Heterogametic sex chromosomes have evolved independently in various lineages of vertebrates. Such sex chromosome pairs often contain nonrecombining regions, with one of the chromosomes harboring a master sex-determining (SD) gene. It is hypothesized that these sex chromosomes evolved from a pair of autosomes that diverged after acquiring the SD gene. By linkage and association mapping of the SD locus in fugu (Takifugu rubripes), we show that a SNP (C/G) in the anti-Müllerian hormone receptor type II (Amhr2) gene is the only polymorphism associated with phenotypic sex. This SNP changes an amino acid (His/Asp384) in the kinase domain. While females are homozygous (His/His384), males are heterozygous. Sex in fugu is most likely determined by a combination of the two alleles of Amhr2. Consistent with this model, the medaka hotei mutant carrying a substitution in the kinase domain of Amhr2 causes a female phenotype. The association of the Amhr2 SNP with phenotypic sex is conserved in two other species of Takifugu but not in Tetraodon. The fugu SD locus shows no sign of recombination suppression between X and Y chromosomes. Thus, fugu sex chromosomes represent an unusual example of proto–sex chromosomes. Such undifferentiated X-Y chromosomes may be more common in vertebrates than previously thought.

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In a recent issue of Molecular Cell (15, 279-285), there was an error in the author affiliations. The corrected version appears below.

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To analyze the mechanism of formation of tubular myelin (TM), we reconstituted TM from synthetic lipids and two surfactant-associated proteins (SP15 and SP35). SP15 was extracted from lyophilized pig pulmonary surfactant with 5% Triton X-100 and purified by DEAE-cellulose, CM-cellulose, and affinity chromatography with a specific antibody. SP35 was extracted from the precipitate of the 5% Triton X-100 extraction with pH 10 borate buffer and purified by DEAE-cellulose column chromatography. Lipid-SP15 complex was formed by a detergent dialysis method using octylglucopyranoside, and to this complex were added various concentrations of SP35 at 37 degrees C. Structures similar to TM were formed when lipid-SP15 complex containing dipalmitoylphosphatidylcholine:phosphatidylglycerol from egg lecithin (2:1) and SP15 (lipid/protein, 5:1) was incubated with SP35 at concentrations of 0.15 to 0.22 mg/ml in CaCl2-containing buffer. At higher concentrations of SP35, many six-sided lattices were formed; the addition of EDTA abolished the formation of these lattice structures. The results suggest that SP15 and SP35 have an important function in the structural organization of lipid membranes to form lattices.