Absence of the figla-like gene is concordant with femaleness in cichlids harboring the LG1 sex-determination system
Signaling pathways involved in gonad development play essential roles in sex-determination (SD). Oreochromis niloticus has been used as a reference genome for studies of tilapia SD disclos-ing linkage-groups (LGs) 1, 3 and 23 but missing the master-key-regulator gene (MKR) that un-derlies LG1 SD-signaling. Using the O. aureus reference genome and data from genomic libraries of cichlids with LG1 SD-systems including Coptodon zillii and Sarotherodon galilaeus, we observed figla-like orthologs only in males. We further validated this observation in O. mossambicus and in the commercial Amherst and Chitralada hybrid stocks. In the previously-defined critical-region of LG1 SD, additional coding variations were undetected. Meta-analysis of the figla-like expression in gonads of the Amherst stock revealed that it was exclusively expressed in testes. Genome alignment of the 1252-bp transcript showed that the size of the figla-like gene was 2664 bp and its three exons were capable of encoding 99 aminoacids including a 45-aminoacid basic-helix-loop-helix domain that is found in factor-in-the-germline-alpha (FIGLA), which is known to regulate ovary development. Thus, figla-like absence is compatible with default-female developmental program, whereas, its presence determines male fate. Indicating that figla-like is the long-sought LG1 SD MKR, our findings provide tools for controlling sex in commercial ti-lapia production