From competition to cooperation: Kin selection against selfish shade avoidance behaviour promotes plant invasions

There is conflicting evidence from studies on the fitness consequences of plant kin interactions, suggesting that kinship is not the only important factor in determining the outcome of kin interaction. Here, we tested whether density‐dependent kin interactions have evolved from competition to cooperation by kin selection of reduced shade avoidance responses following the introduction of an invasive plant. We measured how plants respond to kin neighbours (the same accession) and simulated vegetation shade (lower PPFD and R:FR ratio treatments using transparent green plastic filters) and determine whether shade avoidance responses affect the outcome of kin interactions using five native (Argentina) and five invasive (USA) accessions of Alternanthera philoxeroides. Our results showed that invasive accessions exhibited constant stem elongation but increased total biomass as canopy density increased, while native accessions exhibited the opposite. Furthermore, invasive accessions reduced stem elongation, but increased biomass and defence allocation in response to an experimental treatment causing a low R:FR ratio, while native accessions exhibited the opposite. Synthesis . Our results support the hypothesis that kin selection against shade avoidance may effectively promote the evolution of density‐dependent kin cooperation. This suggests that kin selection pressures may play a more important role in fine‐tuning plant shade avoidance responses than generally thought. Kin selection against selfish shade avoidance behaviour and release from negative density‐dependent regulation provides a new mechanism to explain plant invasion success for future studies. 摘要 关于植物亲属互作的适合度后果研究结果长期存在巨大争议，表明亲缘关系并不是决定亲属互作结果的唯一重要因素。 本研究验证密度依赖的亲属互作是否会通过亲属选择降低避荫行为的方式促进由竞争到合作的进化。我们利用喜旱莲子草(Alternanthera philoxeroides)的5个原产地(阿根廷)和5个入侵地基因型(美国)，测定了它们对不同密度亲属邻体(相同基因型)和模拟邻体遮荫(使用绿色透明滤光片降低PPFD和R:FR)的主枝伸长反应、生物量和防御性状的改变。 我们发现，随着冠层密度增加，入侵地基因型具有恒定的茎伸长和较高的总生物量，而原产地基因型则表现出相反的趋势；在低R:FR处理下，入侵地基因型具有较低的茎伸长以及较高的总生物量和防御水平，而原产地基因型则表现相反。 本研究结果表明:通过亲属选择对避荫行为进行约束可以有效地促进密度依赖的亲属合作的进化；亲属选择对植物种内避荫反应大小和方向的影响是巨大的。因此，通过亲属选择对自私的避荫反应行为进行约束，从而使种群从负密度依赖的调节中释放，是外来植物成功入侵的新机制。 Our results support the hypothesis that kin selection against shade avoidance may effectively promote the evolution of density‐dependent kin cooperation. This suggests that kin selection pressures may play a more important role in fine‐tuning plant shade avoidance responses than generally thought. Kin selection against selfish shade avoidance behaviour an