Abstract
Fish species have a variety of sex determination systems. Tunas (genus Thunnus) have an XY genetic sex determination system. However, the Y chromosome or responsible locus has not yet been identified in males. In a previous study, a female genome of Pacific bluefin tuna (T. orientalis) was sequenced, and candidates for sex-associated DNA polymorphisms were identified by a genome-wide association study using resequencing data. In the present study, we sequenced a male genome of Pacific bluefin tuna by long-read and linked-read sequencing technologies and explored male-specific loci through a comparison with the female genome. As a result, we found a unique region carrying the male-specific haplotype, where a homolog of estrogen sulfotransferase gene was predicted to be encoded. The genome-wide mapping of previously resequenced data indicated that, among the functionally annotated genes, only this gene, named sult1st6y, was paternally inherited in the males of Pacific bluefin tuna. We reviewed the RNA-seq data of southern bluefin tuna (T. maccoyii) in the public database and found that sult1st6y of southern bluefin tuna was expressed in all male testes, but absent or suppressed in the female ovary. Since estrogen sulfotransferase is responsible for the inactivation of estrogens, it is reasonable to assume that the expression of sult1st6y in gonad cells may inhibit female development, thereby inducing the individuals to become males. Thus, our results raise a promising hypothesis that sult1st6y is the sex determination gene in Thunnus fishes or at least functions at a crucial point in the sex-differentiation cascade.
Highlights
Fish species have a substantial variety of sex determination systems compared to other animals, ranging from environmentally to genetically modulated ones
We found the counterparts of M44 and M175 in the southern bluefin tuna (SBT) male genome, respectively, which were located on different chromosomes, namely, chromosome 4
We have found a paralog of sult1st6y, namely, sult1st6a, in a locus unassociated to PBT’s sex in both of males and of females, and hypothesize that this gene may be involved in common estrogen metabolism
Summary
Fish species have a substantial variety of sex determination systems compared to other animals, ranging from environmentally to genetically modulated ones. Sex determination genes have been reported in several fish species, including DMY/Dmrt in medaka [4, 5] and Chinese tongue sole [6], Amhr in fugu [7], sdY in rainbow trout [8], Hsd17b1 in yellowtail [9], and amh in Patagonian pejerrey [10] and northern pike [11] These studies suggest that a master gene for sex determination in fish can be replaced by another gene, which may be followed by the above-mentioned turnover of the sex chromosome. The sex determination genes of fish are often different for each lineage and are not always identified by deductive approaches from model organisms (that is, case studies are required to identify the genes)
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