新的表现型在演化过程中是怎样被引入而又不会失去适应性的，这个问题在分子层面上基本上仍未得到解释。在三种子囊菌酵母中对一个众所周知的过程（交配类型的确定）的分子细节进行比较的一系列实验，显示了一个转录回路（一种新的调控因子的插入）的构型所发生的一个演化上的变化，这种变化保留了一种古老的调控形式，但产生了一个重要的新表现型。这个新变种使得“奶酪酵母”（乳酸克鲁维斯酵母）只有在养分短缺时才交配，从而将食物供应状况也考虑了进来，这种行为在酿酒酵母或人类病原体白念珠菌中是不存在的。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09560
Intercalation of a new tier of transcription regulation into an ancient circuit
Lauren N. Booth,Brian B. Tuch& Alexander D. Johnson
Changes in gene regulatory networks are a major source of evolutionary novelty1, 2, 3. Here we describe a specific type of network rewiring event, one that intercalates a new level of transcriptional control into an ancient circuit. We deduce that, over evolutionary time, the direct ancestral connections between a regulator and its target genes were broken and replaced by indirect connections, preserving the overall logic of the ancestral circuit but producing a new behaviour. The example was uncovered through a series of experiments in three ascomycete yeasts: the bakers’ yeast Saccharomyces cerevisiae, the dairy yeast Kluyveromyces lactis and the human pathogen Candida albicans. All three species have three cell types: two mating-competent cell forms (a and α) and the product of their mating (a/α), which is mating-incompetent. In the ancestral mating circuit, two homeodomain proteins, Mata1 and Matα2, form a heterodimer that directly represses four genes that are expressed only in a and α cells and are required for mating4, 5, 6. In a relatively recent ancestor of K. lactis, a reorganization occurred. The Mata1–Matα2 heterodimer represses the same four genes (known as the core haploid-specific genes) but now does so indirectly through an intermediate regulatory protein, Rme1. The overall logic of the ancestral circuit is preserved (haploid-specific genes ON in a and α cells and OFF in a/α cells), but a new phenotype was produced by the rewiring: unlike S. cerevisiae and C. albicans, K. lactis integrates nutritional signals, by means of Rme1, into the decision of whether or not to mate.
