Inhibition of Aromatase Induces Partial Sex Change in a Cichlid Fish: Distinct Functions for Sex Steroids in Brains and Gonads

Sex steroids are major drivers of sexual development and also responsible for the maintenance of the established gender. Especially fishes exhibit great plasticity and less conservation in sex determination and sexual development compared to other vertebrate groups. In addition, fishes have a constant sex steroid production throughout their entire lifespan, which makes them particularly susceptible to interferences with the endogenous sex steroid system. This susceptibility has recently been used to show that inhibition of the key enzyme of estrogen synthesis, aromatase Cyp19a1, can induce functional sex reversal even in adult fish. Here, we investigated the impact of the aromatase inhibitor (AI) fadrozole in adult females of the East African cichlid fish Astatotilapia burtoni. Using gene expression, phenotypic measurements, behavioral experiments, and hormone measurements, we assessed if females treated with fadrozole develop a male-like phenotype. We found that AI treatment has a different effect on gene expression in the gonad compared to the brain, the 2 tissues mostly implicated in sexual development. In contrast to observations in other gonochoristic species, A. burtoni ovaries cannot be transformed into functional testis by AI. However, rapid changes towards a male-like phenotype can be induced with AI in coloration, hormone levels, and behavior

Genetics and timing of sex determination in the East African cichlid fish Astatotilapia burtoni

The factors determining sex are diverse in vertebrates and especially so in teleost fishes. Only a handful of master sex-determining genes have been identified, however great efforts have been undertaken to characterize the subsequent genetic network of sex differentiation in various organisms. East African cichlids offer an ideal model system to study the complexity of sexual development, since many different sex-determining mechanisms occur in closely related species of this fish family. Here, we investigated the sex-determining system and gene expression profiles during male development of Astatotilapia burtoni, a member of the rapidly radiating and exceptionally species-rich haplochromine lineage.; Crossing experiments with hormonally sex-reversed fish provided evidence for an XX-XY sex determination system in A. burtoni. Resultant all-male broods were used to assess gene expression patterns throughout development of a set of candidate genes, previously characterized in adult cichlids only.; We could identify the onset of gonad sexual differentiation at 11-12 dpf. The expression profiles identified wnt4B and wt1A as the earliest gonad markers in A. burtoni. Furthermore we identified late testis genes (cyp19a1A, gsdf, dmrt1 and gata4), and brain markers (ctnnb1A, ctnnb1B, dax1A, foxl2, foxl3, nanos1A, nanos1B, rspo1, sf-1, sox9A and sox9B)