Adaptation to selfing in the genus Capsella: Inferring the evolutionary history and adaptive value of selfing-syndrome alleles

开花植物进化中的一个常见事件是从远缘繁殖到自交的过渡。在许多情况下，这种转变之后是花形态的趋同进化，朝着显著减小的花大小和不那么开放的结构进化。为了解释这种所谓的自闭症候群的出现，人们提出了几种适应性假设。然而，到目前为止，由于缺乏已确定的随机潜在基因，它们很难进行严格的测试。此外，目前尚不清楚同源基因的反复突变在多大程度上导致了自体综合征的频繁独立出现。最近，Capsella属已经成为研究自体综合征进化的模型。向自交的转变在该属中独立发生了两次，在这两种情况下都伴随着花大小的强烈缩小。使用数量遗传学方法，我已经确定了一个基因，该基因导致自交物种风疹杆菌的花瓣尺寸比近缘近缘的远交物种格兰迪弗拉病毒的花瓣尺寸小。群体遗传学分析表明，小花瓣等位基因已从异交物种的现有遗传变异中捕捉到。这项建议将追求两个主要目标：(1)研究小花瓣等位基因对适合度的影响；(2)评估东方锦鸡儿自交综合征的独立出现是否涉及与风疹病毒相同的基因座突变。小花瓣等位基因在自然环境中对植物繁殖成功的影响将通过自然种群的关联性研究和共同园艺实验来调查，并将研究其在祖先异交物种后代中的等位基因频率和地理分布。通过比较QTL定位、比较转录分析和通过转基因实验直接检测侧柏等位基因，比较侧柏和风疹自交综合征的遗传基础。因此，该项目将有助于理解为什么自交综合征等位基因在自交物种中变得固定，并将确定在向自交过渡后花形态趋同进化的遗传基础。

A common event in the evolution of flowering plants is the transition from outbreeding to selfing. In many cases, this transition has been followed by convergent evolution of the floral morphology towards a dramatically reduced flower size and less open structure. Several adaptive hypotheses have been formulated to explain the emergence of this so-called selfing syndrome. They have however been difficult to test rigorously up to now due to the lack of identified casual underlying genes. Also, the extent to which recurrent mutations in orthologous loci have contributed to the frequent independent emergence of the selfing syndrome is currently unknown. The genus Capsella has recently emerged as a model to study the evolution of the selfing syndrome. The transition to selfing has occurred independently twice in this genus, and in both cases this has been followed by a strong reduction of flower size. Using a quantitative-genetics approach, I have identified a gene underlying the reduction of petal size in the selfing species C. rubella relative to the closely related outbreeding species C. grandiflora. Population-genetic analyses indicate that the small-petal allele has been captured from standing genetic variation in the outcrossing species.This proposal will pursue two major objectives: (1) to investigate the effects of the small-petal allele on fitness and (2) to assess whether the independent emergence of the selfing syndrome in C. orientalis involved mutations to the same loci identified in C. rubella. The effect of the small-petal allele on plant reproductive success in a natural environment will be investigated by association studies in natural populations and a common-garden experiment, and its allele frequency and geographical distribution in the descendants of the ancestral outcrossing species will be studied. The genetic basis of the selfing syndrome in C. orientalis will be compared to that in C. rubella by comparative QTL mapping, comparative transcriptomic analyses and direct testing of C. orientalis alleles by transgenic experiments. As a result, the proposed project will help to understand why selfing-syndrome alleles become fixed in selfing species, and will define the genetic basis underlying the convergent evolution of floral morphology after the transition to selfing.