Cardamine pratensis early/late ecotype transects Dibbinsdale Nature Reserve 2012-2014

Phenological escape, whereby species alter the timing of life-history events to avoid seasonal antagonists, is usually analyzed either as a potential evolutionary outcome given current selection coefficients or as a realized outcome in response to known enemies. We here gain mechanistic insights into the evolutionary trajectory of phenological escape in the brassicaceous herb Cardamine pratensis, by comparing the flowering schedules of two sympatric ecotypes in different stages of a disruptive response to egg-laying pressure imposed by the pierid butterfly Anthocharis cardamines, whose larvae are pre-dispersal seed predators (reducing realized fecundity by ~70%). When the focal point of highest intensity selection (peak egg-laying) occurs early in the flowering schedule, selection for late flowering dependent on reduced egg-laying combined with selection for early flowering dependent on reduced predator survival results in a symmetrical bimodal flowering curve; when the focal point occurs late, an asymmetrical flowering curve results with a large early flowering mode due to selection for reduced egg-laying augmented by selection for infested plants to outrun larval development and dehisce before seed-pod consumption. Unequal selection pressures on high and low fecundity ramets, due to asynchronous flowering and morphologically targeted (size-dependent) egg-laying, constrain phenological escape, with bimodal flowering evolving primarily in response to disruptive selection on high fecundity phenotypes. These results emphasize the importance of analyzing variation in selection coefficients among morphological phenotypes over the entire flowering schedule to predict how populations will evolve in response to altered phenologies resulting from climate change.物候逃逸（phenological escape）指物种通过改变生活史事件的发生时序以规避季节性拮抗生物的现象，此类研究通常可分为两类：一类是基于当前选择系数（selection coefficients），将其视为潜在的进化结果；另一类则是将其作为响应已知天敌的已实现进化结果进行分析。本研究以十字花科草本植物（brassicaceous herb）碎米荠（Cardamine pratensis）为研究对象，通过比较两种同域生态型（sympatric ecotypes）的开花物候期（flowering schedule），解析其物候逃逸的进化轨迹。这两种生态型正处于由粉蝶科蝴蝶（pierid butterfly）橙尖粉蝶（Anthocharis cardamines）施加的产卵压力（egg-laying pressure）所引发的歧化响应（disruptive response）的不同阶段，该蝴蝶的幼虫为种子扩散前捕食者（pre-dispersal seed predators），可使宿主的实际繁殖力（realized fecundity）降低约70%。当选择强度最高的核心节点（产卵峰值）出现在开花物候期早期时，依赖于低产卵量的晚花选择，与依赖于天敌低存活率的早花选择共同作用，将形成对称双峰开花曲线（symmetrical bimodal flowering curve）；而当该核心节点出现在开花晚期时，则会形成不对称开花曲线（asymmetrical flowering curve），早花模式（early flowering mode）占据主导——这一方面源于选择偏向于降低产卵量，另一方面受侵染植株（infested plants）可通过赶超幼虫发育（outrun larval development）、在豆荚被取食前开裂（dehisce before seed-pod consumption）来规避损失，进一步强化了早花选择优势。由于开花异步性（asynchronous flowering）与体型依赖型产卵（size-dependent egg-laying）的影响，高、低繁殖力分株（ramets）所承受的选择压力并不均等，这一因素会限制物候逃逸的演化进程；而双峰开花表型的演化，主要是对高繁殖力表型（high fecundity phenotypes）产生歧化选择（disruptive selection）的响应结果。本研究结果强调，若要准确预测种群如何响应气候变化（climate change）引发的物候改变而发生进化，需对整个开花物候期内不同形态表型的选择系数差异进行全面分析。