The nature of spontaneous mutational variation for fitness in Chlamydomonas

许多特征，包括生存和繁殖的能力（适应性），在一个物种内的个体之间存在差异。这种适应性和其他性状的变异有一个可遗传的遗传基础。适应性的遗传变异通过自然选择推动进化，正是通过这个过程，生物形式的惊人多样性出现了。重要的是，遗传变异的起源最终来自新的突变，这些突变改变了DNA序列。然而，只有一小部分突变被认为是有利的，并导致适应。很大一部分新的突变是有害的，因为它们破坏了适应良好的基因，因此会被自然选择不断地从种群中清除。了解有害和有益突变的影响对于生物学中一系列尚未解决的现象至关重要。例如，罕见和复发性突变是许多复杂遗传疾病的原因。此外，有性生殖可能会持续下去，因为它可以让种群摆脱有害的突变。否则，这些有害的突变可能会在近亲繁殖或无性繁殖的种群中积累，并可能使它们容易灭绝。尽管突变在这些重要过程中发挥着核心作用，但我们对它的了解相对较少，主要是因为新的突变非常罕见，一代人中每十亿个DNA位置中只有一到十个发生。因此，无论是有害和有益突变的相对频率，还是它们对适应性的影响强度，都没有得到很好的描述。如果没有这些信息，我们就不能开始研究突变效应的根本原因，并最终预测整个基因组突变的后果。在拟议的项目中，我们将研究显微镜下的莱茵衣藻中新的自发突变的性质。这种单细胞植物被广泛用作光合作用、细胞生物学研究的模型，并且越来越多地用于产生绿色能源或生物燃料的潜力。基因组学领域的新兴技术将使我们能够以前所未有的细节研究新的突变。我们现在能够对许多个体的完整基因组进行测序，从而识别罕见的突变事件。其他技术使我们能够同时监测基因组中每个基因的表达，以评估突变对遗传调控的影响。我们的项目可以分为三个互补的部分：（1）首先，我们计划研究这些品系携带什么类型的突变，以及这些不同类型的突变如何影响适应性。这将帮助我们深入了解基因组的哪些部分对适应性最重要，并且容易发生突变。(2)然后，我们将测量数百种突变的个体效应。这将通过比较许多携带突变株系与其非突变祖先的生长并从统计学上解开每个单独突变的影响来实现。与早期尝试估计突变的影响不同，我们的研究将是第一个直接捕捉自发突变效应的复杂性。(3)最后，我们将比较基因是如何在我们的突变株系和它们的非突变祖先之间调节的。然后，我们将测试一些假设的重要性，基因调控的健身，包括如何敏感的生物体是基因调控的变化，是否更高表达的基因更重要，以及是否基因之间的相互联系预测其意义或突变的易感性。这项研究描述了突变的基本过程，将有助于生物研究人员解决与疾病、保护和进化有关的重要问题。

Many traits, including the ability to survive and reproduce (fitness), vary amongst individuals within a species. Much of this variation for fitness and other traits has a heritable genetic basis. Heritable variation in fitness fuels evolution by natural selection and it is through this process that the staggering diversity of biological form has arisen. Importantly, the origin of genetic variation is ultimately from new mutations, which alter the DNA sequence. However, only a small fraction of mutations are believed to be advantageous and lead to adaptation. A high proportion of new mutations are harmful, as they damage well adapted genes and therefore are continually purged from populations by natural selection. Understanding the impact of both harmful and beneficial mutations is crucially important for a range of unresolved phenomena in biology. For example, rare and recurrent mutations are responsible for many complex genetic disorders. Moreover, sexual reproduction may persist because it allows populations to rid themselves of harmful mutations. Otherwise, these harmful mutations may build up in inbred or asexual populations and potentially leave them vulnerable to extinction. Despite the central role of mutation in these important processes, we know relatively little about it, mostly because new mutations are very rare, occurring at only about one to ten per billion DNA positions in a generation. As a result, neither the relative frequency of harmful and beneficial mutations nor the strength of their effects on fitness has been well described. Without this information, we cannot begin to investigate the underlying causes of mutational effects and ultimately predict the consequences of mutations across the genome.In the proposed project, we will investigate the nature of new spontaneous mutations in the microscopic alga, Chlamydomonas reinhardtii. This single-celled plant is widely used as a model for the study of photosynthesis, cell biology, and increasingly for its potential to generate green energy or biofuel. Emerging technologies in the field of genomics will allow us to study new mutations in unprecedented detail. We are now able to sequence the complete genomes of many individuals and therefore to identify rare mutational events. Other techniques allow us to simultaneously monitor the expression of every gene in the genome to assess the impact of mutation on genetic regulation. Our project can be divided into three complementary sections: (1) First, we plan to look at what kinds of mutations the lines carry and how these different types of mutations affect fitness. This will help us gain insights into what parts of the genome are, on average, most important to fitness and prone to mutation. (2) We will then measure the individual effects of hundreds of mutations. This will be achieved by comparing the growth of many mutant-bearing lines with their non-mutated ancestors and statistically disentangling the effect of each individual mutation. Unlike earlier attempts to estimate the effects of mutations, our study will be the first to directly capture the complexity of spontaneous mutational effects. (3) Lastly, we will compare how genes are regulated amongst our mutant-bearing lines and their non-mutated ancestors. We will then test a number of hypotheses about the importance of gene regulation for fitness, including how sensitive organisms are to changes in gene regulation, whether more highly expressed genes are more important and whether the interconnectedness amongst genes predicts their significance or susceptibility to mutation. This research, describing the fundamental process of mutation, will help biological researchers to address important questions pertaining to disease, conservation and evolution.