Heat inducible nuclear translocation of Kdm6bb drives temperature dependent sex reversal in Nile tilapia.

Understanding the primary molecular events driving temperature-dependent sex reversal (TSR) has proven challenging, particularly in distinguishing these from secondary effects of sexual differentiation. The mechanisms translating temperature into a sex-determining signal in fish are still largely unknown. Through combined transcriptomic and genome-wide histone methylation analyses of gonads in Nile tilapia (Oreochromis niloticus) exposed to normal and elevated temperatures, we observed significant upregulation of male-promoting genes (amh, dmrt1, gsdf) and suppression of female-promoting genes (wt1a and foxl3) at high temperature. These changes were correlated with methylation changes in H3K27 and H3K4 in the promoter regions of these genes. Among the histone methylation enzymes induced by high temperature, we identified the H3K27 demethylase Kdm6bb to be a key factor. Gene deletion and biochemical studies confirmed that Kdm6bb significantly impacts the H3K27 methylation level, that influences sex determination. Crucially, we discovered that the TSR function of Kdm6bb is mediated by the alternative inclusion of a previously unrecognized intron, enabling nuclear translocation of the demethylase to perform its function. Our findings refute the previously proposed "translation deficiency" mechanism of kdm6bb, and highlight the critical role of mRNA alternative splicing and subcellular localization of the demethylase in temperature-induced sex reversal.