Depicting the phenotypic space of the annual plant Diplotaxis acris in hyper-arid deserts

The phenotypic space encompasses the assemblage of trait combinations yielding well-suited integrated phenotypes. At the population level, understanding phenotypic space structure requires the quantification of among- and within-population variation in traits and the correlation pattern among them. Here, we studied the phenotypic space of the annual plant Diplotaxis acris occurring in hyper-arid deserts. Given the advance of warming and aridity in vast regions occupied by drylands, D. acris can indicate the successful evolutionary trajectory that many other annual plant species may follow in expanding drylands. To this end, we conducted a greenhouse experiment with 176 D. acris individuals from five Saudi populations to quantify the genetic component of variation in architectural and life-history traits. We found low among-population divergence but high among-individual variation in all traits. In addition, all traits showed a high degree of genetic determination in our study experimental conditions. We did not find significant effects of recruitment and fecundity on fitness. Finally, all architectural traits exhibited a strong correlation pattern among them, whereas for life-history traits, only higher seed germination implied earlier flowering. Seed weight appeared to be an important trait in D. acris, as individuals with heavier seeds tended to advance flowering and have a more vigorous branching pattern, which led to higher fecundity. Population divergence in D. acris might be constrained by the severity of the hyper-arid environment, but populations maintain high among-individual genetic variation in all traits. Furthermore, D. acris showed phenotypic integration for architectural traits and, to a lesser extent, for life-history traits. Overall, we hypothesize that D. acris may be fine-tuned to its demanding extreme environments. Evolutionary speaking, annual plants facing increasing warming, aridity and environmental seasonality might modify their phenotypic spaces towards new phenotypic configurations strongly dominated by correlated architectural traits enhancing fecundity and seed-related traits advancing flowering time.

表型空间（phenotypic space）涵盖了所有能够产生适配性良好的整合型表型的性状组合集合。在种群层面，解析表型空间的结构需要量化种群间与种群内的性状变异及其间的相关模式。本研究以分布于超干旱荒漠的一年生植物香芥（Diplotaxis acris）的表型空间为研究对象。鉴于干旱区广袤区域正经历变暖与干旱化加剧，香芥可作为指示物种，反映众多其他一年生植物在干旱区扩张过程中可能遵循的成功演化轨迹。为此，我们开展了温室栽培实验，选取来自沙特阿拉伯5个种群的176株香芥个体，以量化其株型性状（architectural traits）与生活史性状（life-history traits）变异的遗传组分（genetic component）。研究发现，所有性状的种群间分化程度较低，但个体间变异程度较高。此外，在本实验条件下，所有性状均表现出较高的遗传决定度（genetic determination）。未发现定植（recruitment）与繁殖力（fecundity）对适合度（fitness）存在显著影响。最后，所有株型性状间均呈现显著的相关模式；而在生活史性状中，仅较高的种子萌发率（seed germination）与更早的开花时间相关。种子重量（seed weight）在香芥中是关键性状：种子更重的个体往往开花更早，且分枝模式更为旺盛，进而带来更高的繁殖力。香芥的种群间分化可能受限于超干旱环境的严酷性，但各种群在所有性状上均保留了较高的个体间遗传变异。此外，香芥的株型性状存在表型整合（phenotypic integration），而生活史性状的表型整合程度相对较弱。综上，我们推测香芥可能已被精细适配于其所处的极端严酷环境。从演化视角来看，面临变暖、干旱化与环境季节性加剧的一年生植物，可能会调整其表型空间，转向以强化繁殖力的相关株型性状与提前开花的种子相关性状为主导的全新表型构型。