Hawthorne D, Philippsen P
Department of Genetics, University of Washington, Seattle 98195.
Yeast. 1994 Oct;10(10):1285-96. doi: 10.1002/yea.320101005.
We have studied the phenomenon of infertility of yeast hybrids obtained with physiological conditions under the control of compatible mating systems. The yeasts investigated are three Saccharomyces species: S. cerevisiae, S. uvarum and a new species, S. douglasii. The diploid hybrids from crosses between these species sporulate well but are essentially infertile. The rare viable spores, one per 10(4) to 10(5) asci, that have been examined carry a complete genome comprised of chromosomes contributed by both parents but invariably have extra chromosomes, i.e. they are generally disomic for at least two or three chromosomes. This observation is consistent with a failure, in meiosis I, of the pairing and disjunction of homologous chromosomes which in most cases results in spores with an incomplete set of chromosomes. This apparent lack of pairing of 'homeologous' chromosomes in meiosis I was analysed in most detail with S. cerevisiae/S. douglasii hybrids. As a genetic tool we studied frequencies of recombination, taking advantage of an S. douglasii breeding stock of some 50 identified mutations in non-switching haploids. Recombination, although markedly reduced, could be observed at both the chromosomal and allelic levels, implying a sporadic pairing in meiosis to allow genetic exchange. Meiotic recombination frequencies were studied for 14 gene pairs and generally found to be reduced ten-fold. Heteroallelic recombination (gene conversion) frequencies were measured at 22 loci and were judged to be reduced at least two- to 100-fold. DNA hybridization experiments with S. cerevisiae gene probes gave results consistent with low DNA sequence homologies between S. cerevisiae and S. douglasii. Moreover, by change, our experiments disclosed another Saccharomyces strain (CBS2908, originally classified as S. cerevisiae) with hybridization patterns identical to S. douglasii except for the hybridization with the Ty transposon probes. Crosses between CBS2908 and S. douglasii yielded diploid hybrids with 80-90% spore viability, thus establishing a second member of the S. douglasii species.
我们研究了在相容交配系统控制的生理条件下获得的酵母杂种不育现象。所研究的酵母是三种酿酒酵母：酿酒酵母、葡萄汁酵母和一个新物种道格拉斯酿酒酵母。这些物种之间杂交产生的二倍体杂种能很好地形成孢子，但基本上不育。已检测到的罕见活孢子，每10⁴至10⁵个孢子囊中出现一个，携带由双亲贡献的染色体组成的完整基因组，但总是有额外的染色体，即它们通常至少有两三条染色体是二体的。这一观察结果与减数分裂I中同源染色体配对和分离失败一致，在大多数情况下，这会导致孢子的染色体组不完整。我们利用约50个已鉴定的非转换单倍体突变的道格拉斯酿酒酵母育种群体作为遗传工具，最详细地分析了酿酒酵母/道格拉斯酿酒酵母杂种减数分裂I中“部分同源”染色体明显缺乏配对的情况。尽管重组明显减少，但在染色体和等位基因水平都能观察到重组，这意味着减数分裂中偶尔会发生配对以允许基因交换。我们研究了14个基因对的减数分裂重组频率，发现通常降低了10倍。在22个位点测量了异等位基因重组（基因转换）频率，判断至少降低了2至100倍。用酿酒酵母基因探针进行的DNA杂交实验结果与酿酒酵母和道格拉斯酿酒酵母之间低DNA序列同源性一致。此外，偶然地，我们的实验发现了另一种酿酒酵母菌株（CBS2908，最初归类为酿酒酵母），其杂交模式与道格拉斯酿酒酵母相同，只是与Ty转座子探针的杂交情况不同。CBS2908和道格拉斯酿酒酵母之间的杂交产生了孢子活力为80 - 90%的二倍体杂种，从而确定了道格拉斯酿酒酵母物种的第二个成员。
