Nishikawa Ken, Kinjo Akira R
National Institute of Genetics, 1111 Yata, Mishima, Shizuoka, 411-8540, Japan.
Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka, 565-0871, Japan.
Biophys Rev. 2018 Apr;10(2):667-676. doi: 10.1007/s12551-018-0403-x. Epub 2018 Feb 21.
Conrad H. Waddington discovered the phenomenon of genetic assimilation through a series of experiments on fruit flies. In those experiments, artificially exerted environmental stress induced plastic phenotypic changes in the fruit flies, but after some generations, the same phenotypic variant started to appear without the environmental stress. Both the initial state (where the phenotypic changes were environmentally induced and plastic) and the final state (where the phenotypic changes were genetically fixed and constitutive) are experimental facts. However, it remains unclear how the environmentally induced phenotypic change in the first generation becomes genetically fixed in the central process of genetic assimilation itself. We have argued that the key to understanding the mechanism of genetic assimilation lies in epigenetics, and proposed the "cooperative model" in which the evolutionary process depends on both genetic and epigenetic factors. Evolutionary simulations based on the cooperative model reproduced the process of genetic assimilation. Detailed analysis of the trajectories has revealed genetic assimilation is a process in which epigenetically induced phenotypic changes are incrementally and statistically replaced with multiple minor genetic mutations through natural selection. In this scenario, epigenetic and genetic changes may be considered as mutually independent but equivalent in terms of their effects on phenotypic changes. This finding rejects the common (and confused) hypothesis that epigenetically induced phenotypic changes depend on genetic mutations. Furthermore, we argue that transgenerational epigenetic inheritance is not required for evolution by genetic assimilation.
康拉德·H·沃丁顿通过一系列对果蝇的实验发现了遗传同化现象。在那些实验中，人为施加的环境压力在果蝇中诱导出可塑性表型变化，但经过几代之后，相同的表型变异在没有环境压力的情况下开始出现。初始状态（表型变化是由环境诱导且具有可塑性）和最终状态（表型变化是由基因固定且具有组成性）都是实验事实。然而，第一代中由环境诱导的表型变化在遗传同化自身的核心过程中如何变得基因固定仍不清楚。我们认为理解遗传同化机制的关键在于表观遗传学，并提出了“合作模型”，其中进化过程取决于遗传和表观遗传因素。基于合作模型的进化模拟重现了遗传同化过程。对轨迹的详细分析表明，遗传同化是一个过程，在这个过程中，表观遗传诱导的表型变化通过自然选择逐渐且统计学上被多个微小基因突变所取代。在这种情况下，表观遗传和基因变化在对表型变化的影响方面可被视为相互独立但等效。这一发现驳斥了常见（且混淆不清）的假设，即表观遗传诱导的表型变化依赖于基因突变。此外，我们认为遗传同化导致的进化不需要跨代表观遗传遗传。
