Data from: Loss of heterosis and family-dependent inbreeding depression in plant performance and resistance against multiple herbivores under drought stress

1. Inbreeding depression (ID), outbreeding depression (OD) and heterosis can occur concurrently in plant populations. ID often increases under environmental stress, but the combined effects of inbreeding, outbreeding between populations and environmental stress, such as drought, on plant performance and herbivore resistance remain unclear. 2. In order to determine environment-dependent and family-dependent ID, OD and heterosis we conducted a common garden experiment with plants from five populations of Brassica nigra. Inbred, within-population outbred and between-population outbred plant families were exposed to drought or ambient water levels. We recorded the abundance and damage caused by specialist herbivores from contrasting feeding guilds, i.e. the phloem-feeding Brevicoryne brassicae, the leaf-chewing Psylliodes chrysocephalus and the stem-boring Ceutorhynchus quadridens larvae. 3. Drought stress had negative effects on growth, herbivore resistance and resistance against B. brassicae and positive effects on investments in reproductive output and plant secondary metabolites (sinigrin). We found drought stress-induced loss of heterosis for plant height and investment in reproductive output. Between-population outbred plants were more sensitive to drought stress in terms of above-ground biomass compared to within-population outbred plants. 4. Drought and inbreeding synergistically negatively influenced traits related to growth and reproductive output (environment-dependent inbreeding depression, EDID). There was high variation among families within populations in the degree of ID and EDID. Genetic variation in EDID could buffer the negative effects of genetic stress associated with habitat fragmentation and concurrent environmental stress. In order to capture fully the effects of both inbreeding and between-population outbreeding under stress the different spatial scales of the effects of inbreeding and between-population outbreeding should be taken into account. 5. Synthesis. Our results indicate that drought stress influences not only inbreeding depression (ID), but also heterosis. These findings shed new light on the combined effects of anthropogenic environmental change and the genetic consequences of habitat fragmentation on plants and their interactions with other organisms. Conservation programmes aiming to restore genetically degraded populations with the translocation of individuals between populations should consider environmental stress as a risk factor.1. 近交衰退（Inbreeding depression, ID）、远交衰退（Outbreeding depression, OD）与杂种优势可在植物种群中同时发生。环境胁迫下近交衰退通常会加剧，但近交、种群间远交以及干旱这类环境胁迫对植物表现及植食动物抗性的联合效应仍不明晰。 2. 为探明环境依赖型与家系依赖型的近交衰退、远交衰退及杂种优势，本研究以5个种群的黑芥（*Brassica nigra*）为研究材料开展同质园实验。将近交、种群内远交及种群间远交的植物家系分别置于干旱胁迫与正常水分水平环境中，记录不同取食功能群专性植食动物的种群丰度与植株受害程度，具体涵盖取食韧皮部的*Brevicoryne brassicae*、取食叶片的*Psylliodes chrysocephalus*以及蛀食茎秆的*Ceutorhynchus quadridens*幼虫。 3. 干旱胁迫对植物生长、植食动物抗性以及对*B. brassicae*的抗性均产生负面影响，而对繁殖投入与植物次生代谢物（黑芥子苷，sinigrin）的合成具有促进作用。本研究发现，干旱胁迫会导致植物株高与繁殖投入的杂种优势丧失；相较于种群内远交植株，种群间远交植株的地上生物量对干旱胁迫更为敏感。 4. 干旱胁迫与近交可协同对植物生长与繁殖相关性状产生负面影响，即环境依赖型近交衰退（EDID）。种群内不同家系的近交衰退程度与环境依赖型近交衰退程度均存在显著差异。环境依赖型近交衰退的遗传变异可缓冲生境破碎化伴随的遗传胁迫与协同环境胁迫带来的负面影响。为全面阐明胁迫条件下近交与种群间远交的联合效应，需纳入考虑近交与种群间远交效应的不同空间尺度。 5. 综合分析：本研究结果表明，干旱胁迫不仅会影响近交衰退（ID），还会对杂种优势产生调控作用。该研究为阐明人为环境变化与生境破碎化的遗传效应对植物及其与其他生物互作的联合影响提供了新视角。旨在通过种群间个体迁置修复遗传退化种群的保护计划，应将环境胁迫视为一项风险因子纳入考量。