Parental care results in a greater mutation load, for which it is also a phenotypic antidote

Benevolent social behaviours, such as parental care, are predicted to relax selection against deleterious mutations, enabling them to persist. We tested this prediction experimentally using burying beetles, Nicrophorus vespilloides, which make an edible nest for their larvae, whom they nourish and defend. For 20 generations, we allowed replicate experimental burying beetle populations to evolve either with post-hatching care (‘Full Care’ populations) or without it (‘No Care’ populations). Lineages were seeded from these experimental populations and then inbred to expose differences in their mutation load. Outbred lineages served as controls. Half the lineages received post-hatching care, half did not. We found that inbred lineages derived from the Full Care populations had lower breeding success and went extinct more quickly than lineages derived from the No Care populations – but only when offspring received no post-hatching care. We infer that Full Care lineages carried more recessive deleterious mutations. When parents provided care, the developmental environment was sufficiently benign that broods had higher survival, whether the population had a high mutation load or not. We suggest that the increased mutation load caused by parental care increases a population’s dependence upon care. This could explain why care is seldom lost once it has evolved.

诸如育幼行为在内的亲社会性行为，据预测可缓解对有害突变的选择压力，使其得以存续。我们以葬甲（Nicrophorus vespilloides）为实验对象开展验证实验，该物种会为幼虫构筑可食用巢穴，并对幼虫进行抚育与防御。我们将重复实验的葬甲种群分为两组，分别在有孵化后抚育（post-hatching care）的「全抚育（Full Care）」组与无孵化后抚育的「无抚育（No Care）」组条件下，连续进化20个世代。从上述实验种群中分离得到的谱系随后进行近交繁殖，以揭示其突变负荷（mutation load）的差异；远交谱系则作为对照群体。实验中，一半谱系提供孵化后抚育，另一半则不提供。我们发现，仅当子代未获得孵化后抚育时，源自全抚育组种群的近交谱系其繁殖成功率更低，且灭绝速度快于源自无抚育组种群的谱系。据此我们推断，全抚育组谱系携带更多隐性有害突变。当亲代提供抚育时，无论种群突变负荷高低，发育环境均足够适宜，幼虫群存活率均更高。我们认为，亲代抚育所导致的突变负荷升高，会增强种群对抚育行为的依赖，这一机制或可解释为何抚育行为一旦演化形成便极少丢失。