Department of Biotechnology and Food Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel.
Mol Ecol. 2010 Jan;19(1):121-31. doi: 10.1111/j.1365-294X.2009.04436.x. Epub 2009 Dec 3.
Understanding the evolution of sex and recombination, key factors in the evolution of life, is a major challenge in biology. Studies of reproduction strategies of natural populations are important to complement the theoretical and experimental models. Fungi with both sexual and asexual life cycles are an interesting system for understanding the evolution of sex. In a study of natural populations of yeast Saccharomyces cerevisiae, we found that the isolates are heterothallic, meaning their mating type is stable, while the general belief is that natural S. cerevisiae strains are homothallic (can undergo mating-type switching). Mating-type switching is a gene-conversion process initiated by a site-specific endonuclease HO; this process can be followed by mother-daughter mating. Heterothallic yeast can mate with unrelated haploids (amphimixis), or undergo mating between spores from the same tetrad (intratetrad mating, or automixis), but cannot undergo mother-daughter mating as homothallic yeasts can. Sequence analysis of HO gene in a panel of natural S. cerevisiae isolates revealed multiple mutations. Good correspondence was found in the comparison of population structure characterized using 19 microsatellite markers spread over eight chromosomes and the HO sequence. Experiments that tested whether the mating-type switching pathway upstream and downstream of HO is functional, together with the detected HO mutations, strongly suggest that loss of function of HO is the cause of heterothallism. Furthermore, our results support the hypothesis that clonal reproduction and intratetrad mating may predominate in natural yeast populations, while mother-daughter mating might not be as significant as was considered.
理解性和重组的进化，生命进化的关键因素，是生物学的一个主要挑战。对自然种群生殖策略的研究对于补充理论和实验模型非常重要。具有有性和无性生命周期的真菌是理解性进化的一个有趣系统。在对酵母酿酒酵母自然种群的研究中，我们发现分离株是异配性的，这意味着它们的交配型是稳定的，而普遍的观点是自然酿酒酵母菌株是同配性的（可以进行交配型转换）。交配型转换是由一种特定的内切核酸酶 HO 引发的基因转换过程；这个过程可以通过母女交配来完成。异配性酵母可以与不相关的单倍体交配（异核交配），或者在同一个四分孢子体之间进行交配（自交，或自动交配），但不能像同配性酵母那样进行母女交配。对一组自然酿酒酵母分离株的 HO 基因进行序列分析显示存在多种突变。通过对分布在八个染色体上的 19 个微卫星标记进行特征描述的群体结构进行比较，发现了很好的一致性HO 序列。测试 HO 上下游的交配型转换途径是否具有功能的实验，以及检测到的 HO 突变，强烈表明 HO 的功能丧失是异配性的原因。此外，我们的结果支持这样的假设，即无性繁殖和自交可能在自然酵母种群中占主导地位，而母女交配可能不如人们认为的那么重要。
