Evolution on ecological timescales: a role for non-genetic inheritance in adapting to novel anthropogenic stressors?

越来越多的研究现在证明了“当代进化”，即种群进化非常迅速，有时不到10代。现代进化通常与人类活动有关，例如细菌的抗生素抗性、植物的金属污染耐受性、杀虫剂和除草剂抗性以及鱼类种群因过度捕捞而发生的变化。了解种群如何适应快速而持续的人为变化对于预测种群持续性的极限和降低物种灭绝率至关重要。我们传统的进化观认为，适应是通过后代继承父母的基因来实现的，这些基因增加了他们在当前环境中的生存和繁殖（适应性）。遗传变异被认为是通过随机基因突变产生的。但是，由于大多数随机突变都会降低生物体的适应性，即使是增加适应性的随机突变最初也只发生在单个个体中，而这些个体可能只是偶然未能繁殖，因此很难看到随机突变如何单独解释当代进化对快速人为变化的反应。如果种群足够大，一个有利的突变可能已经存在于种群中，可以“拯救”它免于灭绝。然而，本研究将关注的一个新出现的替代假设是，种群最初是通过非遗传遗传过程进化的，其中新的表型是通过DNA序列以外的因素的传递产生的。当父母有区别地将非遗传物质分配给他们的后代，改变他们的行为，形态或生理时，就会出现“父母效应”。例如，在许多物种中，雌性在经历恶劣环境时产生较少的较大后代。或者，当负责改变DNA在细胞中折叠方式的表观遗传标记从一代遗传到下一代时，“表观遗传遗传”发生，改变了在任何给定时间打开或关闭的基因模式。从本质上讲，这与解释我们体内不同细胞类型如何从一次细胞分裂到下一次细胞分裂的机制是相同的。目前，我们对非遗传在使种群能够应对快速环境变化方面的重要性以及可能涉及的机制知之甚少。这是因为在有性生殖的生物体中，每个个体都有自己独特的一组基因，这使得很难将遗传遗传与非遗传遗传分开，其次是因为非遗传遗传的研究必须进行至少3代，这在许多研究系统中是不可行的。在本研究中，我们将克服这些问题，通过研究非遗传的水蚤，蚤状蚤;一种以无性繁殖方式繁殖的生物，其世代时间为10天，即所有后代的基因都与其母亲相同。我们将通过比较不同常见淡水污染物暴露后的生长、存活无论祖先是否暴露于污染，其遗传相同的后代的繁殖和基因表达模式。父母的影响和表观遗传的相对重要性将通过确定暴露和非暴露谱系中后代性状的差异是否可以通过后代供应的差异或基因开启或关闭模式的变化以及负责折叠细胞中DNA分子的表观遗传标记来解释来评估。最后，我们将通过确定祖先暴露于污染物是否会改变复制的遗传相同的种群之间的竞争实验的结果来评估非遗传的进化意义。

Increasing numbers of studies are now demonstrating 'contemporary evolution' in which populations evolve very quickly, sometimes in less than 10 generations. Frequently, contemporary evolution is associated with human activities for example antibiotic resistance in bacteria, metal pollution tolerance in plants, pesticide and herbicide resistance, and changes in fish stocks in response to over-fishing. Understanding how populations adapt to rapid and sustained anthropogenic change is imperative for predicting limits to population persistence, and reducing species extinction rates.Our traditional view of evolution suggests that adaptation occurs through offspring inheriting genes from their parents that increase their survival and reproduction (fitness) in the current environment. Genetic variation is assumed to arise through random genetic mutations. But since most random mutations decrease an organism's fitness, and even random mutations that increase fitness initially only occur in a single individual, which might fail to breed just by chance, it is hard to see how random mutations alone can explain contemporary evolution in response to rapid anthropogenic change. If populations are large enough, an advantageous mutation may already exist in the population that can 'rescue' it from extinction. However, an emerging alternative hypothesis that this study will focus on is that populations initially evolve through a process of non-genetic inheritance, in which novel phenotypes are generated by the transmission of factors other than DNA sequence. 'Parental effects' arise when parents differentially allocate non-genetic materials to their offspring altering their behaviour, morphology, or physiology. For example, in many species females produce fewer larger offspring when they experience harsh conditions. Alternatively, 'Epigenetic inheritance' occurs when the epigenetic marks responsible for changing the way that DNA is folded in a cell is inherited from one generation to the next, changing the pattern of genes that are switched on or off at any given time. This is essentially the same mechanism that explains how distinct cell types in our bodies are maintained from one cell division to the next.Currently we have little idea how important non-genetic inheritance is in enabling populations to evolve in response to rapid environmental change, or which mechanism(s) might be involved. This is because in sexually reproducing organisms each individual has its own unique set of genes making it difficult to separate genetic inheritance from non-genetic inheritance and secondly because studies of non-genetic inheritance must be conducted over at least 3 generations, something which is not feasible in many study systems. In this study we will overcome these problems by studying non-genetic inheritance in the water flea, Daphnia pulex; an organism that has a generation time of 10 days and reproduces clonally, meaning that all offspring are genetically identical to their mothers.We will quantify the non-genetic inheritance generated by exposure to different common freshwater pollutants by comparing the growth, survival, reproduction and patterns of gene expression of genetically identical offspring from ancestors exposed to pollution or not. The relative importance of parental effects and epigenetic inheritance will be assessed by determining whether differences in offspring traits in exposed and non-exposed lineages are explained by differences in offspring provisioning or changes in the patterns of genes switched on or off and the epigenetic marks responsible for folding DNA molecules in the cell. Finally, we will assess the evolutionary implications of non-genetic inheritance by determining whether the exposure of ancestors to pollutants changes the outcome of competition experiments between replicated genetically identical populations.