Distribution of estrogen sulfotransferase genes in tunas and their relatives

Estrogen sulfotransferase is an enzyme involved in the inactivation of estrogen, and the encoding gene, sult1st6, is widely conserved among animal species. Recently, it was reported that a paralog of the estrogen sulfotransferase gene is paternally inherited in Pacific bluefin tuna (Thunnus orientalis), suggesting that this gene, sult1st6y, may be involved in sex determination or differentiation of Thunnus species as a suppressor of feminization. However, current knowledge on fish sult1st6 is still limited. This study aims to update the previous findings by thoroughly examining the distribution of sult1st6 in tuna (the species of tribe Thunnini) and their relatives (the species of family Scombridae).

Based on a survey of the sult1st6 loci in the available chromosome-level genomes of Scombridae species, it was found that tuna species carry two copies of sult1st6. Based on phylogenetic analysis and genome comparison, these paralogs were identified as sult1st6y and its paralog, namely sult1st6a, respectively, suggesting that sult1st6y may have originated from sult1st6a by duplication prior to the divergence of the Thunnini tribe. In addition, the sult1st6y locus may have undergone more structural changes than has the sult1st6a locus. These results provide insight into the evolutionary scenario of the sex determination system of tunas.