importance of genetic tipping points in developmental biology

The doublesex and mab-3 related transcription factor 1 (dmrt1) plays a crucial role in metazoan sexual differentiation. This gene, or its paralogs, independently became triggers for sex determination several times, including in the tetraploid African clawed frog Xenopus laevis. To explore functional evolution of this gene, we generated knockout lines of each of two dmrt1 homeologs in X. laevis and an ortholog in the closely related diploid Western clawed frog X. tropicalis. Our findings evidence sex-specific functional evolution following duplication by allotetraploidization in an ancestor of X. laevis. In females, dmrt1 was essential for fertility and oogenesis in the Xenopus ancestor, but this important function was lost (subfunctionalized) in one X. laevis homeolog (dmrt1.S) after allotetraploidization. In males - in sharp contrast - dmrt1 was not essential for fertility and spermatogenesis in the Xenopus ancestor, but this essentiality was acquired (neofunctionalized) in the other X. laevis homeolog (dmrt1.L) after allotetraploidization. Transcriptomic analysis of the mesonephros/gonad complex during sexual differentiation identifies distinctive patterns of dysregulation in male and female knockouts of dmrt1.L and dmrt1.S relative to same-sex wildtype siblings, including possible autocatalysis of dmrt1.L and activation of the female-determining gene dm-w. Previous work demonstrates that dm-w was recently derived from partial gene duplication of dmrt1.S - a gene that our analysis demonstrates is non-essential in both sexes. Thus, in X. laevis, a developmental system was pushed past a "tipping point" to a novel state where sexual differentiation is now orchestrated by a sex-specific duplicate of a dispensable gene.