Effect of multilevel selection on the evolution of gynodioecy

大多数特征被认为是进化的，因为它们增加了个体的生存和繁殖。然而，有些特征似乎会降低个体的适合度，但仍然会反复进化。其中一个特征是雌雄异株，这是一种两性植物和雌性植物在一个种群中共存的一种性别多态。雌性有一个与生俱来的劣势，因为它们只能通过种子繁殖，而两性动物可以通过种子和花粉繁殖。尽管存在这一劣势，但雌性植物以及雌雄异株在开花植物中反复进化。雌性可能会对个体水平的选择做出反应，这种选择只在一组有限的条件下起作用，或者对个体上方和/或下方的选择做出反应，包括群体和配子。在多个水平上衡量选择是很重要的，因为特征根据选择行为的水平而不同地演变：即使选择的强度和方向在不同的水平上相似，选择的原因和反应也可能不同。 我们的长期目标是确定雌雄异体的进化是如何受到多层次选择的影响的。我们最近的工作表明，雌雄异株的进化可以响应频率依赖的选择(其中个体的适合性部分取决于其群体的平均表型)和配子选择(其中配子的生存能力取决于它携带的等位基因)。然而，我们也发现雌雄异株种群中雌性的频率差异很大，这表明选择的强度是不同的。 为了确定为什么雌雄异株种群的选择强度不同，我们将重点关注两个短期目标。首先，我们将通过测试为什么雌性植物具有频率依赖的适合度以及频率依赖选择的强度是否取决于同种密度来确定导致雌性植物频率依赖选择的机制。其次，我们将通过测试配子选择强度的变化是否由生态或遗传因素引起，以及这些遗传因素是否存在于细胞质或核基因组中，来确定导致性别决定基因配子选择的机制。所有这些工作都将在雌雄异株物种Lobelia siphilitica上完成。 虽然雌雄同体最先是由达尔文研究的，但对于它为什么在一些种群中进化，而在另一些种群中不进化，仍然存在不确定性。这项拟议的研究集中在导致频率依赖选择和配子选择的机制上，将使我们能够预测雌雄异株进化的生态和遗传背景。更广泛地说，这项拟议的研究将展示如何在多个水平上衡量选择，从而为重复进化的特征带来新的解释，这些特征似乎会降低个人的适应能力。

Most traits are thought to evolve because they increase an individual's survival and reproduction. However, there are some traits that appear to decrease an individual's fitness, yet still evolve repeatedly. One such trait is gynodioecy, a type of sexual polymorphism in which hermaphroditic and female plants coexist within a population. Females have an inherent fitness disadvantage because they can reproduce only via seeds, whereas hermaphrodites can reproduce via both seeds and pollen. Despite this disadvantage, females, and thus gynodioecy, have evolved repeatedly in the flowering plants. Females could evolve in response to individual-level selection that acts only under a restricted set of conditions, or in response to selection at levels above and/or below the individual, including the group and the gamete. Measuring selection at multiple levels is important because traits evolve differently based on the level at which selection acts: even when the strength and direction of selection are similar across levels, both the causes of and responses to selection can differ. Our long-term goal is to determine how the evolution of gynodioecy has been affected by selection at multiple levels. Our recent work suggests that gynodioecy can evolve in response to frequency-dependent selection (where an individual's fitness depends in part on the mean phenotype of its group) and gametic selection (where the viability of a gamete depends on which allele it carries). However, we also found that the frequency of females varies widely among gynodioecious populations, suggesting that the strength of selection varies. To determine the why the strength of selection varies among gynodioecious populations, we will focus on two short-term goals. First, we will identify the mechanisms that cause frequency-dependent selection on female plants by testing why females have frequency-dependent fitness and whether the strength of frequency-dependent selection depends on conspecific density. Second, we will identify the mechanisms that cause gametic selection on sex-determining genes by testing whether variation in the strength of gametic selection is caused by ecological or genetic factors and whether these genetic factors are in the cytoplasmic or nuclear genome. All of this work will be done in the gynodioecious species Lobelia siphilitica. Although gynodioecy was first studied by Darwin, there is still uncertainty as to why it evolves in some populations but not in others. The proposed research, by focusing on the mechanisms that cause frequency-dependent selection and gametic selection, will allow us to predict the ecological and genetic contexts in which gynodioecy is more and less likely to evolve. More generally, the proposed research will demonstrate how measuring selection at multiple levels can lead to new explanations for the repeated evolution of traits that appear to decrease an individual's fitness.