A Y-linked duplication of anti-Mullerian hormone is the sex determination gene in threespine stickleback
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Many taxa have independently evolved genetic sex determination where a single gene located on a sex chromosome controls gonadal differentiation. The gene anti-Mullerian hormone ( amh ) has convergently evolved as a sex determination gene in numerous vertebrate species, but how this gene has repeatedly evolved this novel function is not well understood. In the threespine stickleback ( Gasterosteus aculeatus ), amh was duplicated onto the Y chromosome ( amhy ) ∼22 million years ago. To determine whether amhy is the primary sex determination gene, we used CRISPR/Cas9 and transgenesis to show that amhy is necessary and sufficient for male sex determination, consistent with the function of a primary sex determination gene. While we find substantial regulatory evolution has occurred in amhy , we did not observe an increase in amhy expression across early development relative to its autosomal paralog, amha or a significant difference in total amh dosage between males and females around the time of sex determination. This indicates the mechanism of sex determination may involve subtle changes in cell-specific expression or coding sequence evolution. The creation of sex reversed lines also allowed us to investigate the genetic basis of secondary sex characteristics. Threespine stickleback have striking differences in behavior and morphology between sexes. Here we show one of the classic traits important for reproductive success, blue male nuptial coloration, is controlled by both Y-linked genetic factors as well as hormonal factors independent of sex chromosome genotype. This research establishes stickleback as a model to investigate how amh regulates gonadal development and how this gene repeatedly evolves novel function in sex determination. Analogous to the “four core genotypes” model in house mice, sex-reversed threespine stickleback offer a new vertebrate model for investigating the separate contributions of gonadal sex and sex chromosomes to sexual dimorphism.
