Kin-selected conflict and the evolution of lifespan and ageing

为什么生物会衰老，也就是随着年龄的增长而变得更虚弱，这是进化生态学的一个中心话题。既然身体是自我修复的，为什么每个生物体都不能不断地自我更新，尽管它的年龄按时间顺序排列，但它仍然处于巅峰状态呢？衰老的进化论用自然选择回答了这个问题。意外不可避免地确保了老年人比年轻人少。这意味着老年人对后代的基因贡献较小。反过来，这意味着自然选择不那么看重他们，或者，换句话说，在老年人中，具有有害影响的基因受到的选择不那么强烈。结果是针对年龄的选择。然而，这种关于老龄化演变的传统观点是不完整的，因为它忽略了社会影响。通常情况下，一个人的死亡只会让那些获得其留下的资源的无关特定者受益。但在一个由亲属组成的群体中，一个人的死亡会让其相关的群体成员受益。这通过亲属选择影响寿命和衰老的进化，即因为亲属拥有共同的基因。例如，在自然界中常见的情况是，父母的死亡释放了后代繁殖所需的资源，如巢或领地(资源继承)。最终，这有利于后代，促使父母过早死亡，但有一个转折：父母和子女在父母死亡和资源移交的确切时间上没有达成一致。当父母死亡时，它会有效地用自己的后代(亲缘关系，r=0.5)来交换与其后代亲缘关系较小的后代，即其大后代(r=0.25)。但后代用它的兄弟姐妹(r=0.5)交换自己的同等血缘关系的后代(r=0.5)。由此得出的结论是，如果父母的繁殖力下降，后代会倾向于继承资源，而不是父母倾向于生产资源。因此，在满足这些条件的社会制度中，后代应该骚扰甚至杀死生育能力下降的父母。如果父母的繁殖力充分下降，父母通过死亡并允许后代繁殖比通过活着获得更多的健康。这样的父母应该迅速变老和死亡。在这个方案中，我们的目标是以大黄蜂为实验系统，检验亲属选择的资源遗传冲突影响寿命和衰老的假设。这一物种非常适合这项工作，因为工蚁从它们的母亲--女王那里继承了这个巢，并在里面生下了自己的后代。此外，白纹伊蚊的工作人员有时确实会在繁殖前杀死自己的母亲(工作人员杀母)。我们将检验这一假说的两个核心预测。第一个是，当工人们评估他们从后代生产中获得的健康程度将比通过维持女王的生命和抚养兄弟姐妹获得更大的健康时，他们骚扰女王，并杀害母亲。第二种是，当蜂王被选中将巢的控制权让给工蚁时，在或接近门槛时，蜂王的衰老应该加快。为了验证第一个预测，我们将进行实验，比如比较有攻击性、可能有生殖能力的工作人员和没有工作人员的蜂王的寿命。为了测试第二个问题，我们将确认在其他群居昆虫中被认为是衰老指标的基因在B.terrestris中也起到了同样的作用。然后，我们将测试在被工作人员骚扰的蜂王身上，这些基因是否显示出表达水平的变化，表明衰老加速。这项拟议的研究是新的，因为焦点假说以前没有被检验过，也没有实施过行为和基因相结合的方法来研究这些问题。这是根本的，因为了解社会性如何影响衰老具有理论和实践上的重要性。因此，这项工作应该产生对几个学科有价值和相关性的结果。

Why organisms age, i.e. grow feebler, as they grow old is a central topic in evolutionary ecology. Since bodies self-repair, why cannot every organism constantly renew itself and remain at peak condition despite its chronological age? The evolutionary theory of ageing answers this question with natural selection. Accidents inevitably ensure that there are fewer old individuals than young ones. This means that old individuals make a smaller genetic contribution to future generations. In turn, this means that natural selection values them less highly, or, put another way, that genes with harmful effects are less strongly selected against in older individuals. The result is selection for ageing. However, this conventional view of the evolution of ageing is incomplete, because it omits social effects. Normally an individual's death benefits only the unrelated conspecifics who gain access to the resources it leaves behind. But in a population made up of groups of relatives, death of one individual benefits its related group-mates. This affects the evolution of lifespan and ageing through kin selection, i.e. because relatives share genes in common. An example is the case, common in nature, of a parent whose death releases a resource, such as a nest or territory, required by an offspring to breed (resource inheritance). Eventually this benefits the offspring, creating an incentive for the parent to die prematurely, but there is a twist; parent and offspring do not 'agree' on the exact timing of parental death and resource handover. When the parent dies, it effectively 'trades' its own offspring (relatedness, r, = 0,5) for the less-related offspring of its young, namely its grandoffspring (r = 0.25). But the offspring trades its siblings (r = 0.5) for its own, equally-related offspring (r = 0.5). It follows that, if the parent is declining in fecundity, the offspring favours inheriting the resource before the parent favours yielding it. So, in social systems meeting these conditions, offspring should harass and even kill a parent whose fecundity is declining. If parental fecundity declines sufficiently, the parent gains more fitness by dying and allowing the offspring to reproduce than by remaining alive. Such a parent should rapidly age and die. In this proposal, we aim to test the hypothesis that kin-selected conflict over resource inheritance affects lifespan and ageing using the bumble bee Bombus terrestris as our experimental system. This species is highly suitable for the work because workers inherit the nest from their mother, the queen, and produce their own offspring within it. In addition, B. terrestris workers do sometimes kill their own mother before reproducing (worker matricide). We will test the two central predictions of the hypothesis. The first is that workers harass the queen and commit matricide when they assess that they will gain greater fitness from offspring production than from keeping the queen alive and rearing siblings. The second is that, at or approaching the threshold when the queen is selected to cede control of the nest to workers, queen ageing should be accelerated. To test the first prediction, we will carry out experiments such as comparing the lifespans of queens with and without aggressive, potentially reproductive workers. To test the second, we will confirm that genes known to be indicators of ageing in other social insects act likewise in B. terrestris. We will then test whether, in queens being harassed by workers, these genes show changes in expression level indicating accelerated ageing. The proposed research is novel because the focal hypothesis has not previously been tested, and nor has a combined behavioural and genetic approach to investigating such issues been implemented. It is fundamental because of the theoretical and practical importance of understanding how sociality affects ageing. The work should therefore yield results of value and relevance to several disciplines.