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

在开花植物的进化过程中，一个共同的事件是从近亲繁殖到自交的转变。在许多情况下，这种转变伴随着花形态的趋同进化，花的大小急剧缩小，结构不那么开放。人们提出了几个适应性假说来解释这种所谓的自恋综合症的出现。然而，到目前为止，由于缺乏确定的偶然潜在基因，它们很难进行严格的测试。此外，同源位点的复发性突变在多大程度上导致了自交综合征的频繁独立出现，目前尚不清楚。菜花属最近成为研究自交综合征进化的一个模型。在这个属中，向自交的过渡独立地发生了两次，在这两次情况下，花的大小都大大缩小了。使用定量遗传学方法，我已经确定了一个基因，在自交物种风疹C.相对于密切相关的远交物种大花C.花瓣大小的减少。群体遗传分析表明，小花瓣等位基因是从异交种的直立遗传变异中捕获的。本研究将有两个主要目的：(1)研究小花瓣等位基因对适应度的影响；(2)评估东方蓟马自花综合征的独立出现是否涉及风疹蓟马自花综合征中相同位点的突变。通过自然群体关联研究和普通园林实验，探讨小花瓣等位基因在自然环境下对植物繁殖成功的影响，并研究其等位基因频率和在祖先异交种后代中的地理分布。通过比较QTL定位、比较转录组分析和转基因实验直接检测东方花自交综合征等位基因，比较东方花自交综合征与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是固定的，并将定义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.