No evidence for thermal transgenerational plasticity in metabolism when minimizing the potential for confounding effects

Environmental change may cause phenotypic changes that are inherited across generations through transgenerational plasticity (TGP). If TGP is adaptive, offspring fitness increases with an increasing match between parent and offspring environment. Here we test for adaptive TGP in somatic growth and metabolic rate in response to temperature in the clonal zooplankton Daphnia pulex. Animals of the first focal generation experienced thermal transgenerational ‘mismatch’ (parental and offspring temperatures differed), whereas conditions of the next two generations matched the (grand)maternal thermal conditions. Adjustments of metabolic rate occurred during the lifetime of the first generation (i.e. within-generation plasticity). However, no further change was observed during the subsequent two generations, as would be expected under TGP. Furthermore, we observed no tendency for increased juvenile somatic growth (a trait highly correlated with fitness in Daphnia) over the three generations when reared at new temperatures. These results are inconsistent with existing studies of thermal TGP, and we describe how previous experimental designs may have confounded TGP with within-generation plasticity and selective mortality. We suggest that the current evidence for thermal TGP is weak. To increase our understanding of the ecological and evolutionary role of TGP, future studies should more carefully identify possible confounding factors.

环境变化可诱发表型改变，这类改变可通过跨代可塑性（transgenerational plasticity, TGP）实现跨代传递。若TGP具备适应性，则后代适合度将随亲本与后代环境的匹配程度升高而提升。本研究以克隆型浮游动物蚤状溞（Daphnia pulex）为研究对象，检验其躯体生长与代谢率对温度响应的适应性TGP。首个目标世代的实验个体经历了热跨代“错配”处理——即亲本与后代的培养温度存在差异，而后续两个世代的培养条件则与（曾）祖母代的热环境保持一致。首代个体的代谢率在其生命周期内发生了调整，这属于代内可塑性范畴。但在后续两个世代中，并未观察到符合TGP理论预期的进一步变化。此外，当在新温度环境下饲养时，我们未发现幼体躯体生长（该性状在蚤状溞中与适合度高度相关）在三代培养周期内出现增强的趋势。本研究结果与已发表的热适应性TGP相关研究结论并不一致，我们进一步指出，此前的实验设计可能存在将TGP与代内可塑性、选择性死亡相混淆的问题。我们认为，当前关于热适应性TGP的研究证据尚且薄弱。为深化对TGP的生态与进化作用的认知，未来的相关研究应更为严谨地甄别可能存在的混杂因素。