Sake is an alcoholic beverage of Japan, with a tradition lasting more than 1,300 years; it is produced from rice and water by fermenting with the koji mold Aspergillus oryzae and sake yeast Saccharomyces cerevisiae. Breeding research on sake yeasts was originally developed in Japan by incorporating microbiological and genetic research methodologies adopted in other scientific areas. Since the advent of a genetic paradigm, isolation of yeast mutants has been a dominant approach for the breeding of favorable sake yeasts. These sake yeasts include (a) those that do not form foams (produced by isolating a mutant that does not stick to foams, thus decreasing the cost of sake production); (b) those that do not produce urea, which leads to the formation of ethyl carbamate, a possible carcinogen (isolated by positive selection in a canavanine-, arginine-, and ornithine-containing medium); (c) those that produce an increased amount of ethyl caproate, an apple-like flavor (produced by isolating a mutant resistant to cerulenin, an inhibitor of fatty-acid synthesis); and (d) those that produce a decreased amount of pyruvate (produced by isolating a mutant resistant to an inhibitor of mitochondrial transport, thus decreasing the amount of diacetyl). Given that sake yeasts perform sexual reproduction, sporulation and mating are potent approaches for their breeding. Recently, the genome sequences of sake yeasts have been determined and made publicly accessible. By utilizing this information, the quantitative trait loci (QTLs) for the brewing characteristics of sake yeasts have been identified, which paves a way to DNA marker-assisted selection of the mated strains. Genetic engineering technologies for experimental yeast strains have recently been established by academic groups, and these technologies have also been applied to the breeding of sake yeasts. Sake yeasts whose genomes have been modified with these technologies correspond to genetically modified organisms (GMOs). However, technologies that enable the elimination of extraneous DNA sequences from the genome of sake yeast have been developed. Sake yeasts genetically modified with these technologies are called self-cloning yeasts and do not contain extraneous DNA sequences. These yeasts were exempted from the Japanese government's guidelines for genetically modified food. Protoplast fusion has also been utilized to breed favorable sake yeasts. Future directions for the breeding of sake yeasts are also proposed in this review. The reviewed research provides perspectives for the breeding of brewery yeasts in other fermentation industries.

中文翻译：

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日本清酒酵母的育种研究：历史，最新技术进步和未来展望。

清酒是日本的酒精饮料，已有1300多年的悠久历史。它是用米曲霉米曲霉和清酒酵母酿酒酵母发酵而成的，由米和水制成。清酒酵母的育种研究最初是在日本通过结合其他科学领域采用的微生物学和遗传学研究方法进行的。自从遗传学范式问世以来，分离酵母突变体一直是有利于清酒酵母育种的主要方法。这些清酒酵母包括：（a）不形成泡沫的酵母（通过分离不粘在泡沫上的突变体生产，从而降低了清酒生产成本）；（b）不产生尿素的物质，这些物质会导致氨基甲酸乙酯的形成，氨基甲酸乙酯是一种可能的致癌物（通过在正黄花氨酸，精氨酸，和含鸟氨酸的培养基）；（c）产生增加量的己酸乙酯（一种苹果样风味剂）的产品（通过分离出一种对铜蓝蛋白有抵抗力的突变体（一种脂肪酸合成抑制剂）生产）；（d）产生减少量的丙酮酸的那些（通过分离对线粒体运输抑制剂有抗性的突变体而产生，从而减少了二乙酰基的量）。考虑到缘故，酵母可以进行有性繁殖，因此孢子形成和交配是繁殖的有效方法。最近，已经确定了清酒酵母的基因组序列并使其可公开获得。通过利用这些信息，已经确定了清酒酵母酿造特性的定量性状基因座（QTL），这为DNA标记辅助菌株的选择铺平了道路。最近，学术团体已经建立了用于实验酵母菌株的基因工程技术，这些技术也已应用于清酒酵母的育种。其基因组已被这些技术修饰的清酒酵母对应于转基因生物（GMO）。然而，已经开发出能够从清酒酵母的基因组中消除无关的DNA序列的技术。用这些技术进行基因修饰的清酒酵母被称为自克隆酵母，不包含外源DNA序列。这些酵母菌不受日本政府关于转基因食品的指导方针的约束。原生质体融合也已被用于繁殖有利的清酒酵母。本文还提出了清酒酵母育种的未来方向。