Data from: Secondary reproduction in the herbaceous monocarp Lobelia inflata: time-constrained primary reproduction does not result in increased deferral of reproductive effort

Background: Although semelparity is a life history characterized by a single reproductive episode within a single reproductive season, some semelparous organisms facultatively express a second bout of reproduction, either in a subsequent season (“facultative iteroparity”) or later within the same season as the primary bout (“secondary reproduction”). Secondary reproduction has been explained as the adaptive deferral of reproductive potential under circumstances in which some fraction of reproductive success would otherwise have been lost (due, for example, to inopportune timing). This deferral hypothesis predicts a positive relationship between constraints on primary reproduction and expression of secondary reproduction. The herbaceous monocarp Lobelia inflata has been observed occasionally to express a secondary reproductive episode in the field. However, it is unknown whether secondary reproduction is an example of adaptive reproductive deferral, or is more parsimoniously explained as the vestigial expression of iteroparity after a recent transition to semelparity. Here, we experimentally manipulate effective season length in each of three years to test whether secondary reproduction is a form of adaptive plasticity consistent with the deferral hypothesis. Results: Our results were found to be inconsistent with the adaptive deferral explanation: first, plants whose primary reproduction was time-constrained exhibited decreased (not increased) allocation to subsequent secondary reproduction, a result that was consistent across all three years; second, secondary offspring—although viable in the laboratory—would not have the opportunity for expression under field conditions, and would thus not contribute to reproductive success. Conclusions: Although alternative adaptive explanations for secondary reproduction cannot be precluded, we conclude that the characteristics of secondary reproduction found in L. inflata are consistent with predictions of incomplete or transitional evolution to annual semelparity.

研究背景：一次结实（semelparity）是指在单个生殖季内仅完成一次生殖事件的生活史策略，但部分一次结实生物可兼性表达第二次生殖事件——既可在后续生殖季发生（称为“兼性多次结实（facultative iteroparity）”），也可在同一生殖季内的初次生殖事件之后发生（称为“次生生殖（secondary reproduction）”）。次生生殖曾被解释为：当部分生殖成功率因特定因素（如不适宜的发育时机）本会流失时，对生殖潜力的适应性延迟策略。该延迟假说预测，初次生殖受到的约束强度与次生生殖的表达程度呈正相关。已有观测显示，草本单次结实植物紫花半边莲（Lobelia inflata）在野外环境中偶发次生生殖事件，但目前尚不明确该次生生殖究竟属于适应性生殖延迟策略，还是更符合进化简约性原则的、近期向一次结实演化过渡后残留的多次结实表达特征。本研究通过三年间的实验操控有效生殖季长度，验证次生生殖是否符合延迟假说所预测的适应性可塑性特征。研究结果：本研究结果与适应性延迟解释不符：其一，初次生殖受时间约束的植株，其对后续次生生殖的资源分配反而降低（而非升高），该结果在三年实验中均保持一致；其二，尽管次生后代在实验室条件下具备生存能力，但在野外环境中无法获得表达机会，因此无法对种群生殖成功率产生贡献。研究结论：尽管无法排除次生生殖存在其他适应性解释的可能性，但我们认为，紫花半边莲（L. inflata）中观测到的次生生殖特征，符合向一年生一次结实演化过程中不完全过渡或过渡阶段的预测。