Season-dependent effect of cleistogamy in Lamium amplexicaule: flower type origin versus inbreeding status

Premise – Inbreeding depression is traditionally considered a major factor favoring outcrossing in hermaphrodites. Recent experiments have shown that environmental conditions can influence the magnitude of inbreeding depression, but their relevance in natural populations is unclear. To investigate this, we studied the cleistogamous species Lamium amplexicaule, an annual species with individuals experiencing either spring or autumn environmental conditions. In this species, the proportion of cleistogamous/chasmogamous flowers changes according to seasonal cues (e.g. temperature, photoperiod, etc.). Our hypothesis was that the plasticity of cleistogamy is an adaptation to seasonal fitness variation in different flower progenies. Methods – To test this, we produced the three possible types of progenies through controlled crosses: (1) selfed progeny from cleistogamous flowers, (2) chasmogamous selfed progeny, and (3) chasmogamous outcrossed progeny. Progeny fitness was then measured in common garden in the two reproduction seasons (autumn and spring). Results – The results showed that season had a major impact on fitness. The fitness of the different progeny types changed across seasons, indicating that the effect of cleistogamy on progeny performance is season-dependent, consistent with a previous study in a similar environment. Surprisingly, the flower from which the progeny issued (cleistogamous or chasmogamous) had more impact on fitness than the inbred status of the progeny (selfed versus outcrossed), suggesting a potential role of epigenetic processes. Conclusions – The observed patterns of variation were not consistent either with adaptation to environment-dependent inbreeding depression or to variation in resource availability, but were possibly consistent with adaptation to seasonal pollinator activity.

研究背景——近交衰退（inbreeding depression）传统上被视为推动雌雄同体植物（hermaphrodites）选择异交（outcrossing）的核心驱动因素。近期实验证实，环境条件可调控近交衰退的强度，但该效应在自然种群中的实际相关性仍有待明确。为探究该问题，我们以闭花受精物种宝盖草（Lamium amplexicaule）为研究对象，该物种为一年生植物，其个体分别经历春季或秋季的环境条件。该物种的闭花受精（cleistogamy）与开花受精（chasmogamy）花朵的比例，会随温度、光周期等季节信号发生动态变化。我们提出的研究假说为：闭花受精的表型可塑性，是对不同花子代的季节适合度变异的适应。
研究方法——为验证上述假说，我们通过控制性杂交获得了三种可育子代：（1）闭花受精花朵产生的自交子代；（2）开花受精花朵产生的自交子代；（3）开花受精花朵产生的异交子代。随后在同质园（common garden）试验中，于两个繁殖季节（秋季与春季）分别测定子代的适合度。
研究结果——结果表明，季节对子代适合度具有显著影响。不同子代类型的适合度随季节发生显著变化，提示闭花受精对子代表现的效应具有季节依赖性，这与此前一项相似环境下的研究结果相符。值得注意的是，子代来源的花朵类型（闭花受精或开花受精）对子代适合度的影响，强于子代自身的近交状态（自交与异交），这暗示表观遗传过程（epigenetic processes）可能在此发挥了潜在作用。
研究结论——本次观测到的适合度变异模式，既不符合适应环境依赖性近交衰退的假说，也不符合资源可用性变化的适应假说，但可能与适应季节传粉者活动（pollinator activity）的变化相一致。