ddRADseq reveals determinants for temperature-dependent sex reversal in Nile tilapia on LG23

In Nile tilapia sex determination is governed by a male heterogametic system XX/XY either on LG1 or LG23. The latter carries a Y-specific duplicate of the amh gene, which is a testis-determining factor. Allelic variants in the amh gene demonstrated to be major triggers for autosomal and temperature-dependent sex reversal. Further, QTL on LG23 and LG20 show a temperature-responsiveness with influence on the phenotypic sex relative to the sex chromosomes. Here we present a ddRADseq based approach to identify genomic regions that show unusual large differentiation in terms of fixation index (F_ST) between temperature-treated pseudomales and non-masculinized females using a comparative genome-scan. Genome-wide associations were identified for the temperature-dependent sex using a genetically all-female population devoid of amh-ΔY.

Twenty-two thousand three hundred ninety-two SNPs were interrogated for the comparison of temperature-treated pseudomales and females, which revealed the largest differentiation on LG23. Outlier F_ST-values (0.35-0.44) were determined for six SNPs in the genomic interval (9,190,077-11,065,693) harbouring the amh gene (9,602,693-9,605,808), exceeding the genome-wide low F_ST of 0.013. Association analysis with a set of 9104 selected SNPs confirmed that the same genomic region on LG23 exerts a significant effect on the temperature-dependent sex.

This study highlights the role of LG23 in sex determination, harbouring major determinants for temperature-dependent sex reversal in Nile tilapia. Furthermore F_ST outlier detection proves a powerful tool for detection of sex-determining regions in fish genomes.