Maltotriose consumption by hybrid Saccharomyces pastorianus is heterotic and results from regulatory cross-talk between parental sub-genomes

S. pastorianus strains are hybrids of S. cerevisiae and S. eubayanus that have been domesticated for several centuries in lager-beer brewing environments. As sequences and structures of S. pastorianus genomes are being resolved, molecular mechanisms and evolutionary origin of several industrially relevant phenotypes remain unknown. This study investigates how maltotriose metabolism, a key feature in brewing, may have arisen in early S. eubayanus x S. cerevisiae hybrids. To address this question, we generated a near-complete genome assembly of Himalayan S. eubayanus strains of the Holarctic subclade. This group of strains have been proposed to be the origin of the S. eubayanus subgenome of current S. pastorianus strains. The Himalayan S. eubayanus genomes harbored several copies of an SeAGT1 -oligoglucoside transporter gene with high sequence identity to genes encountered in S. pastorianus. Although Himalayan S. eubayanus strains are unable to grown on maltose and maltotriose, their maltose-hydrolase and SeMALT1 and SeAGT1 maltose-transporter genes complemented the corresponding null mutants of S. cerevisiae. Expression, in a Himalayan S. eubayanus strain, of a functional S. cerevisiae maltose-metabolism regulator gene (MALx3) enabled growth on oligoglucosides. The hypothesis that the maltotriose-positive phenotype in S. pastorianus is a result of heterosis was experimentally tested by constructing a S. cerevisiae x S. eubayanus laboratory hybrid with a complement of maltose-metabolism genes that resembles that of current S. pastorianus strains. The ability of this hybrid to consume maltotriose in brewer's wort demonstrated regulatory cross talk between sub-genomes and thereby validated this hypothesis. These results provide experimental evidence of the evolutionary origin of an essential phenotype of lager-brewing strains and valuable knowledge for industrial exploitation of laboratory-made S. pastorianus-like hybrids. Overall design: Compare expression of maltose metabolism genes in S. eubayanus and S. cerevisiae x S. eubayanus strains on various carbon sources

巴氏酵母（S. pastorianus）是酿酒酵母（S. cerevisiae）与埃氏酵母（S. eubayanus）的杂交物种，已在拉格啤酒（lager-beer）酿造环境中被驯化数个世纪。随着巴氏酵母基因组序列与结构的解析工作持续推进，若干工业相关表型的分子机制与进化起源仍未明确。本研究聚焦于酿造工业的关键特征——麦芽三糖（maltotriose）代谢，探究其在早期埃氏酵母×酿酒酵母杂交菌株中的起源路径。
为解答该科学问题，我们对全北极亚支的喜马拉雅地区埃氏酵母菌株完成了近完整基因组组装（genome assembly）。该类菌株此前被推测为现存巴氏酵母菌株中埃氏酵母亚基因组（subgenome）的供体来源。喜马拉雅埃氏酵母基因组中，携有多份SeAGT1寡葡糖苷转运蛋白（oligoglucoside transporter）基因拷贝，其序列与巴氏酵母中的同源基因相似度极高。
尽管喜马拉雅埃氏酵母菌株无法在以麦芽糖、麦芽三糖为唯一碳源的培养基上生长，但其编码麦芽糖水解酶、SeMALT1与SeAGT1麦芽糖转运蛋白的基因，可互补酿酒酵母的对应缺失突变体（null mutants）。将功能性酿酒酵母麦芽糖代谢调控基因（MALx3）导入喜马拉雅埃氏酵母菌株后，该菌株即可在寡葡糖苷培养基上实现正常生长。
本研究针对"巴氏酵母的麦芽三糖阳性表型源于杂种优势（heterosis）"这一假说开展实验验证：通过构建携带与现存巴氏酵母菌株相似的麦芽糖代谢基因组合的酿酒酵母×埃氏酵母实验室杂交株，检测其在啤酒麦芽汁（brewer's wort）中消耗麦芽三糖的能力，结果证实了亚基因组间存在调控交叉对话，从而验证了该假说。
上述研究结果为拉格啤酒酿造菌株核心表型的进化起源提供了实验证据，同时为工业开发实验室构建的类巴氏酵母杂交菌株提供了宝贵的理论支撑。总体实验设计：对比埃氏酵母与酿酒酵母×埃氏酵母杂交菌株在多种碳源条件下的麦芽糖代谢基因表达水平。