Reproductive isolation and drift load in selfing populations of an otherwise obligately outcrossing species

从理论上讲，两性植物可以通过自交(自交)繁殖，但大多数是自交不亲和的，或者已经进化出其他机制来促进交叉受精(异交)。从异交到自交的转变是植物进化中最频繁的转变之一。然而，只有相对较小的比例(10%-15%)的植物主要通过自交繁殖。那么为什么会这样呢？作为一种可能的解释，有人提出了自交促进物种形成，但这种多样化的影响被更高的灭绝率所抵消。尽管这一假说得到了一些系统发育学的支持，但一方面促进自交谱系物种形成的机制，另一方面它们增加的灭绝率，在很大程度上仍然是一个黑匣子。在这里，我提出了三项研究，以解决这个问题，使用北美物种拟南芥Lyrata。该物种的大多数种群是自交不亲和的，因此必须进行异交，但已发现6个种群完全由自交率高的自交亲和植物组成(自交种群)。这为研究自交进化的生态驱动因素及其在漂移负荷方面的负面后果提供了一个很好的模型。在第一项研究中，我们将进行一项常见的花园实验，以测试物候和昆虫访问的变化是否会将自交种群与其他种群隔离开来。在第二项研究中，我们将测试授粉后(但仍在受精之前)的作用机制是否有助于自交种群的生殖隔离。具体地说，我们将测试来自自交群体的植物与来自其他群体的植物之间的杂交是否会产生比群体内参考杂交更少的种子。第三项研究将基于多年的种子生产并考虑种子活力，测试在普通花园中，来自自交群体的植物是否比来自同种异交群体的植物表现更差。总而言之，前两项研究将提高我们对生态过程的概念性理解，这些过程在允许自交种群发展成生殖隔离的、不同的谱系方面发挥了作用。此外，第三项研究将允许测试这种多样化的过程是否会被漂移负荷造成的健康状况恶化所抵消。

Hermaphroditic plants can theoretically reproduce through self-fertilisation (selfing), but most are self-incompatible or have evolved other mechanisms to promote cross-fertilisation (outcrossing). Transitions from outcrossing to selfing are among the most frequent transitions in plant evolution. Nevertheless, only a relatively small proportion of plant species (10-15%) reproduce mainly by selfing. So why is this? As a potential explanation, it was proposed that selfing promotes speciation, but that this diversifying effect is countered by even higher extinction rates. Although there is some phylogenetic support for this hypothesis, the mechanisms that promote speciation of selfing lineages on the one hand, and their increased extinction rates on the other hand, are still largely a black box. Here, I propose three studies to address this using the North American species Arabidopsis lyrata. Most populations of this species are self-incompatible and thus obligately outcrossing, but six populations have been discovered that exclusively consist of self-compatible plants with high selfing rates (selfing populations). This provides an excellent model to study the ecological drivers of the evolution of selfing, and its negative consequences in terms of drift load. In the first study, we will perform a common garden experiment to test whether changes in phenology and insect visitation reproductively isolate selfing populations from other populations. In the second study, we will test whether mechanisms that act after pollination (but still before fertilisation) contribute to reproductive isolation of selfing populations. Specifically, we will test whether crosses between plants from selfing populations and plants from other populations yield fewer seeds than within-population reference crosses. The third study will test whether, in a common garden, plants from selfing populations have a reduced performance compared to those from conspecific outcrossing populations, based on multi-year seed production and taking into account seed viability. Together, the first two studies will improve our conceptual understanding of the ecological processes that play a role in allowing selfing populations to develop into reproductively isolated, divergent lineages. Additionally, the third study will allow to test whether this diversifying process is counteracted by fitness deterioration due to drift load.