Significance Although turnover of sex chromosomes is very common in many vertebrate lineages, the transition process is still elusive. We studied the sex-determining region (SDR) of 12 congeneric fish species of Takifugu fish that compose an important model for the study of genomics and sex determination. We found that while nine species retained their ancestral SDR, three species had acquired derived SDRs. Although the derived SDRs resided in three different chromosomes, they harbored a shared supergene flanked by two putative transposable elements. The results highlight the underestimated role of a mobile supergene in turnover of sex chromosomes in vertebrates.

Recent studies have revealed a surprising diversity of sex chromosomes in vertebrates. However, the detailed mechanism of their turnover is still elusive. To understand this process, it is necessary to compare closely related species in terms of sex-determining genes and the chromosomes harboring them. Here, we explored the genus Takifugu, in which one strong candidate sex-determining gene, Amhr2, has been identified. To trace the processes involved in transitions in the sex determination system in this genus, we studied 12 species and found that while the Amhr2 locus likely determines sex in the majority of Takifugu species, three species have acquired sex-determining loci at different chromosomal locations. Nevertheless, the generation of genome assemblies for the three species revealed that they share a portion of the male-specific supergene that contains a candidate sex-determining gene, GsdfY, along with genes that potentially play a role in male fitness. The shared supergene span approximately 100 kb and are flanked by two duplicated regions characterized by CACTA transposable elements. These results suggest that the shared supergene has taken over the role of sex-determining locus from Amhr2 in lineages leading to the three species, and repeated translocations of the supergene underlie the turnover of sex chromosomes in these lineages. These findings highlight the underestimated role of a mobile supergene in the turnover of sex chromosomes in vertebrates.SignificanceAlthough turnover of sex chromosomes is very common in many vertebrate lineages, the transition process is still elusive. We studied the sex-determining region (SDR) of 12 congeneric fish species. We found that while nine species retained their ancestral SDR, three species had acquired derived SDRs. Although the derived SDRs resided in three different chromosomes, they harbored a shared supergene flanked by two putative transposable elements. The results highlight the underestimated role of a mobile supergene in turnover of sex chromosomes in vertebrates.

Heavy metal contamination of fish is a global safety concern for the consumers. Atomic absorption spectrometry (AAS) was used to measure the concentration of three HMs (As, Pb, and Cr) in two commercially farmed fish species that are widely consumed: Oreochromis niloticus (tilapia) and Pangasianodon hypophthalmus (pangasius). This investigation was done to determine whether there was any potential health risk. The concentration of the studied heavy metals was within standard acceptable limits. As, Cr, and Pb had respective mean concentrations of 0.037 ± 0.023 mg/kg, 0.005 ± 0.002 mg/kg and 0.029 ± 0.015 mg/kg in the case of tilapia; and 0.049 ± 0.017 mg/kg, 0.007 ± 0.005 mg/kg and 0.024 ± 0.009 mg/kg, respectively, in the case of pangasius. Hierarchical clustering revealed that As may come through seepage and groundwater, while Cr and Pb are the products of contaminated feeds. ALP and ATPase activity varied significantly amongst the fishes' various organs. However, multiple regression analysis revealed that the current concentration of HMs in tilapia and pangasius was insufficient to predict enzymatic activity except for ALP in pangasius gill. The results of calculations for estimated daily intake (EDI), target hazard quotient (THQ), hazard index (HI), and carcinogenic risk (CR) indices made it abundantly evident that eating the fish under study posed no significant harm to the consumers' health. In conclusion, the contamination level of farmed tilapia and pangasius sold in the study area was within the permissible limit, however regular monitoring is needed to assure safe production.

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