Coprecipitation mechanisms of Zn by birnessite formation and its mineralogy under neutral pH conditions.

Birnessite (δ-Mn(IV)O2) is a great manganese (Mn) adsorbent for dissolved divalent metals. In this study, we investigated the coprecipitation mechanism of δ-MnO2 in the presence of Zn(II) and an oxidizing agent (sodium hypochlorite) under two neutral pH values (6.0 and 7.5). The mineralogical characteristics and Zn-Mn mixed products were compared with simple surface complexation by adsorption modeling and structural analysis. Batch coprecipitation experiments at different Zn/Mn molar ratios showed a Langmuir-type isotherm at pH 6.0, which was similar to the result of adsorption experiments at pH 6.0 and 7.5. X-ray diffraction and X-ray absorption fine structure analysis revealed triple-corner-sharing inner-sphere complexation on the vacant sites was the dominant Zn sorption mechanism on δ-MnO2 under these experimental conditions. A coprecipitation experiment at pH 6.0 produced some hetaerolite (ZnMn(III)2O4) and manganite (γ-Mn(III)OOH), but only at low Zn/Mn molar ratios (< 1). These secondary precipitates disappeared because of crystal dissolution at higher Zn/Mn molar ratios because they were thermodynamically unstable. Woodruffite (ZnMn(IV)3O7•2H2O) was produced in the coprecipitation experiment at pH 7.5 with a high Zn/Mn molar ratio of 5. This resulted in a Brunauer-Emmett-Teller (BET)-type sorption isotherm, in which formation was explained by transformation of the crystalline structure of δ-MnO2 to a tunnel structure. Our experiments demonstrate that abiotic coprecipitation reactions can induce Zn-Mn compound formation on the δ-MnO2 surface, and that the pH is an important controlling factor for the crystalline structures and thermodynamic stabilities.

Repeated translocation of a supergene underlying rapid sex chromosome turnover in Takifugu pufferfish.

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 spans ∼100 kb and is 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.

Identification of the sex-determining locus in grass puffer (Takifugu niphobles) provides evidence for sex-chromosome turnover in a subset of Takifugu species.

There is increasing evidence for frequent turnover in sex chromosomes in vertebrates. Yet experimental systems suitable for tracing the detailed process of turnover are rare. In theory, homologous turnover is possible if the new sex-determining locus is established on the existing sex-chromosome. However, there is no empirical evidence for such an event. The genus Takifugu includes fugu (Takifugu rubripes) and its two closely-related species whose sex is most likely determined by a SNP at the Amhr2 locus. In these species, males are heterozygous, with G and C alleles at the SNP site, while females are homozygous for the C allele. To determine if a shift in the sex-determining locus occurred in another member of this genus, we used genetic mapping to characterize the sex-chromosome systems of Takifugu niphobles. We found that the G allele of Amhr2 is absent in T. niphobles. Nevertheless, our initial mapping suggests a linkage between the phenotypic sex and the chromosome 19, which harbors the Amhr2 locus. Subsequent high-resolution analysis using a sex-reversed fish demonstrated that the sex-determining locus maps to the proximal end of chromosome 19, far from the Amhr2 locus. Thus, it is likely that homologous turnover involving these species has occurred. The data also showed that there is a male-specific reduction of recombination around the sex-determining locus. Nevertheless, no evidence for sex-chromosome differentiation was detected: the reduced recombination depended on phenotypic sex rather than genotypic sex; no X- or Y-specific maker was obtained; the YY individual was viable. Furthermore, fine-scale mapping narrowed down the new sex-determining locus to the interval corresponding to approximately 300-kb of sequence in the fugu genome. Thus, T. niphobles is determined to have a young and small sex-determining region that is suitable for studying an early phase of sex-chromosome evolution and the mechanisms underlying turnover of sex chromosome.