Department of Basic Science, Univ, of Tokyo, and Complex Systems Biology Project, ERATO, JST, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan.
BMC Evol Biol. 2011 Jan 26;11:27. doi: 10.1186/1471-2148-11-27.
Characterization of robustness and plasticity of phenotypes is a basic issue in evolutionary and developmental biology. The robustness and plasticity are concerned with changeability of a biological system against external perturbations. The perturbations are either genetic, i.e., due to mutations in genes in the population, or epigenetic, i.e., due to noise during development or environmental variations. Thus, the variances of phenotypes due to genetic and epigenetic perturbations provide quantitative measures for such changeability during evolution and development, respectively.
Using numerical models simulating the evolutionary changes in the gene regulation network required to achieve a particular expression pattern, we first confirmed that gene expression dynamics robust to mutation evolved in the presence of a sufficient level of transcriptional noise. Under such conditions, the two types of variances in the gene expression levels, i.e. those due to mutations to the gene regulation network and those due to noise in gene expression dynamics were found to be proportional over a number of genes. The fraction of such genes with a common proportionality coefficient increased with an increase in the robustness of the evolved network. This proportionality was generally confirmed, also under the presence of environmental fluctuations and sexual recombination in diploids, and was explained from an evolutionary robustness hypothesis, in which an evolved robust system suppresses the so-called error catastrophe--the destabilization of the single-peaked distribution in gene expression levels. Experimental evidences for the proportionality of the variances over genes are also discussed.
The proportionality between the genetic and epigenetic variances of phenotypes implies the correlation between the robustness (or plasticity) against genetic changes and against noise in development, and also suggests that phenotypic traits that are more variable epigenetically have a higher evolutionary potential.
表型稳健性和可塑性的特征是进化和发育生物学中的一个基本问题。稳健性和可塑性涉及生物系统对外界干扰的可变性。这些干扰要么是遗传的，即由于群体中基因的突变，要么是表观遗传的，即由于发育过程中的噪声或环境变化。因此，由于遗传和表观遗传干扰而导致的表型方差分别为进化和发育过程中的这种可变性提供了定量的度量。
使用模拟基因调控网络进化变化所需的数值模型来实现特定的表达模式，我们首先证实了在转录噪声足够水平存在的情况下，对突变具有稳健性的基因表达动力学会进化。在这种情况下，基因表达水平的两种类型的方差，即由于基因突变导致的方差和由于基因表达动力学中的噪声导致的方差，在许多基因中被发现是成比例的。具有共同比例系数的此类基因的分数随着进化网络稳健性的增加而增加。这种比例性在存在环境波动和二倍体中的性重组的情况下也得到了普遍证实，并从进化稳健性假说中得到了解释，其中进化的稳健系统抑制了所谓的错误灾难--基因表达水平单峰分布的不稳定性。还讨论了实验证据表明基因之间的方差具有比例性。
表型的遗传和表观遗传方差之间的比例性意味着对遗传变化和发育过程中噪声的稳健性（或可塑性）之间的相关性，也表明在表观遗传上更具可变性的表型特征具有更高的进化潜力。
